core/iter/traits/iterator.rs
1use super::super::{
2 ArrayChunks, ByRefSized, Chain, Cloned, Copied, Cycle, Enumerate, Filter, FilterMap, FlatMap,
3 Flatten, Fuse, Inspect, Intersperse, IntersperseWith, Map, MapWhile, MapWindows, Peekable,
4 Product, Rev, Scan, Skip, SkipWhile, StepBy, Sum, Take, TakeWhile, TrustedRandomAccessNoCoerce,
5 Zip, try_process,
6};
7use crate::array;
8use crate::cmp::{self, Ordering};
9use crate::num::NonZero;
10use crate::ops::{ChangeOutputType, ControlFlow, FromResidual, Residual, Try};
11
12fn _assert_is_dyn_compatible(_: &dyn Iterator<Item = ()>) {}
13
14/// A trait for dealing with iterators.
15///
16/// This is the main iterator trait. For more about the concept of iterators
17/// generally, please see the [module-level documentation]. In particular, you
18/// may want to know how to [implement `Iterator`][impl].
19///
20/// [module-level documentation]: crate::iter
21/// [impl]: crate::iter#implementing-iterator
22#[stable(feature = "rust1", since = "1.0.0")]
23#[rustc_on_unimplemented(
24 on(
25 _Self = "core::ops::range::RangeTo<Idx>",
26 note = "you might have meant to use a bounded `Range`"
27 ),
28 on(
29 _Self = "core::ops::range::RangeToInclusive<Idx>",
30 note = "you might have meant to use a bounded `RangeInclusive`"
31 ),
32 label = "`{Self}` is not an iterator",
33 message = "`{Self}` is not an iterator"
34)]
35#[doc(notable_trait)]
36#[lang = "iterator"]
37#[rustc_diagnostic_item = "Iterator"]
38#[must_use = "iterators are lazy and do nothing unless consumed"]
39pub trait Iterator {
40 /// The type of the elements being iterated over.
41 #[rustc_diagnostic_item = "IteratorItem"]
42 #[stable(feature = "rust1", since = "1.0.0")]
43 type Item;
44
45 /// Advances the iterator and returns the next value.
46 ///
47 /// Returns [`None`] when iteration is finished. Individual iterator
48 /// implementations may choose to resume iteration, and so calling `next()`
49 /// again may or may not eventually start returning [`Some(Item)`] again at some
50 /// point.
51 ///
52 /// [`Some(Item)`]: Some
53 ///
54 /// # Examples
55 ///
56 /// ```
57 /// let a = [1, 2, 3];
58 ///
59 /// let mut iter = a.iter();
60 ///
61 /// // A call to next() returns the next value...
62 /// assert_eq!(Some(&1), iter.next());
63 /// assert_eq!(Some(&2), iter.next());
64 /// assert_eq!(Some(&3), iter.next());
65 ///
66 /// // ... and then None once it's over.
67 /// assert_eq!(None, iter.next());
68 ///
69 /// // More calls may or may not return `None`. Here, they always will.
70 /// assert_eq!(None, iter.next());
71 /// assert_eq!(None, iter.next());
72 /// ```
73 #[lang = "next"]
74 #[stable(feature = "rust1", since = "1.0.0")]
75 fn next(&mut self) -> Option<Self::Item>;
76
77 /// Advances the iterator and returns an array containing the next `N` values.
78 ///
79 /// If there are not enough elements to fill the array then `Err` is returned
80 /// containing an iterator over the remaining elements.
81 ///
82 /// # Examples
83 ///
84 /// Basic usage:
85 ///
86 /// ```
87 /// #![feature(iter_next_chunk)]
88 ///
89 /// let mut iter = "lorem".chars();
90 ///
91 /// assert_eq!(iter.next_chunk().unwrap(), ['l', 'o']); // N is inferred as 2
92 /// assert_eq!(iter.next_chunk().unwrap(), ['r', 'e', 'm']); // N is inferred as 3
93 /// assert_eq!(iter.next_chunk::<4>().unwrap_err().as_slice(), &[]); // N is explicitly 4
94 /// ```
95 ///
96 /// Split a string and get the first three items.
97 ///
98 /// ```
99 /// #![feature(iter_next_chunk)]
100 ///
101 /// let quote = "not all those who wander are lost";
102 /// let [first, second, third] = quote.split_whitespace().next_chunk().unwrap();
103 /// assert_eq!(first, "not");
104 /// assert_eq!(second, "all");
105 /// assert_eq!(third, "those");
106 /// ```
107 #[inline]
108 #[unstable(feature = "iter_next_chunk", reason = "recently added", issue = "98326")]
109 fn next_chunk<const N: usize>(
110 &mut self,
111 ) -> Result<[Self::Item; N], array::IntoIter<Self::Item, N>>
112 where
113 Self: Sized,
114 {
115 array::iter_next_chunk(self)
116 }
117
118 /// Returns the bounds on the remaining length of the iterator.
119 ///
120 /// Specifically, `size_hint()` returns a tuple where the first element
121 /// is the lower bound, and the second element is the upper bound.
122 ///
123 /// The second half of the tuple that is returned is an <code>[Option]<[usize]></code>.
124 /// A [`None`] here means that either there is no known upper bound, or the
125 /// upper bound is larger than [`usize`].
126 ///
127 /// # Implementation notes
128 ///
129 /// It is not enforced that an iterator implementation yields the declared
130 /// number of elements. A buggy iterator may yield less than the lower bound
131 /// or more than the upper bound of elements.
132 ///
133 /// `size_hint()` is primarily intended to be used for optimizations such as
134 /// reserving space for the elements of the iterator, but must not be
135 /// trusted to e.g., omit bounds checks in unsafe code. An incorrect
136 /// implementation of `size_hint()` should not lead to memory safety
137 /// violations.
138 ///
139 /// That said, the implementation should provide a correct estimation,
140 /// because otherwise it would be a violation of the trait's protocol.
141 ///
142 /// The default implementation returns <code>(0, [None])</code> which is correct for any
143 /// iterator.
144 ///
145 /// # Examples
146 ///
147 /// Basic usage:
148 ///
149 /// ```
150 /// let a = [1, 2, 3];
151 /// let mut iter = a.iter();
152 ///
153 /// assert_eq!((3, Some(3)), iter.size_hint());
154 /// let _ = iter.next();
155 /// assert_eq!((2, Some(2)), iter.size_hint());
156 /// ```
157 ///
158 /// A more complex example:
159 ///
160 /// ```
161 /// // The even numbers in the range of zero to nine.
162 /// let iter = (0..10).filter(|x| x % 2 == 0);
163 ///
164 /// // We might iterate from zero to ten times. Knowing that it's five
165 /// // exactly wouldn't be possible without executing filter().
166 /// assert_eq!((0, Some(10)), iter.size_hint());
167 ///
168 /// // Let's add five more numbers with chain()
169 /// let iter = (0..10).filter(|x| x % 2 == 0).chain(15..20);
170 ///
171 /// // now both bounds are increased by five
172 /// assert_eq!((5, Some(15)), iter.size_hint());
173 /// ```
174 ///
175 /// Returning `None` for an upper bound:
176 ///
177 /// ```
178 /// // an infinite iterator has no upper bound
179 /// // and the maximum possible lower bound
180 /// let iter = 0..;
181 ///
182 /// assert_eq!((usize::MAX, None), iter.size_hint());
183 /// ```
184 #[inline]
185 #[stable(feature = "rust1", since = "1.0.0")]
186 fn size_hint(&self) -> (usize, Option<usize>) {
187 (0, None)
188 }
189
190 /// Consumes the iterator, counting the number of iterations and returning it.
191 ///
192 /// This method will call [`next`] repeatedly until [`None`] is encountered,
193 /// returning the number of times it saw [`Some`]. Note that [`next`] has to be
194 /// called at least once even if the iterator does not have any elements.
195 ///
196 /// [`next`]: Iterator::next
197 ///
198 /// # Overflow Behavior
199 ///
200 /// The method does no guarding against overflows, so counting elements of
201 /// an iterator with more than [`usize::MAX`] elements either produces the
202 /// wrong result or panics. If debug assertions are enabled, a panic is
203 /// guaranteed.
204 ///
205 /// # Panics
206 ///
207 /// This function might panic if the iterator has more than [`usize::MAX`]
208 /// elements.
209 ///
210 /// # Examples
211 ///
212 /// ```
213 /// let a = [1, 2, 3];
214 /// assert_eq!(a.iter().count(), 3);
215 ///
216 /// let a = [1, 2, 3, 4, 5];
217 /// assert_eq!(a.iter().count(), 5);
218 /// ```
219 #[inline]
220 #[stable(feature = "rust1", since = "1.0.0")]
221 fn count(self) -> usize
222 where
223 Self: Sized,
224 {
225 self.fold(
226 0,
227 #[rustc_inherit_overflow_checks]
228 |count, _| count + 1,
229 )
230 }
231
232 /// Consumes the iterator, returning the last element.
233 ///
234 /// This method will evaluate the iterator until it returns [`None`]. While
235 /// doing so, it keeps track of the current element. After [`None`] is
236 /// returned, `last()` will then return the last element it saw.
237 ///
238 /// # Examples
239 ///
240 /// ```
241 /// let a = [1, 2, 3];
242 /// assert_eq!(a.iter().last(), Some(&3));
243 ///
244 /// let a = [1, 2, 3, 4, 5];
245 /// assert_eq!(a.iter().last(), Some(&5));
246 /// ```
247 #[inline]
248 #[stable(feature = "rust1", since = "1.0.0")]
249 fn last(self) -> Option<Self::Item>
250 where
251 Self: Sized,
252 {
253 #[inline]
254 fn some<T>(_: Option<T>, x: T) -> Option<T> {
255 Some(x)
256 }
257
258 self.fold(None, some)
259 }
260
261 /// Advances the iterator by `n` elements.
262 ///
263 /// This method will eagerly skip `n` elements by calling [`next`] up to `n`
264 /// times until [`None`] is encountered.
265 ///
266 /// `advance_by(n)` will return `Ok(())` if the iterator successfully advances by
267 /// `n` elements, or a `Err(NonZero<usize>)` with value `k` if [`None`] is encountered,
268 /// where `k` is remaining number of steps that could not be advanced because the iterator ran out.
269 /// If `self` is empty and `n` is non-zero, then this returns `Err(n)`.
270 /// Otherwise, `k` is always less than `n`.
271 ///
272 /// Calling `advance_by(0)` can do meaningful work, for example [`Flatten`]
273 /// can advance its outer iterator until it finds an inner iterator that is not empty, which
274 /// then often allows it to return a more accurate `size_hint()` than in its initial state.
275 ///
276 /// [`Flatten`]: crate::iter::Flatten
277 /// [`next`]: Iterator::next
278 ///
279 /// # Examples
280 ///
281 /// ```
282 /// #![feature(iter_advance_by)]
283 ///
284 /// use std::num::NonZero;
285 ///
286 /// let a = [1, 2, 3, 4];
287 /// let mut iter = a.iter();
288 ///
289 /// assert_eq!(iter.advance_by(2), Ok(()));
290 /// assert_eq!(iter.next(), Some(&3));
291 /// assert_eq!(iter.advance_by(0), Ok(()));
292 /// assert_eq!(iter.advance_by(100), Err(NonZero::new(99).unwrap())); // only `&4` was skipped
293 /// ```
294 #[inline]
295 #[unstable(feature = "iter_advance_by", reason = "recently added", issue = "77404")]
296 fn advance_by(&mut self, n: usize) -> Result<(), NonZero<usize>> {
297 for i in 0..n {
298 if self.next().is_none() {
299 // SAFETY: `i` is always less than `n`.
300 return Err(unsafe { NonZero::new_unchecked(n - i) });
301 }
302 }
303 Ok(())
304 }
305
306 /// Returns the `n`th element of the iterator.
307 ///
308 /// Like most indexing operations, the count starts from zero, so `nth(0)`
309 /// returns the first value, `nth(1)` the second, and so on.
310 ///
311 /// Note that all preceding elements, as well as the returned element, will be
312 /// consumed from the iterator. That means that the preceding elements will be
313 /// discarded, and also that calling `nth(0)` multiple times on the same iterator
314 /// will return different elements.
315 ///
316 /// `nth()` will return [`None`] if `n` is greater than or equal to the length of the
317 /// iterator.
318 ///
319 /// # Examples
320 ///
321 /// Basic usage:
322 ///
323 /// ```
324 /// let a = [1, 2, 3];
325 /// assert_eq!(a.iter().nth(1), Some(&2));
326 /// ```
327 ///
328 /// Calling `nth()` multiple times doesn't rewind the iterator:
329 ///
330 /// ```
331 /// let a = [1, 2, 3];
332 ///
333 /// let mut iter = a.iter();
334 ///
335 /// assert_eq!(iter.nth(1), Some(&2));
336 /// assert_eq!(iter.nth(1), None);
337 /// ```
338 ///
339 /// Returning `None` if there are less than `n + 1` elements:
340 ///
341 /// ```
342 /// let a = [1, 2, 3];
343 /// assert_eq!(a.iter().nth(10), None);
344 /// ```
345 #[inline]
346 #[stable(feature = "rust1", since = "1.0.0")]
347 fn nth(&mut self, n: usize) -> Option<Self::Item> {
348 self.advance_by(n).ok()?;
349 self.next()
350 }
351
352 /// Creates an iterator starting at the same point, but stepping by
353 /// the given amount at each iteration.
354 ///
355 /// Note 1: The first element of the iterator will always be returned,
356 /// regardless of the step given.
357 ///
358 /// Note 2: The time at which ignored elements are pulled is not fixed.
359 /// `StepBy` behaves like the sequence `self.next()`, `self.nth(step-1)`,
360 /// `self.nth(step-1)`, …, but is also free to behave like the sequence
361 /// `advance_n_and_return_first(&mut self, step)`,
362 /// `advance_n_and_return_first(&mut self, step)`, …
363 /// Which way is used may change for some iterators for performance reasons.
364 /// The second way will advance the iterator earlier and may consume more items.
365 ///
366 /// `advance_n_and_return_first` is the equivalent of:
367 /// ```
368 /// fn advance_n_and_return_first<I>(iter: &mut I, n: usize) -> Option<I::Item>
369 /// where
370 /// I: Iterator,
371 /// {
372 /// let next = iter.next();
373 /// if n > 1 {
374 /// iter.nth(n - 2);
375 /// }
376 /// next
377 /// }
378 /// ```
379 ///
380 /// # Panics
381 ///
382 /// The method will panic if the given step is `0`.
383 ///
384 /// # Examples
385 ///
386 /// ```
387 /// let a = [0, 1, 2, 3, 4, 5];
388 /// let mut iter = a.iter().step_by(2);
389 ///
390 /// assert_eq!(iter.next(), Some(&0));
391 /// assert_eq!(iter.next(), Some(&2));
392 /// assert_eq!(iter.next(), Some(&4));
393 /// assert_eq!(iter.next(), None);
394 /// ```
395 #[inline]
396 #[stable(feature = "iterator_step_by", since = "1.28.0")]
397 fn step_by(self, step: usize) -> StepBy<Self>
398 where
399 Self: Sized,
400 {
401 StepBy::new(self, step)
402 }
403
404 /// Takes two iterators and creates a new iterator over both in sequence.
405 ///
406 /// `chain()` will return a new iterator which will first iterate over
407 /// values from the first iterator and then over values from the second
408 /// iterator.
409 ///
410 /// In other words, it links two iterators together, in a chain. 🔗
411 ///
412 /// [`once`] is commonly used to adapt a single value into a chain of
413 /// other kinds of iteration.
414 ///
415 /// # Examples
416 ///
417 /// Basic usage:
418 ///
419 /// ```
420 /// let a1 = [1, 2, 3];
421 /// let a2 = [4, 5, 6];
422 ///
423 /// let mut iter = a1.iter().chain(a2.iter());
424 ///
425 /// assert_eq!(iter.next(), Some(&1));
426 /// assert_eq!(iter.next(), Some(&2));
427 /// assert_eq!(iter.next(), Some(&3));
428 /// assert_eq!(iter.next(), Some(&4));
429 /// assert_eq!(iter.next(), Some(&5));
430 /// assert_eq!(iter.next(), Some(&6));
431 /// assert_eq!(iter.next(), None);
432 /// ```
433 ///
434 /// Since the argument to `chain()` uses [`IntoIterator`], we can pass
435 /// anything that can be converted into an [`Iterator`], not just an
436 /// [`Iterator`] itself. For example, slices (`&[T]`) implement
437 /// [`IntoIterator`], and so can be passed to `chain()` directly:
438 ///
439 /// ```
440 /// let s1 = &[1, 2, 3];
441 /// let s2 = &[4, 5, 6];
442 ///
443 /// let mut iter = s1.iter().chain(s2);
444 ///
445 /// assert_eq!(iter.next(), Some(&1));
446 /// assert_eq!(iter.next(), Some(&2));
447 /// assert_eq!(iter.next(), Some(&3));
448 /// assert_eq!(iter.next(), Some(&4));
449 /// assert_eq!(iter.next(), Some(&5));
450 /// assert_eq!(iter.next(), Some(&6));
451 /// assert_eq!(iter.next(), None);
452 /// ```
453 ///
454 /// If you work with Windows API, you may wish to convert [`OsStr`] to `Vec<u16>`:
455 ///
456 /// ```
457 /// #[cfg(windows)]
458 /// fn os_str_to_utf16(s: &std::ffi::OsStr) -> Vec<u16> {
459 /// use std::os::windows::ffi::OsStrExt;
460 /// s.encode_wide().chain(std::iter::once(0)).collect()
461 /// }
462 /// ```
463 ///
464 /// [`once`]: crate::iter::once
465 /// [`OsStr`]: ../../std/ffi/struct.OsStr.html
466 #[inline]
467 #[stable(feature = "rust1", since = "1.0.0")]
468 fn chain<U>(self, other: U) -> Chain<Self, U::IntoIter>
469 where
470 Self: Sized,
471 U: IntoIterator<Item = Self::Item>,
472 {
473 Chain::new(self, other.into_iter())
474 }
475
476 /// 'Zips up' two iterators into a single iterator of pairs.
477 ///
478 /// `zip()` returns a new iterator that will iterate over two other
479 /// iterators, returning a tuple where the first element comes from the
480 /// first iterator, and the second element comes from the second iterator.
481 ///
482 /// In other words, it zips two iterators together, into a single one.
483 ///
484 /// If either iterator returns [`None`], [`next`] from the zipped iterator
485 /// will return [`None`].
486 /// If the zipped iterator has no more elements to return then each further attempt to advance
487 /// it will first try to advance the first iterator at most one time and if it still yielded an item
488 /// try to advance the second iterator at most one time.
489 ///
490 /// To 'undo' the result of zipping up two iterators, see [`unzip`].
491 ///
492 /// [`unzip`]: Iterator::unzip
493 ///
494 /// # Examples
495 ///
496 /// Basic usage:
497 ///
498 /// ```
499 /// let a1 = [1, 2, 3];
500 /// let a2 = [4, 5, 6];
501 ///
502 /// let mut iter = a1.iter().zip(a2.iter());
503 ///
504 /// assert_eq!(iter.next(), Some((&1, &4)));
505 /// assert_eq!(iter.next(), Some((&2, &5)));
506 /// assert_eq!(iter.next(), Some((&3, &6)));
507 /// assert_eq!(iter.next(), None);
508 /// ```
509 ///
510 /// Since the argument to `zip()` uses [`IntoIterator`], we can pass
511 /// anything that can be converted into an [`Iterator`], not just an
512 /// [`Iterator`] itself. For example, slices (`&[T]`) implement
513 /// [`IntoIterator`], and so can be passed to `zip()` directly:
514 ///
515 /// ```
516 /// let s1 = &[1, 2, 3];
517 /// let s2 = &[4, 5, 6];
518 ///
519 /// let mut iter = s1.iter().zip(s2);
520 ///
521 /// assert_eq!(iter.next(), Some((&1, &4)));
522 /// assert_eq!(iter.next(), Some((&2, &5)));
523 /// assert_eq!(iter.next(), Some((&3, &6)));
524 /// assert_eq!(iter.next(), None);
525 /// ```
526 ///
527 /// `zip()` is often used to zip an infinite iterator to a finite one.
528 /// This works because the finite iterator will eventually return [`None`],
529 /// ending the zipper. Zipping with `(0..)` can look a lot like [`enumerate`]:
530 ///
531 /// ```
532 /// let enumerate: Vec<_> = "foo".chars().enumerate().collect();
533 ///
534 /// let zipper: Vec<_> = (0..).zip("foo".chars()).collect();
535 ///
536 /// assert_eq!((0, 'f'), enumerate[0]);
537 /// assert_eq!((0, 'f'), zipper[0]);
538 ///
539 /// assert_eq!((1, 'o'), enumerate[1]);
540 /// assert_eq!((1, 'o'), zipper[1]);
541 ///
542 /// assert_eq!((2, 'o'), enumerate[2]);
543 /// assert_eq!((2, 'o'), zipper[2]);
544 /// ```
545 ///
546 /// If both iterators have roughly equivalent syntax, it may be more readable to use [`zip`]:
547 ///
548 /// ```
549 /// use std::iter::zip;
550 ///
551 /// let a = [1, 2, 3];
552 /// let b = [2, 3, 4];
553 ///
554 /// let mut zipped = zip(
555 /// a.into_iter().map(|x| x * 2).skip(1),
556 /// b.into_iter().map(|x| x * 2).skip(1),
557 /// );
558 ///
559 /// assert_eq!(zipped.next(), Some((4, 6)));
560 /// assert_eq!(zipped.next(), Some((6, 8)));
561 /// assert_eq!(zipped.next(), None);
562 /// ```
563 ///
564 /// compared to:
565 ///
566 /// ```
567 /// # let a = [1, 2, 3];
568 /// # let b = [2, 3, 4];
569 /// #
570 /// let mut zipped = a
571 /// .into_iter()
572 /// .map(|x| x * 2)
573 /// .skip(1)
574 /// .zip(b.into_iter().map(|x| x * 2).skip(1));
575 /// #
576 /// # assert_eq!(zipped.next(), Some((4, 6)));
577 /// # assert_eq!(zipped.next(), Some((6, 8)));
578 /// # assert_eq!(zipped.next(), None);
579 /// ```
580 ///
581 /// [`enumerate`]: Iterator::enumerate
582 /// [`next`]: Iterator::next
583 /// [`zip`]: crate::iter::zip
584 #[inline]
585 #[stable(feature = "rust1", since = "1.0.0")]
586 fn zip<U>(self, other: U) -> Zip<Self, U::IntoIter>
587 where
588 Self: Sized,
589 U: IntoIterator,
590 {
591 Zip::new(self, other.into_iter())
592 }
593
594 /// Creates a new iterator which places a copy of `separator` between adjacent
595 /// items of the original iterator.
596 ///
597 /// In case `separator` does not implement [`Clone`] or needs to be
598 /// computed every time, use [`intersperse_with`].
599 ///
600 /// # Examples
601 ///
602 /// Basic usage:
603 ///
604 /// ```
605 /// #![feature(iter_intersperse)]
606 ///
607 /// let mut a = [0, 1, 2].iter().intersperse(&100);
608 /// assert_eq!(a.next(), Some(&0)); // The first element from `a`.
609 /// assert_eq!(a.next(), Some(&100)); // The separator.
610 /// assert_eq!(a.next(), Some(&1)); // The next element from `a`.
611 /// assert_eq!(a.next(), Some(&100)); // The separator.
612 /// assert_eq!(a.next(), Some(&2)); // The last element from `a`.
613 /// assert_eq!(a.next(), None); // The iterator is finished.
614 /// ```
615 ///
616 /// `intersperse` can be very useful to join an iterator's items using a common element:
617 /// ```
618 /// #![feature(iter_intersperse)]
619 ///
620 /// let hello = ["Hello", "World", "!"].iter().copied().intersperse(" ").collect::<String>();
621 /// assert_eq!(hello, "Hello World !");
622 /// ```
623 ///
624 /// [`Clone`]: crate::clone::Clone
625 /// [`intersperse_with`]: Iterator::intersperse_with
626 #[inline]
627 #[unstable(feature = "iter_intersperse", reason = "recently added", issue = "79524")]
628 fn intersperse(self, separator: Self::Item) -> Intersperse<Self>
629 where
630 Self: Sized,
631 Self::Item: Clone,
632 {
633 Intersperse::new(self, separator)
634 }
635
636 /// Creates a new iterator which places an item generated by `separator`
637 /// between adjacent items of the original iterator.
638 ///
639 /// The closure will be called exactly once each time an item is placed
640 /// between two adjacent items from the underlying iterator; specifically,
641 /// the closure is not called if the underlying iterator yields less than
642 /// two items and after the last item is yielded.
643 ///
644 /// If the iterator's item implements [`Clone`], it may be easier to use
645 /// [`intersperse`].
646 ///
647 /// # Examples
648 ///
649 /// Basic usage:
650 ///
651 /// ```
652 /// #![feature(iter_intersperse)]
653 ///
654 /// #[derive(PartialEq, Debug)]
655 /// struct NotClone(usize);
656 ///
657 /// let v = [NotClone(0), NotClone(1), NotClone(2)];
658 /// let mut it = v.into_iter().intersperse_with(|| NotClone(99));
659 ///
660 /// assert_eq!(it.next(), Some(NotClone(0))); // The first element from `v`.
661 /// assert_eq!(it.next(), Some(NotClone(99))); // The separator.
662 /// assert_eq!(it.next(), Some(NotClone(1))); // The next element from `v`.
663 /// assert_eq!(it.next(), Some(NotClone(99))); // The separator.
664 /// assert_eq!(it.next(), Some(NotClone(2))); // The last element from `v`.
665 /// assert_eq!(it.next(), None); // The iterator is finished.
666 /// ```
667 ///
668 /// `intersperse_with` can be used in situations where the separator needs
669 /// to be computed:
670 /// ```
671 /// #![feature(iter_intersperse)]
672 ///
673 /// let src = ["Hello", "to", "all", "people", "!!"].iter().copied();
674 ///
675 /// // The closure mutably borrows its context to generate an item.
676 /// let mut happy_emojis = [" ❤️ ", " 😀 "].iter().copied();
677 /// let separator = || happy_emojis.next().unwrap_or(" 🦀 ");
678 ///
679 /// let result = src.intersperse_with(separator).collect::<String>();
680 /// assert_eq!(result, "Hello ❤️ to 😀 all 🦀 people 🦀 !!");
681 /// ```
682 /// [`Clone`]: crate::clone::Clone
683 /// [`intersperse`]: Iterator::intersperse
684 #[inline]
685 #[unstable(feature = "iter_intersperse", reason = "recently added", issue = "79524")]
686 fn intersperse_with<G>(self, separator: G) -> IntersperseWith<Self, G>
687 where
688 Self: Sized,
689 G: FnMut() -> Self::Item,
690 {
691 IntersperseWith::new(self, separator)
692 }
693
694 /// Takes a closure and creates an iterator which calls that closure on each
695 /// element.
696 ///
697 /// `map()` transforms one iterator into another, by means of its argument:
698 /// something that implements [`FnMut`]. It produces a new iterator which
699 /// calls this closure on each element of the original iterator.
700 ///
701 /// If you are good at thinking in types, you can think of `map()` like this:
702 /// If you have an iterator that gives you elements of some type `A`, and
703 /// you want an iterator of some other type `B`, you can use `map()`,
704 /// passing a closure that takes an `A` and returns a `B`.
705 ///
706 /// `map()` is conceptually similar to a [`for`] loop. However, as `map()` is
707 /// lazy, it is best used when you're already working with other iterators.
708 /// If you're doing some sort of looping for a side effect, it's considered
709 /// more idiomatic to use [`for`] than `map()`.
710 ///
711 /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
712 ///
713 /// # Examples
714 ///
715 /// Basic usage:
716 ///
717 /// ```
718 /// let a = [1, 2, 3];
719 ///
720 /// let mut iter = a.iter().map(|x| 2 * x);
721 ///
722 /// assert_eq!(iter.next(), Some(2));
723 /// assert_eq!(iter.next(), Some(4));
724 /// assert_eq!(iter.next(), Some(6));
725 /// assert_eq!(iter.next(), None);
726 /// ```
727 ///
728 /// If you're doing some sort of side effect, prefer [`for`] to `map()`:
729 ///
730 /// ```
731 /// # #![allow(unused_must_use)]
732 /// // don't do this:
733 /// (0..5).map(|x| println!("{x}"));
734 ///
735 /// // it won't even execute, as it is lazy. Rust will warn you about this.
736 ///
737 /// // Instead, use for:
738 /// for x in 0..5 {
739 /// println!("{x}");
740 /// }
741 /// ```
742 #[rustc_diagnostic_item = "IteratorMap"]
743 #[inline]
744 #[stable(feature = "rust1", since = "1.0.0")]
745 fn map<B, F>(self, f: F) -> Map<Self, F>
746 where
747 Self: Sized,
748 F: FnMut(Self::Item) -> B,
749 {
750 Map::new(self, f)
751 }
752
753 /// Calls a closure on each element of an iterator.
754 ///
755 /// This is equivalent to using a [`for`] loop on the iterator, although
756 /// `break` and `continue` are not possible from a closure. It's generally
757 /// more idiomatic to use a `for` loop, but `for_each` may be more legible
758 /// when processing items at the end of longer iterator chains. In some
759 /// cases `for_each` may also be faster than a loop, because it will use
760 /// internal iteration on adapters like `Chain`.
761 ///
762 /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
763 ///
764 /// # Examples
765 ///
766 /// Basic usage:
767 ///
768 /// ```
769 /// use std::sync::mpsc::channel;
770 ///
771 /// let (tx, rx) = channel();
772 /// (0..5).map(|x| x * 2 + 1)
773 /// .for_each(move |x| tx.send(x).unwrap());
774 ///
775 /// let v: Vec<_> = rx.iter().collect();
776 /// assert_eq!(v, vec![1, 3, 5, 7, 9]);
777 /// ```
778 ///
779 /// For such a small example, a `for` loop may be cleaner, but `for_each`
780 /// might be preferable to keep a functional style with longer iterators:
781 ///
782 /// ```
783 /// (0..5).flat_map(|x| x * 100 .. x * 110)
784 /// .enumerate()
785 /// .filter(|&(i, x)| (i + x) % 3 == 0)
786 /// .for_each(|(i, x)| println!("{i}:{x}"));
787 /// ```
788 #[inline]
789 #[stable(feature = "iterator_for_each", since = "1.21.0")]
790 fn for_each<F>(self, f: F)
791 where
792 Self: Sized,
793 F: FnMut(Self::Item),
794 {
795 #[inline]
796 fn call<T>(mut f: impl FnMut(T)) -> impl FnMut((), T) {
797 move |(), item| f(item)
798 }
799
800 self.fold((), call(f));
801 }
802
803 /// Creates an iterator which uses a closure to determine if an element
804 /// should be yielded.
805 ///
806 /// Given an element the closure must return `true` or `false`. The returned
807 /// iterator will yield only the elements for which the closure returns
808 /// `true`.
809 ///
810 /// # Examples
811 ///
812 /// Basic usage:
813 ///
814 /// ```
815 /// let a = [0i32, 1, 2];
816 ///
817 /// let mut iter = a.iter().filter(|x| x.is_positive());
818 ///
819 /// assert_eq!(iter.next(), Some(&1));
820 /// assert_eq!(iter.next(), Some(&2));
821 /// assert_eq!(iter.next(), None);
822 /// ```
823 ///
824 /// Because the closure passed to `filter()` takes a reference, and many
825 /// iterators iterate over references, this leads to a possibly confusing
826 /// situation, where the type of the closure is a double reference:
827 ///
828 /// ```
829 /// let a = [0, 1, 2];
830 ///
831 /// let mut iter = a.iter().filter(|x| **x > 1); // need two *s!
832 ///
833 /// assert_eq!(iter.next(), Some(&2));
834 /// assert_eq!(iter.next(), None);
835 /// ```
836 ///
837 /// It's common to instead use destructuring on the argument to strip away
838 /// one:
839 ///
840 /// ```
841 /// let a = [0, 1, 2];
842 ///
843 /// let mut iter = a.iter().filter(|&x| *x > 1); // both & and *
844 ///
845 /// assert_eq!(iter.next(), Some(&2));
846 /// assert_eq!(iter.next(), None);
847 /// ```
848 ///
849 /// or both:
850 ///
851 /// ```
852 /// let a = [0, 1, 2];
853 ///
854 /// let mut iter = a.iter().filter(|&&x| x > 1); // two &s
855 ///
856 /// assert_eq!(iter.next(), Some(&2));
857 /// assert_eq!(iter.next(), None);
858 /// ```
859 ///
860 /// of these layers.
861 ///
862 /// Note that `iter.filter(f).next()` is equivalent to `iter.find(f)`.
863 #[inline]
864 #[stable(feature = "rust1", since = "1.0.0")]
865 #[cfg_attr(not(test), rustc_diagnostic_item = "iter_filter")]
866 fn filter<P>(self, predicate: P) -> Filter<Self, P>
867 where
868 Self: Sized,
869 P: FnMut(&Self::Item) -> bool,
870 {
871 Filter::new(self, predicate)
872 }
873
874 /// Creates an iterator that both filters and maps.
875 ///
876 /// The returned iterator yields only the `value`s for which the supplied
877 /// closure returns `Some(value)`.
878 ///
879 /// `filter_map` can be used to make chains of [`filter`] and [`map`] more
880 /// concise. The example below shows how a `map().filter().map()` can be
881 /// shortened to a single call to `filter_map`.
882 ///
883 /// [`filter`]: Iterator::filter
884 /// [`map`]: Iterator::map
885 ///
886 /// # Examples
887 ///
888 /// Basic usage:
889 ///
890 /// ```
891 /// let a = ["1", "two", "NaN", "four", "5"];
892 ///
893 /// let mut iter = a.iter().filter_map(|s| s.parse().ok());
894 ///
895 /// assert_eq!(iter.next(), Some(1));
896 /// assert_eq!(iter.next(), Some(5));
897 /// assert_eq!(iter.next(), None);
898 /// ```
899 ///
900 /// Here's the same example, but with [`filter`] and [`map`]:
901 ///
902 /// ```
903 /// let a = ["1", "two", "NaN", "four", "5"];
904 /// let mut iter = a.iter().map(|s| s.parse()).filter(|s| s.is_ok()).map(|s| s.unwrap());
905 /// assert_eq!(iter.next(), Some(1));
906 /// assert_eq!(iter.next(), Some(5));
907 /// assert_eq!(iter.next(), None);
908 /// ```
909 #[inline]
910 #[stable(feature = "rust1", since = "1.0.0")]
911 fn filter_map<B, F>(self, f: F) -> FilterMap<Self, F>
912 where
913 Self: Sized,
914 F: FnMut(Self::Item) -> Option<B>,
915 {
916 FilterMap::new(self, f)
917 }
918
919 /// Creates an iterator which gives the current iteration count as well as
920 /// the next value.
921 ///
922 /// The iterator returned yields pairs `(i, val)`, where `i` is the
923 /// current index of iteration and `val` is the value returned by the
924 /// iterator.
925 ///
926 /// `enumerate()` keeps its count as a [`usize`]. If you want to count by a
927 /// different sized integer, the [`zip`] function provides similar
928 /// functionality.
929 ///
930 /// # Overflow Behavior
931 ///
932 /// The method does no guarding against overflows, so enumerating more than
933 /// [`usize::MAX`] elements either produces the wrong result or panics. If
934 /// debug assertions are enabled, a panic is guaranteed.
935 ///
936 /// # Panics
937 ///
938 /// The returned iterator might panic if the to-be-returned index would
939 /// overflow a [`usize`].
940 ///
941 /// [`zip`]: Iterator::zip
942 ///
943 /// # Examples
944 ///
945 /// ```
946 /// let a = ['a', 'b', 'c'];
947 ///
948 /// let mut iter = a.iter().enumerate();
949 ///
950 /// assert_eq!(iter.next(), Some((0, &'a')));
951 /// assert_eq!(iter.next(), Some((1, &'b')));
952 /// assert_eq!(iter.next(), Some((2, &'c')));
953 /// assert_eq!(iter.next(), None);
954 /// ```
955 #[inline]
956 #[stable(feature = "rust1", since = "1.0.0")]
957 #[cfg_attr(not(test), rustc_diagnostic_item = "enumerate_method")]
958 fn enumerate(self) -> Enumerate<Self>
959 where
960 Self: Sized,
961 {
962 Enumerate::new(self)
963 }
964
965 /// Creates an iterator which can use the [`peek`] and [`peek_mut`] methods
966 /// to look at the next element of the iterator without consuming it. See
967 /// their documentation for more information.
968 ///
969 /// Note that the underlying iterator is still advanced when [`peek`] or
970 /// [`peek_mut`] are called for the first time: In order to retrieve the
971 /// next element, [`next`] is called on the underlying iterator, hence any
972 /// side effects (i.e. anything other than fetching the next value) of
973 /// the [`next`] method will occur.
974 ///
975 ///
976 /// # Examples
977 ///
978 /// Basic usage:
979 ///
980 /// ```
981 /// let xs = [1, 2, 3];
982 ///
983 /// let mut iter = xs.iter().peekable();
984 ///
985 /// // peek() lets us see into the future
986 /// assert_eq!(iter.peek(), Some(&&1));
987 /// assert_eq!(iter.next(), Some(&1));
988 ///
989 /// assert_eq!(iter.next(), Some(&2));
990 ///
991 /// // we can peek() multiple times, the iterator won't advance
992 /// assert_eq!(iter.peek(), Some(&&3));
993 /// assert_eq!(iter.peek(), Some(&&3));
994 ///
995 /// assert_eq!(iter.next(), Some(&3));
996 ///
997 /// // after the iterator is finished, so is peek()
998 /// assert_eq!(iter.peek(), None);
999 /// assert_eq!(iter.next(), None);
1000 /// ```
1001 ///
1002 /// Using [`peek_mut`] to mutate the next item without advancing the
1003 /// iterator:
1004 ///
1005 /// ```
1006 /// let xs = [1, 2, 3];
1007 ///
1008 /// let mut iter = xs.iter().peekable();
1009 ///
1010 /// // `peek_mut()` lets us see into the future
1011 /// assert_eq!(iter.peek_mut(), Some(&mut &1));
1012 /// assert_eq!(iter.peek_mut(), Some(&mut &1));
1013 /// assert_eq!(iter.next(), Some(&1));
1014 ///
1015 /// if let Some(mut p) = iter.peek_mut() {
1016 /// assert_eq!(*p, &2);
1017 /// // put a value into the iterator
1018 /// *p = &1000;
1019 /// }
1020 ///
1021 /// // The value reappears as the iterator continues
1022 /// assert_eq!(iter.collect::<Vec<_>>(), vec![&1000, &3]);
1023 /// ```
1024 /// [`peek`]: Peekable::peek
1025 /// [`peek_mut`]: Peekable::peek_mut
1026 /// [`next`]: Iterator::next
1027 #[inline]
1028 #[stable(feature = "rust1", since = "1.0.0")]
1029 fn peekable(self) -> Peekable<Self>
1030 where
1031 Self: Sized,
1032 {
1033 Peekable::new(self)
1034 }
1035
1036 /// Creates an iterator that [`skip`]s elements based on a predicate.
1037 ///
1038 /// [`skip`]: Iterator::skip
1039 ///
1040 /// `skip_while()` takes a closure as an argument. It will call this
1041 /// closure on each element of the iterator, and ignore elements
1042 /// until it returns `false`.
1043 ///
1044 /// After `false` is returned, `skip_while()`'s job is over, and the
1045 /// rest of the elements are yielded.
1046 ///
1047 /// # Examples
1048 ///
1049 /// Basic usage:
1050 ///
1051 /// ```
1052 /// let a = [-1i32, 0, 1];
1053 ///
1054 /// let mut iter = a.iter().skip_while(|x| x.is_negative());
1055 ///
1056 /// assert_eq!(iter.next(), Some(&0));
1057 /// assert_eq!(iter.next(), Some(&1));
1058 /// assert_eq!(iter.next(), None);
1059 /// ```
1060 ///
1061 /// Because the closure passed to `skip_while()` takes a reference, and many
1062 /// iterators iterate over references, this leads to a possibly confusing
1063 /// situation, where the type of the closure argument is a double reference:
1064 ///
1065 /// ```
1066 /// let a = [-1, 0, 1];
1067 ///
1068 /// let mut iter = a.iter().skip_while(|x| **x < 0); // need two *s!
1069 ///
1070 /// assert_eq!(iter.next(), Some(&0));
1071 /// assert_eq!(iter.next(), Some(&1));
1072 /// assert_eq!(iter.next(), None);
1073 /// ```
1074 ///
1075 /// Stopping after an initial `false`:
1076 ///
1077 /// ```
1078 /// let a = [-1, 0, 1, -2];
1079 ///
1080 /// let mut iter = a.iter().skip_while(|x| **x < 0);
1081 ///
1082 /// assert_eq!(iter.next(), Some(&0));
1083 /// assert_eq!(iter.next(), Some(&1));
1084 ///
1085 /// // while this would have been false, since we already got a false,
1086 /// // skip_while() isn't used any more
1087 /// assert_eq!(iter.next(), Some(&-2));
1088 ///
1089 /// assert_eq!(iter.next(), None);
1090 /// ```
1091 #[inline]
1092 #[doc(alias = "drop_while")]
1093 #[stable(feature = "rust1", since = "1.0.0")]
1094 fn skip_while<P>(self, predicate: P) -> SkipWhile<Self, P>
1095 where
1096 Self: Sized,
1097 P: FnMut(&Self::Item) -> bool,
1098 {
1099 SkipWhile::new(self, predicate)
1100 }
1101
1102 /// Creates an iterator that yields elements based on a predicate.
1103 ///
1104 /// `take_while()` takes a closure as an argument. It will call this
1105 /// closure on each element of the iterator, and yield elements
1106 /// while it returns `true`.
1107 ///
1108 /// After `false` is returned, `take_while()`'s job is over, and the
1109 /// rest of the elements are ignored.
1110 ///
1111 /// # Examples
1112 ///
1113 /// Basic usage:
1114 ///
1115 /// ```
1116 /// let a = [-1i32, 0, 1];
1117 ///
1118 /// let mut iter = a.iter().take_while(|x| x.is_negative());
1119 ///
1120 /// assert_eq!(iter.next(), Some(&-1));
1121 /// assert_eq!(iter.next(), None);
1122 /// ```
1123 ///
1124 /// Because the closure passed to `take_while()` takes a reference, and many
1125 /// iterators iterate over references, this leads to a possibly confusing
1126 /// situation, where the type of the closure is a double reference:
1127 ///
1128 /// ```
1129 /// let a = [-1, 0, 1];
1130 ///
1131 /// let mut iter = a.iter().take_while(|x| **x < 0); // need two *s!
1132 ///
1133 /// assert_eq!(iter.next(), Some(&-1));
1134 /// assert_eq!(iter.next(), None);
1135 /// ```
1136 ///
1137 /// Stopping after an initial `false`:
1138 ///
1139 /// ```
1140 /// let a = [-1, 0, 1, -2];
1141 ///
1142 /// let mut iter = a.iter().take_while(|x| **x < 0);
1143 ///
1144 /// assert_eq!(iter.next(), Some(&-1));
1145 ///
1146 /// // We have more elements that are less than zero, but since we already
1147 /// // got a false, take_while() isn't used any more
1148 /// assert_eq!(iter.next(), None);
1149 /// ```
1150 ///
1151 /// Because `take_while()` needs to look at the value in order to see if it
1152 /// should be included or not, consuming iterators will see that it is
1153 /// removed:
1154 ///
1155 /// ```
1156 /// let a = [1, 2, 3, 4];
1157 /// let mut iter = a.iter();
1158 ///
1159 /// let result: Vec<i32> = iter.by_ref()
1160 /// .take_while(|n| **n != 3)
1161 /// .cloned()
1162 /// .collect();
1163 ///
1164 /// assert_eq!(result, &[1, 2]);
1165 ///
1166 /// let result: Vec<i32> = iter.cloned().collect();
1167 ///
1168 /// assert_eq!(result, &[4]);
1169 /// ```
1170 ///
1171 /// The `3` is no longer there, because it was consumed in order to see if
1172 /// the iteration should stop, but wasn't placed back into the iterator.
1173 #[inline]
1174 #[stable(feature = "rust1", since = "1.0.0")]
1175 fn take_while<P>(self, predicate: P) -> TakeWhile<Self, P>
1176 where
1177 Self: Sized,
1178 P: FnMut(&Self::Item) -> bool,
1179 {
1180 TakeWhile::new(self, predicate)
1181 }
1182
1183 /// Creates an iterator that both yields elements based on a predicate and maps.
1184 ///
1185 /// `map_while()` takes a closure as an argument. It will call this
1186 /// closure on each element of the iterator, and yield elements
1187 /// while it returns [`Some(_)`][`Some`].
1188 ///
1189 /// # Examples
1190 ///
1191 /// Basic usage:
1192 ///
1193 /// ```
1194 /// let a = [-1i32, 4, 0, 1];
1195 ///
1196 /// let mut iter = a.iter().map_while(|x| 16i32.checked_div(*x));
1197 ///
1198 /// assert_eq!(iter.next(), Some(-16));
1199 /// assert_eq!(iter.next(), Some(4));
1200 /// assert_eq!(iter.next(), None);
1201 /// ```
1202 ///
1203 /// Here's the same example, but with [`take_while`] and [`map`]:
1204 ///
1205 /// [`take_while`]: Iterator::take_while
1206 /// [`map`]: Iterator::map
1207 ///
1208 /// ```
1209 /// let a = [-1i32, 4, 0, 1];
1210 ///
1211 /// let mut iter = a.iter()
1212 /// .map(|x| 16i32.checked_div(*x))
1213 /// .take_while(|x| x.is_some())
1214 /// .map(|x| x.unwrap());
1215 ///
1216 /// assert_eq!(iter.next(), Some(-16));
1217 /// assert_eq!(iter.next(), Some(4));
1218 /// assert_eq!(iter.next(), None);
1219 /// ```
1220 ///
1221 /// Stopping after an initial [`None`]:
1222 ///
1223 /// ```
1224 /// let a = [0, 1, 2, -3, 4, 5, -6];
1225 ///
1226 /// let iter = a.iter().map_while(|x| u32::try_from(*x).ok());
1227 /// let vec = iter.collect::<Vec<_>>();
1228 ///
1229 /// // We have more elements which could fit in u32 (4, 5), but `map_while` returned `None` for `-3`
1230 /// // (as the `predicate` returned `None`) and `collect` stops at the first `None` encountered.
1231 /// assert_eq!(vec, vec![0, 1, 2]);
1232 /// ```
1233 ///
1234 /// Because `map_while()` needs to look at the value in order to see if it
1235 /// should be included or not, consuming iterators will see that it is
1236 /// removed:
1237 ///
1238 /// ```
1239 /// let a = [1, 2, -3, 4];
1240 /// let mut iter = a.iter();
1241 ///
1242 /// let result: Vec<u32> = iter.by_ref()
1243 /// .map_while(|n| u32::try_from(*n).ok())
1244 /// .collect();
1245 ///
1246 /// assert_eq!(result, &[1, 2]);
1247 ///
1248 /// let result: Vec<i32> = iter.cloned().collect();
1249 ///
1250 /// assert_eq!(result, &[4]);
1251 /// ```
1252 ///
1253 /// The `-3` is no longer there, because it was consumed in order to see if
1254 /// the iteration should stop, but wasn't placed back into the iterator.
1255 ///
1256 /// Note that unlike [`take_while`] this iterator is **not** fused.
1257 /// It is also not specified what this iterator returns after the first [`None`] is returned.
1258 /// If you need fused iterator, use [`fuse`].
1259 ///
1260 /// [`fuse`]: Iterator::fuse
1261 #[inline]
1262 #[stable(feature = "iter_map_while", since = "1.57.0")]
1263 fn map_while<B, P>(self, predicate: P) -> MapWhile<Self, P>
1264 where
1265 Self: Sized,
1266 P: FnMut(Self::Item) -> Option<B>,
1267 {
1268 MapWhile::new(self, predicate)
1269 }
1270
1271 /// Creates an iterator that skips the first `n` elements.
1272 ///
1273 /// `skip(n)` skips elements until `n` elements are skipped or the end of the
1274 /// iterator is reached (whichever happens first). After that, all the remaining
1275 /// elements are yielded. In particular, if the original iterator is too short,
1276 /// then the returned iterator is empty.
1277 ///
1278 /// Rather than overriding this method directly, instead override the `nth` method.
1279 ///
1280 /// # Examples
1281 ///
1282 /// ```
1283 /// let a = [1, 2, 3];
1284 ///
1285 /// let mut iter = a.iter().skip(2);
1286 ///
1287 /// assert_eq!(iter.next(), Some(&3));
1288 /// assert_eq!(iter.next(), None);
1289 /// ```
1290 #[inline]
1291 #[stable(feature = "rust1", since = "1.0.0")]
1292 fn skip(self, n: usize) -> Skip<Self>
1293 where
1294 Self: Sized,
1295 {
1296 Skip::new(self, n)
1297 }
1298
1299 /// Creates an iterator that yields the first `n` elements, or fewer
1300 /// if the underlying iterator ends sooner.
1301 ///
1302 /// `take(n)` yields elements until `n` elements are yielded or the end of
1303 /// the iterator is reached (whichever happens first).
1304 /// The returned iterator is a prefix of length `n` if the original iterator
1305 /// contains at least `n` elements, otherwise it contains all of the
1306 /// (fewer than `n`) elements of the original iterator.
1307 ///
1308 /// # Examples
1309 ///
1310 /// Basic usage:
1311 ///
1312 /// ```
1313 /// let a = [1, 2, 3];
1314 ///
1315 /// let mut iter = a.iter().take(2);
1316 ///
1317 /// assert_eq!(iter.next(), Some(&1));
1318 /// assert_eq!(iter.next(), Some(&2));
1319 /// assert_eq!(iter.next(), None);
1320 /// ```
1321 ///
1322 /// `take()` is often used with an infinite iterator, to make it finite:
1323 ///
1324 /// ```
1325 /// let mut iter = (0..).take(3);
1326 ///
1327 /// assert_eq!(iter.next(), Some(0));
1328 /// assert_eq!(iter.next(), Some(1));
1329 /// assert_eq!(iter.next(), Some(2));
1330 /// assert_eq!(iter.next(), None);
1331 /// ```
1332 ///
1333 /// If less than `n` elements are available,
1334 /// `take` will limit itself to the size of the underlying iterator:
1335 ///
1336 /// ```
1337 /// let v = [1, 2];
1338 /// let mut iter = v.into_iter().take(5);
1339 /// assert_eq!(iter.next(), Some(1));
1340 /// assert_eq!(iter.next(), Some(2));
1341 /// assert_eq!(iter.next(), None);
1342 /// ```
1343 #[inline]
1344 #[stable(feature = "rust1", since = "1.0.0")]
1345 fn take(self, n: usize) -> Take<Self>
1346 where
1347 Self: Sized,
1348 {
1349 Take::new(self, n)
1350 }
1351
1352 /// An iterator adapter which, like [`fold`], holds internal state, but
1353 /// unlike [`fold`], produces a new iterator.
1354 ///
1355 /// [`fold`]: Iterator::fold
1356 ///
1357 /// `scan()` takes two arguments: an initial value which seeds the internal
1358 /// state, and a closure with two arguments, the first being a mutable
1359 /// reference to the internal state and the second an iterator element.
1360 /// The closure can assign to the internal state to share state between
1361 /// iterations.
1362 ///
1363 /// On iteration, the closure will be applied to each element of the
1364 /// iterator and the return value from the closure, an [`Option`], is
1365 /// returned by the `next` method. Thus the closure can return
1366 /// `Some(value)` to yield `value`, or `None` to end the iteration.
1367 ///
1368 /// # Examples
1369 ///
1370 /// ```
1371 /// let a = [1, 2, 3, 4];
1372 ///
1373 /// let mut iter = a.iter().scan(1, |state, &x| {
1374 /// // each iteration, we'll multiply the state by the element ...
1375 /// *state = *state * x;
1376 ///
1377 /// // ... and terminate if the state exceeds 6
1378 /// if *state > 6 {
1379 /// return None;
1380 /// }
1381 /// // ... else yield the negation of the state
1382 /// Some(-*state)
1383 /// });
1384 ///
1385 /// assert_eq!(iter.next(), Some(-1));
1386 /// assert_eq!(iter.next(), Some(-2));
1387 /// assert_eq!(iter.next(), Some(-6));
1388 /// assert_eq!(iter.next(), None);
1389 /// ```
1390 #[inline]
1391 #[stable(feature = "rust1", since = "1.0.0")]
1392 fn scan<St, B, F>(self, initial_state: St, f: F) -> Scan<Self, St, F>
1393 where
1394 Self: Sized,
1395 F: FnMut(&mut St, Self::Item) -> Option<B>,
1396 {
1397 Scan::new(self, initial_state, f)
1398 }
1399
1400 /// Creates an iterator that works like map, but flattens nested structure.
1401 ///
1402 /// The [`map`] adapter is very useful, but only when the closure
1403 /// argument produces values. If it produces an iterator instead, there's
1404 /// an extra layer of indirection. `flat_map()` will remove this extra layer
1405 /// on its own.
1406 ///
1407 /// You can think of `flat_map(f)` as the semantic equivalent
1408 /// of [`map`]ping, and then [`flatten`]ing as in `map(f).flatten()`.
1409 ///
1410 /// Another way of thinking about `flat_map()`: [`map`]'s closure returns
1411 /// one item for each element, and `flat_map()`'s closure returns an
1412 /// iterator for each element.
1413 ///
1414 /// [`map`]: Iterator::map
1415 /// [`flatten`]: Iterator::flatten
1416 ///
1417 /// # Examples
1418 ///
1419 /// ```
1420 /// let words = ["alpha", "beta", "gamma"];
1421 ///
1422 /// // chars() returns an iterator
1423 /// let merged: String = words.iter()
1424 /// .flat_map(|s| s.chars())
1425 /// .collect();
1426 /// assert_eq!(merged, "alphabetagamma");
1427 /// ```
1428 #[inline]
1429 #[stable(feature = "rust1", since = "1.0.0")]
1430 fn flat_map<U, F>(self, f: F) -> FlatMap<Self, U, F>
1431 where
1432 Self: Sized,
1433 U: IntoIterator,
1434 F: FnMut(Self::Item) -> U,
1435 {
1436 FlatMap::new(self, f)
1437 }
1438
1439 /// Creates an iterator that flattens nested structure.
1440 ///
1441 /// This is useful when you have an iterator of iterators or an iterator of
1442 /// things that can be turned into iterators and you want to remove one
1443 /// level of indirection.
1444 ///
1445 /// # Examples
1446 ///
1447 /// Basic usage:
1448 ///
1449 /// ```
1450 /// let data = vec![vec![1, 2, 3, 4], vec![5, 6]];
1451 /// let flattened = data.into_iter().flatten().collect::<Vec<u8>>();
1452 /// assert_eq!(flattened, &[1, 2, 3, 4, 5, 6]);
1453 /// ```
1454 ///
1455 /// Mapping and then flattening:
1456 ///
1457 /// ```
1458 /// let words = ["alpha", "beta", "gamma"];
1459 ///
1460 /// // chars() returns an iterator
1461 /// let merged: String = words.iter()
1462 /// .map(|s| s.chars())
1463 /// .flatten()
1464 /// .collect();
1465 /// assert_eq!(merged, "alphabetagamma");
1466 /// ```
1467 ///
1468 /// You can also rewrite this in terms of [`flat_map()`], which is preferable
1469 /// in this case since it conveys intent more clearly:
1470 ///
1471 /// ```
1472 /// let words = ["alpha", "beta", "gamma"];
1473 ///
1474 /// // chars() returns an iterator
1475 /// let merged: String = words.iter()
1476 /// .flat_map(|s| s.chars())
1477 /// .collect();
1478 /// assert_eq!(merged, "alphabetagamma");
1479 /// ```
1480 ///
1481 /// Flattening works on any `IntoIterator` type, including `Option` and `Result`:
1482 ///
1483 /// ```
1484 /// let options = vec![Some(123), Some(321), None, Some(231)];
1485 /// let flattened_options: Vec<_> = options.into_iter().flatten().collect();
1486 /// assert_eq!(flattened_options, vec![123, 321, 231]);
1487 ///
1488 /// let results = vec![Ok(123), Ok(321), Err(456), Ok(231)];
1489 /// let flattened_results: Vec<_> = results.into_iter().flatten().collect();
1490 /// assert_eq!(flattened_results, vec![123, 321, 231]);
1491 /// ```
1492 ///
1493 /// Flattening only removes one level of nesting at a time:
1494 ///
1495 /// ```
1496 /// let d3 = [[[1, 2], [3, 4]], [[5, 6], [7, 8]]];
1497 ///
1498 /// let d2 = d3.iter().flatten().collect::<Vec<_>>();
1499 /// assert_eq!(d2, [&[1, 2], &[3, 4], &[5, 6], &[7, 8]]);
1500 ///
1501 /// let d1 = d3.iter().flatten().flatten().collect::<Vec<_>>();
1502 /// assert_eq!(d1, [&1, &2, &3, &4, &5, &6, &7, &8]);
1503 /// ```
1504 ///
1505 /// Here we see that `flatten()` does not perform a "deep" flatten.
1506 /// Instead, only one level of nesting is removed. That is, if you
1507 /// `flatten()` a three-dimensional array, the result will be
1508 /// two-dimensional and not one-dimensional. To get a one-dimensional
1509 /// structure, you have to `flatten()` again.
1510 ///
1511 /// [`flat_map()`]: Iterator::flat_map
1512 #[inline]
1513 #[stable(feature = "iterator_flatten", since = "1.29.0")]
1514 fn flatten(self) -> Flatten<Self>
1515 where
1516 Self: Sized,
1517 Self::Item: IntoIterator,
1518 {
1519 Flatten::new(self)
1520 }
1521
1522 /// Calls the given function `f` for each contiguous window of size `N` over
1523 /// `self` and returns an iterator over the outputs of `f`. Like [`slice::windows()`],
1524 /// the windows during mapping overlap as well.
1525 ///
1526 /// In the following example, the closure is called three times with the
1527 /// arguments `&['a', 'b']`, `&['b', 'c']` and `&['c', 'd']` respectively.
1528 ///
1529 /// ```
1530 /// #![feature(iter_map_windows)]
1531 ///
1532 /// let strings = "abcd".chars()
1533 /// .map_windows(|[x, y]| format!("{}+{}", x, y))
1534 /// .collect::<Vec<String>>();
1535 ///
1536 /// assert_eq!(strings, vec!["a+b", "b+c", "c+d"]);
1537 /// ```
1538 ///
1539 /// Note that the const parameter `N` is usually inferred by the
1540 /// destructured argument in the closure.
1541 ///
1542 /// The returned iterator yields 𝑘 − `N` + 1 items (where 𝑘 is the number of
1543 /// items yielded by `self`). If 𝑘 is less than `N`, this method yields an
1544 /// empty iterator.
1545 ///
1546 /// The returned iterator implements [`FusedIterator`], because once `self`
1547 /// returns `None`, even if it returns a `Some(T)` again in the next iterations,
1548 /// we cannot put it into a contiguous array buffer, and thus the returned iterator
1549 /// should be fused.
1550 ///
1551 /// [`slice::windows()`]: slice::windows
1552 /// [`FusedIterator`]: crate::iter::FusedIterator
1553 ///
1554 /// # Panics
1555 ///
1556 /// Panics if `N` is zero. This check will most probably get changed to a
1557 /// compile time error before this method gets stabilized.
1558 ///
1559 /// ```should_panic
1560 /// #![feature(iter_map_windows)]
1561 ///
1562 /// let iter = std::iter::repeat(0).map_windows(|&[]| ());
1563 /// ```
1564 ///
1565 /// # Examples
1566 ///
1567 /// Building the sums of neighboring numbers.
1568 ///
1569 /// ```
1570 /// #![feature(iter_map_windows)]
1571 ///
1572 /// let mut it = [1, 3, 8, 1].iter().map_windows(|&[a, b]| a + b);
1573 /// assert_eq!(it.next(), Some(4)); // 1 + 3
1574 /// assert_eq!(it.next(), Some(11)); // 3 + 8
1575 /// assert_eq!(it.next(), Some(9)); // 8 + 1
1576 /// assert_eq!(it.next(), None);
1577 /// ```
1578 ///
1579 /// Since the elements in the following example implement `Copy`, we can
1580 /// just copy the array and get an iterator over the windows.
1581 ///
1582 /// ```
1583 /// #![feature(iter_map_windows)]
1584 ///
1585 /// let mut it = "ferris".chars().map_windows(|w: &[_; 3]| *w);
1586 /// assert_eq!(it.next(), Some(['f', 'e', 'r']));
1587 /// assert_eq!(it.next(), Some(['e', 'r', 'r']));
1588 /// assert_eq!(it.next(), Some(['r', 'r', 'i']));
1589 /// assert_eq!(it.next(), Some(['r', 'i', 's']));
1590 /// assert_eq!(it.next(), None);
1591 /// ```
1592 ///
1593 /// You can also use this function to check the sortedness of an iterator.
1594 /// For the simple case, rather use [`Iterator::is_sorted`].
1595 ///
1596 /// ```
1597 /// #![feature(iter_map_windows)]
1598 ///
1599 /// let mut it = [0.5, 1.0, 3.5, 3.0, 8.5, 8.5, f32::NAN].iter()
1600 /// .map_windows(|[a, b]| a <= b);
1601 ///
1602 /// assert_eq!(it.next(), Some(true)); // 0.5 <= 1.0
1603 /// assert_eq!(it.next(), Some(true)); // 1.0 <= 3.5
1604 /// assert_eq!(it.next(), Some(false)); // 3.5 <= 3.0
1605 /// assert_eq!(it.next(), Some(true)); // 3.0 <= 8.5
1606 /// assert_eq!(it.next(), Some(true)); // 8.5 <= 8.5
1607 /// assert_eq!(it.next(), Some(false)); // 8.5 <= NAN
1608 /// assert_eq!(it.next(), None);
1609 /// ```
1610 ///
1611 /// For non-fused iterators, they are fused after `map_windows`.
1612 ///
1613 /// ```
1614 /// #![feature(iter_map_windows)]
1615 ///
1616 /// #[derive(Default)]
1617 /// struct NonFusedIterator {
1618 /// state: i32,
1619 /// }
1620 ///
1621 /// impl Iterator for NonFusedIterator {
1622 /// type Item = i32;
1623 ///
1624 /// fn next(&mut self) -> Option<i32> {
1625 /// let val = self.state;
1626 /// self.state = self.state + 1;
1627 ///
1628 /// // yields `0..5` first, then only even numbers since `6..`.
1629 /// if val < 5 || val % 2 == 0 {
1630 /// Some(val)
1631 /// } else {
1632 /// None
1633 /// }
1634 /// }
1635 /// }
1636 ///
1637 ///
1638 /// let mut iter = NonFusedIterator::default();
1639 ///
1640 /// // yields 0..5 first.
1641 /// assert_eq!(iter.next(), Some(0));
1642 /// assert_eq!(iter.next(), Some(1));
1643 /// assert_eq!(iter.next(), Some(2));
1644 /// assert_eq!(iter.next(), Some(3));
1645 /// assert_eq!(iter.next(), Some(4));
1646 /// // then we can see our iterator going back and forth
1647 /// assert_eq!(iter.next(), None);
1648 /// assert_eq!(iter.next(), Some(6));
1649 /// assert_eq!(iter.next(), None);
1650 /// assert_eq!(iter.next(), Some(8));
1651 /// assert_eq!(iter.next(), None);
1652 ///
1653 /// // however, with `.map_windows()`, it is fused.
1654 /// let mut iter = NonFusedIterator::default()
1655 /// .map_windows(|arr: &[_; 2]| *arr);
1656 ///
1657 /// assert_eq!(iter.next(), Some([0, 1]));
1658 /// assert_eq!(iter.next(), Some([1, 2]));
1659 /// assert_eq!(iter.next(), Some([2, 3]));
1660 /// assert_eq!(iter.next(), Some([3, 4]));
1661 /// assert_eq!(iter.next(), None);
1662 ///
1663 /// // it will always return `None` after the first time.
1664 /// assert_eq!(iter.next(), None);
1665 /// assert_eq!(iter.next(), None);
1666 /// assert_eq!(iter.next(), None);
1667 /// ```
1668 #[inline]
1669 #[unstable(feature = "iter_map_windows", reason = "recently added", issue = "87155")]
1670 fn map_windows<F, R, const N: usize>(self, f: F) -> MapWindows<Self, F, N>
1671 where
1672 Self: Sized,
1673 F: FnMut(&[Self::Item; N]) -> R,
1674 {
1675 MapWindows::new(self, f)
1676 }
1677
1678 /// Creates an iterator which ends after the first [`None`].
1679 ///
1680 /// After an iterator returns [`None`], future calls may or may not yield
1681 /// [`Some(T)`] again. `fuse()` adapts an iterator, ensuring that after a
1682 /// [`None`] is given, it will always return [`None`] forever.
1683 ///
1684 /// Note that the [`Fuse`] wrapper is a no-op on iterators that implement
1685 /// the [`FusedIterator`] trait. `fuse()` may therefore behave incorrectly
1686 /// if the [`FusedIterator`] trait is improperly implemented.
1687 ///
1688 /// [`Some(T)`]: Some
1689 /// [`FusedIterator`]: crate::iter::FusedIterator
1690 ///
1691 /// # Examples
1692 ///
1693 /// ```
1694 /// // an iterator which alternates between Some and None
1695 /// struct Alternate {
1696 /// state: i32,
1697 /// }
1698 ///
1699 /// impl Iterator for Alternate {
1700 /// type Item = i32;
1701 ///
1702 /// fn next(&mut self) -> Option<i32> {
1703 /// let val = self.state;
1704 /// self.state = self.state + 1;
1705 ///
1706 /// // if it's even, Some(i32), else None
1707 /// if val % 2 == 0 {
1708 /// Some(val)
1709 /// } else {
1710 /// None
1711 /// }
1712 /// }
1713 /// }
1714 ///
1715 /// let mut iter = Alternate { state: 0 };
1716 ///
1717 /// // we can see our iterator going back and forth
1718 /// assert_eq!(iter.next(), Some(0));
1719 /// assert_eq!(iter.next(), None);
1720 /// assert_eq!(iter.next(), Some(2));
1721 /// assert_eq!(iter.next(), None);
1722 ///
1723 /// // however, once we fuse it...
1724 /// let mut iter = iter.fuse();
1725 ///
1726 /// assert_eq!(iter.next(), Some(4));
1727 /// assert_eq!(iter.next(), None);
1728 ///
1729 /// // it will always return `None` after the first time.
1730 /// assert_eq!(iter.next(), None);
1731 /// assert_eq!(iter.next(), None);
1732 /// assert_eq!(iter.next(), None);
1733 /// ```
1734 #[inline]
1735 #[stable(feature = "rust1", since = "1.0.0")]
1736 fn fuse(self) -> Fuse<Self>
1737 where
1738 Self: Sized,
1739 {
1740 Fuse::new(self)
1741 }
1742
1743 /// Does something with each element of an iterator, passing the value on.
1744 ///
1745 /// When using iterators, you'll often chain several of them together.
1746 /// While working on such code, you might want to check out what's
1747 /// happening at various parts in the pipeline. To do that, insert
1748 /// a call to `inspect()`.
1749 ///
1750 /// It's more common for `inspect()` to be used as a debugging tool than to
1751 /// exist in your final code, but applications may find it useful in certain
1752 /// situations when errors need to be logged before being discarded.
1753 ///
1754 /// # Examples
1755 ///
1756 /// Basic usage:
1757 ///
1758 /// ```
1759 /// let a = [1, 4, 2, 3];
1760 ///
1761 /// // this iterator sequence is complex.
1762 /// let sum = a.iter()
1763 /// .cloned()
1764 /// .filter(|x| x % 2 == 0)
1765 /// .fold(0, |sum, i| sum + i);
1766 ///
1767 /// println!("{sum}");
1768 ///
1769 /// // let's add some inspect() calls to investigate what's happening
1770 /// let sum = a.iter()
1771 /// .cloned()
1772 /// .inspect(|x| println!("about to filter: {x}"))
1773 /// .filter(|x| x % 2 == 0)
1774 /// .inspect(|x| println!("made it through filter: {x}"))
1775 /// .fold(0, |sum, i| sum + i);
1776 ///
1777 /// println!("{sum}");
1778 /// ```
1779 ///
1780 /// This will print:
1781 ///
1782 /// ```text
1783 /// 6
1784 /// about to filter: 1
1785 /// about to filter: 4
1786 /// made it through filter: 4
1787 /// about to filter: 2
1788 /// made it through filter: 2
1789 /// about to filter: 3
1790 /// 6
1791 /// ```
1792 ///
1793 /// Logging errors before discarding them:
1794 ///
1795 /// ```
1796 /// let lines = ["1", "2", "a"];
1797 ///
1798 /// let sum: i32 = lines
1799 /// .iter()
1800 /// .map(|line| line.parse::<i32>())
1801 /// .inspect(|num| {
1802 /// if let Err(ref e) = *num {
1803 /// println!("Parsing error: {e}");
1804 /// }
1805 /// })
1806 /// .filter_map(Result::ok)
1807 /// .sum();
1808 ///
1809 /// println!("Sum: {sum}");
1810 /// ```
1811 ///
1812 /// This will print:
1813 ///
1814 /// ```text
1815 /// Parsing error: invalid digit found in string
1816 /// Sum: 3
1817 /// ```
1818 #[inline]
1819 #[stable(feature = "rust1", since = "1.0.0")]
1820 fn inspect<F>(self, f: F) -> Inspect<Self, F>
1821 where
1822 Self: Sized,
1823 F: FnMut(&Self::Item),
1824 {
1825 Inspect::new(self, f)
1826 }
1827
1828 /// Creates a "by reference" adapter for this instance of `Iterator`.
1829 ///
1830 /// Consuming method calls (direct or indirect calls to `next`)
1831 /// on the "by reference" adapter will consume the original iterator,
1832 /// but ownership-taking methods (those with a `self` parameter)
1833 /// only take ownership of the "by reference" iterator.
1834 ///
1835 /// This is useful for applying ownership-taking methods
1836 /// (such as `take` in the example below)
1837 /// without giving up ownership of the original iterator,
1838 /// so you can use the original iterator afterwards.
1839 ///
1840 /// Uses [impl<I: Iterator + ?Sized> Iterator for &mut I { type Item = I::Item; ...}](https://doc.rust-lang.org/nightly/std/iter/trait.Iterator.html#impl-Iterator-for-%26mut+I).
1841 ///
1842 /// # Examples
1843 ///
1844 /// ```
1845 /// let mut words = ["hello", "world", "of", "Rust"].into_iter();
1846 ///
1847 /// // Take the first two words.
1848 /// let hello_world: Vec<_> = words.by_ref().take(2).collect();
1849 /// assert_eq!(hello_world, vec!["hello", "world"]);
1850 ///
1851 /// // Collect the rest of the words.
1852 /// // We can only do this because we used `by_ref` earlier.
1853 /// let of_rust: Vec<_> = words.collect();
1854 /// assert_eq!(of_rust, vec!["of", "Rust"]);
1855 /// ```
1856 #[stable(feature = "rust1", since = "1.0.0")]
1857 fn by_ref(&mut self) -> &mut Self
1858 where
1859 Self: Sized,
1860 {
1861 self
1862 }
1863
1864 /// Transforms an iterator into a collection.
1865 ///
1866 /// `collect()` can take anything iterable, and turn it into a relevant
1867 /// collection. This is one of the more powerful methods in the standard
1868 /// library, used in a variety of contexts.
1869 ///
1870 /// The most basic pattern in which `collect()` is used is to turn one
1871 /// collection into another. You take a collection, call [`iter`] on it,
1872 /// do a bunch of transformations, and then `collect()` at the end.
1873 ///
1874 /// `collect()` can also create instances of types that are not typical
1875 /// collections. For example, a [`String`] can be built from [`char`]s,
1876 /// and an iterator of [`Result<T, E>`][`Result`] items can be collected
1877 /// into `Result<Collection<T>, E>`. See the examples below for more.
1878 ///
1879 /// Because `collect()` is so general, it can cause problems with type
1880 /// inference. As such, `collect()` is one of the few times you'll see
1881 /// the syntax affectionately known as the 'turbofish': `::<>`. This
1882 /// helps the inference algorithm understand specifically which collection
1883 /// you're trying to collect into.
1884 ///
1885 /// # Examples
1886 ///
1887 /// Basic usage:
1888 ///
1889 /// ```
1890 /// let a = [1, 2, 3];
1891 ///
1892 /// let doubled: Vec<i32> = a.iter()
1893 /// .map(|&x| x * 2)
1894 /// .collect();
1895 ///
1896 /// assert_eq!(vec![2, 4, 6], doubled);
1897 /// ```
1898 ///
1899 /// Note that we needed the `: Vec<i32>` on the left-hand side. This is because
1900 /// we could collect into, for example, a [`VecDeque<T>`] instead:
1901 ///
1902 /// [`VecDeque<T>`]: ../../std/collections/struct.VecDeque.html
1903 ///
1904 /// ```
1905 /// use std::collections::VecDeque;
1906 ///
1907 /// let a = [1, 2, 3];
1908 ///
1909 /// let doubled: VecDeque<i32> = a.iter().map(|&x| x * 2).collect();
1910 ///
1911 /// assert_eq!(2, doubled[0]);
1912 /// assert_eq!(4, doubled[1]);
1913 /// assert_eq!(6, doubled[2]);
1914 /// ```
1915 ///
1916 /// Using the 'turbofish' instead of annotating `doubled`:
1917 ///
1918 /// ```
1919 /// let a = [1, 2, 3];
1920 ///
1921 /// let doubled = a.iter().map(|x| x * 2).collect::<Vec<i32>>();
1922 ///
1923 /// assert_eq!(vec![2, 4, 6], doubled);
1924 /// ```
1925 ///
1926 /// Because `collect()` only cares about what you're collecting into, you can
1927 /// still use a partial type hint, `_`, with the turbofish:
1928 ///
1929 /// ```
1930 /// let a = [1, 2, 3];
1931 ///
1932 /// let doubled = a.iter().map(|x| x * 2).collect::<Vec<_>>();
1933 ///
1934 /// assert_eq!(vec![2, 4, 6], doubled);
1935 /// ```
1936 ///
1937 /// Using `collect()` to make a [`String`]:
1938 ///
1939 /// ```
1940 /// let chars = ['g', 'd', 'k', 'k', 'n'];
1941 ///
1942 /// let hello: String = chars.iter()
1943 /// .map(|&x| x as u8)
1944 /// .map(|x| (x + 1) as char)
1945 /// .collect();
1946 ///
1947 /// assert_eq!("hello", hello);
1948 /// ```
1949 ///
1950 /// If you have a list of [`Result<T, E>`][`Result`]s, you can use `collect()` to
1951 /// see if any of them failed:
1952 ///
1953 /// ```
1954 /// let results = [Ok(1), Err("nope"), Ok(3), Err("bad")];
1955 ///
1956 /// let result: Result<Vec<_>, &str> = results.iter().cloned().collect();
1957 ///
1958 /// // gives us the first error
1959 /// assert_eq!(Err("nope"), result);
1960 ///
1961 /// let results = [Ok(1), Ok(3)];
1962 ///
1963 /// let result: Result<Vec<_>, &str> = results.iter().cloned().collect();
1964 ///
1965 /// // gives us the list of answers
1966 /// assert_eq!(Ok(vec![1, 3]), result);
1967 /// ```
1968 ///
1969 /// [`iter`]: Iterator::next
1970 /// [`String`]: ../../std/string/struct.String.html
1971 /// [`char`]: type@char
1972 #[inline]
1973 #[stable(feature = "rust1", since = "1.0.0")]
1974 #[must_use = "if you really need to exhaust the iterator, consider `.for_each(drop)` instead"]
1975 #[cfg_attr(not(test), rustc_diagnostic_item = "iterator_collect_fn")]
1976 fn collect<B: FromIterator<Self::Item>>(self) -> B
1977 where
1978 Self: Sized,
1979 {
1980 FromIterator::from_iter(self)
1981 }
1982
1983 /// Fallibly transforms an iterator into a collection, short circuiting if
1984 /// a failure is encountered.
1985 ///
1986 /// `try_collect()` is a variation of [`collect()`][`collect`] that allows fallible
1987 /// conversions during collection. Its main use case is simplifying conversions from
1988 /// iterators yielding [`Option<T>`][`Option`] into `Option<Collection<T>>`, or similarly for other [`Try`]
1989 /// types (e.g. [`Result`]).
1990 ///
1991 /// Importantly, `try_collect()` doesn't require that the outer [`Try`] type also implements [`FromIterator`];
1992 /// only the inner type produced on `Try::Output` must implement it. Concretely,
1993 /// this means that collecting into `ControlFlow<_, Vec<i32>>` is valid because `Vec<i32>` implements
1994 /// [`FromIterator`], even though [`ControlFlow`] doesn't.
1995 ///
1996 /// Also, if a failure is encountered during `try_collect()`, the iterator is still valid and
1997 /// may continue to be used, in which case it will continue iterating starting after the element that
1998 /// triggered the failure. See the last example below for an example of how this works.
1999 ///
2000 /// # Examples
2001 /// Successfully collecting an iterator of `Option<i32>` into `Option<Vec<i32>>`:
2002 /// ```
2003 /// #![feature(iterator_try_collect)]
2004 ///
2005 /// let u = vec![Some(1), Some(2), Some(3)];
2006 /// let v = u.into_iter().try_collect::<Vec<i32>>();
2007 /// assert_eq!(v, Some(vec![1, 2, 3]));
2008 /// ```
2009 ///
2010 /// Failing to collect in the same way:
2011 /// ```
2012 /// #![feature(iterator_try_collect)]
2013 ///
2014 /// let u = vec![Some(1), Some(2), None, Some(3)];
2015 /// let v = u.into_iter().try_collect::<Vec<i32>>();
2016 /// assert_eq!(v, None);
2017 /// ```
2018 ///
2019 /// A similar example, but with `Result`:
2020 /// ```
2021 /// #![feature(iterator_try_collect)]
2022 ///
2023 /// let u: Vec<Result<i32, ()>> = vec![Ok(1), Ok(2), Ok(3)];
2024 /// let v = u.into_iter().try_collect::<Vec<i32>>();
2025 /// assert_eq!(v, Ok(vec![1, 2, 3]));
2026 ///
2027 /// let u = vec![Ok(1), Ok(2), Err(()), Ok(3)];
2028 /// let v = u.into_iter().try_collect::<Vec<i32>>();
2029 /// assert_eq!(v, Err(()));
2030 /// ```
2031 ///
2032 /// Finally, even [`ControlFlow`] works, despite the fact that it
2033 /// doesn't implement [`FromIterator`]. Note also that the iterator can
2034 /// continue to be used, even if a failure is encountered:
2035 ///
2036 /// ```
2037 /// #![feature(iterator_try_collect)]
2038 ///
2039 /// use core::ops::ControlFlow::{Break, Continue};
2040 ///
2041 /// let u = [Continue(1), Continue(2), Break(3), Continue(4), Continue(5)];
2042 /// let mut it = u.into_iter();
2043 ///
2044 /// let v = it.try_collect::<Vec<_>>();
2045 /// assert_eq!(v, Break(3));
2046 ///
2047 /// let v = it.try_collect::<Vec<_>>();
2048 /// assert_eq!(v, Continue(vec![4, 5]));
2049 /// ```
2050 ///
2051 /// [`collect`]: Iterator::collect
2052 #[inline]
2053 #[unstable(feature = "iterator_try_collect", issue = "94047")]
2054 fn try_collect<B>(&mut self) -> ChangeOutputType<Self::Item, B>
2055 where
2056 Self: Sized,
2057 Self::Item: Try<Residual: Residual<B>>,
2058 B: FromIterator<<Self::Item as Try>::Output>,
2059 {
2060 try_process(ByRefSized(self), |i| i.collect())
2061 }
2062
2063 /// Collects all the items from an iterator into a collection.
2064 ///
2065 /// This method consumes the iterator and adds all its items to the
2066 /// passed collection. The collection is then returned, so the call chain
2067 /// can be continued.
2068 ///
2069 /// This is useful when you already have a collection and want to add
2070 /// the iterator items to it.
2071 ///
2072 /// This method is a convenience method to call [Extend::extend](trait.Extend.html),
2073 /// but instead of being called on a collection, it's called on an iterator.
2074 ///
2075 /// # Examples
2076 ///
2077 /// Basic usage:
2078 ///
2079 /// ```
2080 /// #![feature(iter_collect_into)]
2081 ///
2082 /// let a = [1, 2, 3];
2083 /// let mut vec: Vec::<i32> = vec![0, 1];
2084 ///
2085 /// a.iter().map(|&x| x * 2).collect_into(&mut vec);
2086 /// a.iter().map(|&x| x * 10).collect_into(&mut vec);
2087 ///
2088 /// assert_eq!(vec, vec![0, 1, 2, 4, 6, 10, 20, 30]);
2089 /// ```
2090 ///
2091 /// `Vec` can have a manual set capacity to avoid reallocating it:
2092 ///
2093 /// ```
2094 /// #![feature(iter_collect_into)]
2095 ///
2096 /// let a = [1, 2, 3];
2097 /// let mut vec: Vec::<i32> = Vec::with_capacity(6);
2098 ///
2099 /// a.iter().map(|&x| x * 2).collect_into(&mut vec);
2100 /// a.iter().map(|&x| x * 10).collect_into(&mut vec);
2101 ///
2102 /// assert_eq!(6, vec.capacity());
2103 /// assert_eq!(vec, vec![2, 4, 6, 10, 20, 30]);
2104 /// ```
2105 ///
2106 /// The returned mutable reference can be used to continue the call chain:
2107 ///
2108 /// ```
2109 /// #![feature(iter_collect_into)]
2110 ///
2111 /// let a = [1, 2, 3];
2112 /// let mut vec: Vec::<i32> = Vec::with_capacity(6);
2113 ///
2114 /// let count = a.iter().collect_into(&mut vec).iter().count();
2115 ///
2116 /// assert_eq!(count, vec.len());
2117 /// assert_eq!(vec, vec![1, 2, 3]);
2118 ///
2119 /// let count = a.iter().collect_into(&mut vec).iter().count();
2120 ///
2121 /// assert_eq!(count, vec.len());
2122 /// assert_eq!(vec, vec![1, 2, 3, 1, 2, 3]);
2123 /// ```
2124 #[inline]
2125 #[unstable(feature = "iter_collect_into", reason = "new API", issue = "94780")]
2126 fn collect_into<E: Extend<Self::Item>>(self, collection: &mut E) -> &mut E
2127 where
2128 Self: Sized,
2129 {
2130 collection.extend(self);
2131 collection
2132 }
2133
2134 /// Consumes an iterator, creating two collections from it.
2135 ///
2136 /// The predicate passed to `partition()` can return `true`, or `false`.
2137 /// `partition()` returns a pair, all of the elements for which it returned
2138 /// `true`, and all of the elements for which it returned `false`.
2139 ///
2140 /// See also [`is_partitioned()`] and [`partition_in_place()`].
2141 ///
2142 /// [`is_partitioned()`]: Iterator::is_partitioned
2143 /// [`partition_in_place()`]: Iterator::partition_in_place
2144 ///
2145 /// # Examples
2146 ///
2147 /// ```
2148 /// let a = [1, 2, 3];
2149 ///
2150 /// let (even, odd): (Vec<_>, Vec<_>) = a
2151 /// .into_iter()
2152 /// .partition(|n| n % 2 == 0);
2153 ///
2154 /// assert_eq!(even, vec![2]);
2155 /// assert_eq!(odd, vec![1, 3]);
2156 /// ```
2157 #[stable(feature = "rust1", since = "1.0.0")]
2158 fn partition<B, F>(self, f: F) -> (B, B)
2159 where
2160 Self: Sized,
2161 B: Default + Extend<Self::Item>,
2162 F: FnMut(&Self::Item) -> bool,
2163 {
2164 #[inline]
2165 fn extend<'a, T, B: Extend<T>>(
2166 mut f: impl FnMut(&T) -> bool + 'a,
2167 left: &'a mut B,
2168 right: &'a mut B,
2169 ) -> impl FnMut((), T) + 'a {
2170 move |(), x| {
2171 if f(&x) {
2172 left.extend_one(x);
2173 } else {
2174 right.extend_one(x);
2175 }
2176 }
2177 }
2178
2179 let mut left: B = Default::default();
2180 let mut right: B = Default::default();
2181
2182 self.fold((), extend(f, &mut left, &mut right));
2183
2184 (left, right)
2185 }
2186
2187 /// Reorders the elements of this iterator *in-place* according to the given predicate,
2188 /// such that all those that return `true` precede all those that return `false`.
2189 /// Returns the number of `true` elements found.
2190 ///
2191 /// The relative order of partitioned items is not maintained.
2192 ///
2193 /// # Current implementation
2194 ///
2195 /// The current algorithm tries to find the first element for which the predicate evaluates
2196 /// to false and the last element for which it evaluates to true, and repeatedly swaps them.
2197 ///
2198 /// Time complexity: *O*(*n*)
2199 ///
2200 /// See also [`is_partitioned()`] and [`partition()`].
2201 ///
2202 /// [`is_partitioned()`]: Iterator::is_partitioned
2203 /// [`partition()`]: Iterator::partition
2204 ///
2205 /// # Examples
2206 ///
2207 /// ```
2208 /// #![feature(iter_partition_in_place)]
2209 ///
2210 /// let mut a = [1, 2, 3, 4, 5, 6, 7];
2211 ///
2212 /// // Partition in-place between evens and odds
2213 /// let i = a.iter_mut().partition_in_place(|&n| n % 2 == 0);
2214 ///
2215 /// assert_eq!(i, 3);
2216 /// assert!(a[..i].iter().all(|&n| n % 2 == 0)); // evens
2217 /// assert!(a[i..].iter().all(|&n| n % 2 == 1)); // odds
2218 /// ```
2219 #[unstable(feature = "iter_partition_in_place", reason = "new API", issue = "62543")]
2220 fn partition_in_place<'a, T: 'a, P>(mut self, ref mut predicate: P) -> usize
2221 where
2222 Self: Sized + DoubleEndedIterator<Item = &'a mut T>,
2223 P: FnMut(&T) -> bool,
2224 {
2225 // FIXME: should we worry about the count overflowing? The only way to have more than
2226 // `usize::MAX` mutable references is with ZSTs, which aren't useful to partition...
2227
2228 // These closure "factory" functions exist to avoid genericity in `Self`.
2229
2230 #[inline]
2231 fn is_false<'a, T>(
2232 predicate: &'a mut impl FnMut(&T) -> bool,
2233 true_count: &'a mut usize,
2234 ) -> impl FnMut(&&mut T) -> bool + 'a {
2235 move |x| {
2236 let p = predicate(&**x);
2237 *true_count += p as usize;
2238 !p
2239 }
2240 }
2241
2242 #[inline]
2243 fn is_true<T>(predicate: &mut impl FnMut(&T) -> bool) -> impl FnMut(&&mut T) -> bool + '_ {
2244 move |x| predicate(&**x)
2245 }
2246
2247 // Repeatedly find the first `false` and swap it with the last `true`.
2248 let mut true_count = 0;
2249 while let Some(head) = self.find(is_false(predicate, &mut true_count)) {
2250 if let Some(tail) = self.rfind(is_true(predicate)) {
2251 crate::mem::swap(head, tail);
2252 true_count += 1;
2253 } else {
2254 break;
2255 }
2256 }
2257 true_count
2258 }
2259
2260 /// Checks if the elements of this iterator are partitioned according to the given predicate,
2261 /// such that all those that return `true` precede all those that return `false`.
2262 ///
2263 /// See also [`partition()`] and [`partition_in_place()`].
2264 ///
2265 /// [`partition()`]: Iterator::partition
2266 /// [`partition_in_place()`]: Iterator::partition_in_place
2267 ///
2268 /// # Examples
2269 ///
2270 /// ```
2271 /// #![feature(iter_is_partitioned)]
2272 ///
2273 /// assert!("Iterator".chars().is_partitioned(char::is_uppercase));
2274 /// assert!(!"IntoIterator".chars().is_partitioned(char::is_uppercase));
2275 /// ```
2276 #[unstable(feature = "iter_is_partitioned", reason = "new API", issue = "62544")]
2277 fn is_partitioned<P>(mut self, mut predicate: P) -> bool
2278 where
2279 Self: Sized,
2280 P: FnMut(Self::Item) -> bool,
2281 {
2282 // Either all items test `true`, or the first clause stops at `false`
2283 // and we check that there are no more `true` items after that.
2284 self.all(&mut predicate) || !self.any(predicate)
2285 }
2286
2287 /// An iterator method that applies a function as long as it returns
2288 /// successfully, producing a single, final value.
2289 ///
2290 /// `try_fold()` takes two arguments: an initial value, and a closure with
2291 /// two arguments: an 'accumulator', and an element. The closure either
2292 /// returns successfully, with the value that the accumulator should have
2293 /// for the next iteration, or it returns failure, with an error value that
2294 /// is propagated back to the caller immediately (short-circuiting).
2295 ///
2296 /// The initial value is the value the accumulator will have on the first
2297 /// call. If applying the closure succeeded against every element of the
2298 /// iterator, `try_fold()` returns the final accumulator as success.
2299 ///
2300 /// Folding is useful whenever you have a collection of something, and want
2301 /// to produce a single value from it.
2302 ///
2303 /// # Note to Implementors
2304 ///
2305 /// Several of the other (forward) methods have default implementations in
2306 /// terms of this one, so try to implement this explicitly if it can
2307 /// do something better than the default `for` loop implementation.
2308 ///
2309 /// In particular, try to have this call `try_fold()` on the internal parts
2310 /// from which this iterator is composed. If multiple calls are needed,
2311 /// the `?` operator may be convenient for chaining the accumulator value
2312 /// along, but beware any invariants that need to be upheld before those
2313 /// early returns. This is a `&mut self` method, so iteration needs to be
2314 /// resumable after hitting an error here.
2315 ///
2316 /// # Examples
2317 ///
2318 /// Basic usage:
2319 ///
2320 /// ```
2321 /// let a = [1, 2, 3];
2322 ///
2323 /// // the checked sum of all of the elements of the array
2324 /// let sum = a.iter().try_fold(0i8, |acc, &x| acc.checked_add(x));
2325 ///
2326 /// assert_eq!(sum, Some(6));
2327 /// ```
2328 ///
2329 /// Short-circuiting:
2330 ///
2331 /// ```
2332 /// let a = [10, 20, 30, 100, 40, 50];
2333 /// let mut it = a.iter();
2334 ///
2335 /// // This sum overflows when adding the 100 element
2336 /// let sum = it.try_fold(0i8, |acc, &x| acc.checked_add(x));
2337 /// assert_eq!(sum, None);
2338 ///
2339 /// // Because it short-circuited, the remaining elements are still
2340 /// // available through the iterator.
2341 /// assert_eq!(it.len(), 2);
2342 /// assert_eq!(it.next(), Some(&40));
2343 /// ```
2344 ///
2345 /// While you cannot `break` from a closure, the [`ControlFlow`] type allows
2346 /// a similar idea:
2347 ///
2348 /// ```
2349 /// use std::ops::ControlFlow;
2350 ///
2351 /// let triangular = (1..30).try_fold(0_i8, |prev, x| {
2352 /// if let Some(next) = prev.checked_add(x) {
2353 /// ControlFlow::Continue(next)
2354 /// } else {
2355 /// ControlFlow::Break(prev)
2356 /// }
2357 /// });
2358 /// assert_eq!(triangular, ControlFlow::Break(120));
2359 ///
2360 /// let triangular = (1..30).try_fold(0_u64, |prev, x| {
2361 /// if let Some(next) = prev.checked_add(x) {
2362 /// ControlFlow::Continue(next)
2363 /// } else {
2364 /// ControlFlow::Break(prev)
2365 /// }
2366 /// });
2367 /// assert_eq!(triangular, ControlFlow::Continue(435));
2368 /// ```
2369 #[inline]
2370 #[stable(feature = "iterator_try_fold", since = "1.27.0")]
2371 fn try_fold<B, F, R>(&mut self, init: B, mut f: F) -> R
2372 where
2373 Self: Sized,
2374 F: FnMut(B, Self::Item) -> R,
2375 R: Try<Output = B>,
2376 {
2377 let mut accum = init;
2378 while let Some(x) = self.next() {
2379 accum = f(accum, x)?;
2380 }
2381 try { accum }
2382 }
2383
2384 /// An iterator method that applies a fallible function to each item in the
2385 /// iterator, stopping at the first error and returning that error.
2386 ///
2387 /// This can also be thought of as the fallible form of [`for_each()`]
2388 /// or as the stateless version of [`try_fold()`].
2389 ///
2390 /// [`for_each()`]: Iterator::for_each
2391 /// [`try_fold()`]: Iterator::try_fold
2392 ///
2393 /// # Examples
2394 ///
2395 /// ```
2396 /// use std::fs::rename;
2397 /// use std::io::{stdout, Write};
2398 /// use std::path::Path;
2399 ///
2400 /// let data = ["no_tea.txt", "stale_bread.json", "torrential_rain.png"];
2401 ///
2402 /// let res = data.iter().try_for_each(|x| writeln!(stdout(), "{x}"));
2403 /// assert!(res.is_ok());
2404 ///
2405 /// let mut it = data.iter().cloned();
2406 /// let res = it.try_for_each(|x| rename(x, Path::new(x).with_extension("old")));
2407 /// assert!(res.is_err());
2408 /// // It short-circuited, so the remaining items are still in the iterator:
2409 /// assert_eq!(it.next(), Some("stale_bread.json"));
2410 /// ```
2411 ///
2412 /// The [`ControlFlow`] type can be used with this method for the situations
2413 /// in which you'd use `break` and `continue` in a normal loop:
2414 ///
2415 /// ```
2416 /// use std::ops::ControlFlow;
2417 ///
2418 /// let r = (2..100).try_for_each(|x| {
2419 /// if 323 % x == 0 {
2420 /// return ControlFlow::Break(x)
2421 /// }
2422 ///
2423 /// ControlFlow::Continue(())
2424 /// });
2425 /// assert_eq!(r, ControlFlow::Break(17));
2426 /// ```
2427 #[inline]
2428 #[stable(feature = "iterator_try_fold", since = "1.27.0")]
2429 fn try_for_each<F, R>(&mut self, f: F) -> R
2430 where
2431 Self: Sized,
2432 F: FnMut(Self::Item) -> R,
2433 R: Try<Output = ()>,
2434 {
2435 #[inline]
2436 fn call<T, R>(mut f: impl FnMut(T) -> R) -> impl FnMut((), T) -> R {
2437 move |(), x| f(x)
2438 }
2439
2440 self.try_fold((), call(f))
2441 }
2442
2443 /// Folds every element into an accumulator by applying an operation,
2444 /// returning the final result.
2445 ///
2446 /// `fold()` takes two arguments: an initial value, and a closure with two
2447 /// arguments: an 'accumulator', and an element. The closure returns the value that
2448 /// the accumulator should have for the next iteration.
2449 ///
2450 /// The initial value is the value the accumulator will have on the first
2451 /// call.
2452 ///
2453 /// After applying this closure to every element of the iterator, `fold()`
2454 /// returns the accumulator.
2455 ///
2456 /// This operation is sometimes called 'reduce' or 'inject'.
2457 ///
2458 /// Folding is useful whenever you have a collection of something, and want
2459 /// to produce a single value from it.
2460 ///
2461 /// Note: `fold()`, and similar methods that traverse the entire iterator,
2462 /// might not terminate for infinite iterators, even on traits for which a
2463 /// result is determinable in finite time.
2464 ///
2465 /// Note: [`reduce()`] can be used to use the first element as the initial
2466 /// value, if the accumulator type and item type is the same.
2467 ///
2468 /// Note: `fold()` combines elements in a *left-associative* fashion. For associative
2469 /// operators like `+`, the order the elements are combined in is not important, but for non-associative
2470 /// operators like `-` the order will affect the final result.
2471 /// For a *right-associative* version of `fold()`, see [`DoubleEndedIterator::rfold()`].
2472 ///
2473 /// # Note to Implementors
2474 ///
2475 /// Several of the other (forward) methods have default implementations in
2476 /// terms of this one, so try to implement this explicitly if it can
2477 /// do something better than the default `for` loop implementation.
2478 ///
2479 /// In particular, try to have this call `fold()` on the internal parts
2480 /// from which this iterator is composed.
2481 ///
2482 /// # Examples
2483 ///
2484 /// Basic usage:
2485 ///
2486 /// ```
2487 /// let a = [1, 2, 3];
2488 ///
2489 /// // the sum of all of the elements of the array
2490 /// let sum = a.iter().fold(0, |acc, x| acc + x);
2491 ///
2492 /// assert_eq!(sum, 6);
2493 /// ```
2494 ///
2495 /// Let's walk through each step of the iteration here:
2496 ///
2497 /// | element | acc | x | result |
2498 /// |---------|-----|---|--------|
2499 /// | | 0 | | |
2500 /// | 1 | 0 | 1 | 1 |
2501 /// | 2 | 1 | 2 | 3 |
2502 /// | 3 | 3 | 3 | 6 |
2503 ///
2504 /// And so, our final result, `6`.
2505 ///
2506 /// This example demonstrates the left-associative nature of `fold()`:
2507 /// it builds a string, starting with an initial value
2508 /// and continuing with each element from the front until the back:
2509 ///
2510 /// ```
2511 /// let numbers = [1, 2, 3, 4, 5];
2512 ///
2513 /// let zero = "0".to_string();
2514 ///
2515 /// let result = numbers.iter().fold(zero, |acc, &x| {
2516 /// format!("({acc} + {x})")
2517 /// });
2518 ///
2519 /// assert_eq!(result, "(((((0 + 1) + 2) + 3) + 4) + 5)");
2520 /// ```
2521 /// It's common for people who haven't used iterators a lot to
2522 /// use a `for` loop with a list of things to build up a result. Those
2523 /// can be turned into `fold()`s:
2524 ///
2525 /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
2526 ///
2527 /// ```
2528 /// let numbers = [1, 2, 3, 4, 5];
2529 ///
2530 /// let mut result = 0;
2531 ///
2532 /// // for loop:
2533 /// for i in &numbers {
2534 /// result = result + i;
2535 /// }
2536 ///
2537 /// // fold:
2538 /// let result2 = numbers.iter().fold(0, |acc, &x| acc + x);
2539 ///
2540 /// // they're the same
2541 /// assert_eq!(result, result2);
2542 /// ```
2543 ///
2544 /// [`reduce()`]: Iterator::reduce
2545 #[doc(alias = "inject", alias = "foldl")]
2546 #[inline]
2547 #[stable(feature = "rust1", since = "1.0.0")]
2548 fn fold<B, F>(mut self, init: B, mut f: F) -> B
2549 where
2550 Self: Sized,
2551 F: FnMut(B, Self::Item) -> B,
2552 {
2553 let mut accum = init;
2554 while let Some(x) = self.next() {
2555 accum = f(accum, x);
2556 }
2557 accum
2558 }
2559
2560 /// Reduces the elements to a single one, by repeatedly applying a reducing
2561 /// operation.
2562 ///
2563 /// If the iterator is empty, returns [`None`]; otherwise, returns the
2564 /// result of the reduction.
2565 ///
2566 /// The reducing function is a closure with two arguments: an 'accumulator', and an element.
2567 /// For iterators with at least one element, this is the same as [`fold()`]
2568 /// with the first element of the iterator as the initial accumulator value, folding
2569 /// every subsequent element into it.
2570 ///
2571 /// [`fold()`]: Iterator::fold
2572 ///
2573 /// # Example
2574 ///
2575 /// ```
2576 /// let reduced: i32 = (1..10).reduce(|acc, e| acc + e).unwrap_or(0);
2577 /// assert_eq!(reduced, 45);
2578 ///
2579 /// // Which is equivalent to doing it with `fold`:
2580 /// let folded: i32 = (1..10).fold(0, |acc, e| acc + e);
2581 /// assert_eq!(reduced, folded);
2582 /// ```
2583 #[inline]
2584 #[stable(feature = "iterator_fold_self", since = "1.51.0")]
2585 fn reduce<F>(mut self, f: F) -> Option<Self::Item>
2586 where
2587 Self: Sized,
2588 F: FnMut(Self::Item, Self::Item) -> Self::Item,
2589 {
2590 let first = self.next()?;
2591 Some(self.fold(first, f))
2592 }
2593
2594 /// Reduces the elements to a single one by repeatedly applying a reducing operation. If the
2595 /// closure returns a failure, the failure is propagated back to the caller immediately.
2596 ///
2597 /// The return type of this method depends on the return type of the closure. If the closure
2598 /// returns `Result<Self::Item, E>`, then this function will return `Result<Option<Self::Item>,
2599 /// E>`. If the closure returns `Option<Self::Item>`, then this function will return
2600 /// `Option<Option<Self::Item>>`.
2601 ///
2602 /// When called on an empty iterator, this function will return either `Some(None)` or
2603 /// `Ok(None)` depending on the type of the provided closure.
2604 ///
2605 /// For iterators with at least one element, this is essentially the same as calling
2606 /// [`try_fold()`] with the first element of the iterator as the initial accumulator value.
2607 ///
2608 /// [`try_fold()`]: Iterator::try_fold
2609 ///
2610 /// # Examples
2611 ///
2612 /// Safely calculate the sum of a series of numbers:
2613 ///
2614 /// ```
2615 /// #![feature(iterator_try_reduce)]
2616 ///
2617 /// let numbers: Vec<usize> = vec![10, 20, 5, 23, 0];
2618 /// let sum = numbers.into_iter().try_reduce(|x, y| x.checked_add(y));
2619 /// assert_eq!(sum, Some(Some(58)));
2620 /// ```
2621 ///
2622 /// Determine when a reduction short circuited:
2623 ///
2624 /// ```
2625 /// #![feature(iterator_try_reduce)]
2626 ///
2627 /// let numbers = vec![1, 2, 3, usize::MAX, 4, 5];
2628 /// let sum = numbers.into_iter().try_reduce(|x, y| x.checked_add(y));
2629 /// assert_eq!(sum, None);
2630 /// ```
2631 ///
2632 /// Determine when a reduction was not performed because there are no elements:
2633 ///
2634 /// ```
2635 /// #![feature(iterator_try_reduce)]
2636 ///
2637 /// let numbers: Vec<usize> = Vec::new();
2638 /// let sum = numbers.into_iter().try_reduce(|x, y| x.checked_add(y));
2639 /// assert_eq!(sum, Some(None));
2640 /// ```
2641 ///
2642 /// Use a [`Result`] instead of an [`Option`]:
2643 ///
2644 /// ```
2645 /// #![feature(iterator_try_reduce)]
2646 ///
2647 /// let numbers = vec!["1", "2", "3", "4", "5"];
2648 /// let max: Result<Option<_>, <usize as std::str::FromStr>::Err> =
2649 /// numbers.into_iter().try_reduce(|x, y| {
2650 /// if x.parse::<usize>()? > y.parse::<usize>()? { Ok(x) } else { Ok(y) }
2651 /// });
2652 /// assert_eq!(max, Ok(Some("5")));
2653 /// ```
2654 #[inline]
2655 #[unstable(feature = "iterator_try_reduce", reason = "new API", issue = "87053")]
2656 fn try_reduce<R>(
2657 &mut self,
2658 f: impl FnMut(Self::Item, Self::Item) -> R,
2659 ) -> ChangeOutputType<R, Option<R::Output>>
2660 where
2661 Self: Sized,
2662 R: Try<Output = Self::Item, Residual: Residual<Option<Self::Item>>>,
2663 {
2664 let first = match self.next() {
2665 Some(i) => i,
2666 None => return Try::from_output(None),
2667 };
2668
2669 match self.try_fold(first, f).branch() {
2670 ControlFlow::Break(r) => FromResidual::from_residual(r),
2671 ControlFlow::Continue(i) => Try::from_output(Some(i)),
2672 }
2673 }
2674
2675 /// Tests if every element of the iterator matches a predicate.
2676 ///
2677 /// `all()` takes a closure that returns `true` or `false`. It applies
2678 /// this closure to each element of the iterator, and if they all return
2679 /// `true`, then so does `all()`. If any of them return `false`, it
2680 /// returns `false`.
2681 ///
2682 /// `all()` is short-circuiting; in other words, it will stop processing
2683 /// as soon as it finds a `false`, given that no matter what else happens,
2684 /// the result will also be `false`.
2685 ///
2686 /// An empty iterator returns `true`.
2687 ///
2688 /// # Examples
2689 ///
2690 /// Basic usage:
2691 ///
2692 /// ```
2693 /// let a = [1, 2, 3];
2694 ///
2695 /// assert!(a.iter().all(|&x| x > 0));
2696 ///
2697 /// assert!(!a.iter().all(|&x| x > 2));
2698 /// ```
2699 ///
2700 /// Stopping at the first `false`:
2701 ///
2702 /// ```
2703 /// let a = [1, 2, 3];
2704 ///
2705 /// let mut iter = a.iter();
2706 ///
2707 /// assert!(!iter.all(|&x| x != 2));
2708 ///
2709 /// // we can still use `iter`, as there are more elements.
2710 /// assert_eq!(iter.next(), Some(&3));
2711 /// ```
2712 #[inline]
2713 #[stable(feature = "rust1", since = "1.0.0")]
2714 fn all<F>(&mut self, f: F) -> bool
2715 where
2716 Self: Sized,
2717 F: FnMut(Self::Item) -> bool,
2718 {
2719 #[inline]
2720 fn check<T>(mut f: impl FnMut(T) -> bool) -> impl FnMut((), T) -> ControlFlow<()> {
2721 move |(), x| {
2722 if f(x) { ControlFlow::Continue(()) } else { ControlFlow::Break(()) }
2723 }
2724 }
2725 self.try_fold((), check(f)) == ControlFlow::Continue(())
2726 }
2727
2728 /// Tests if any element of the iterator matches a predicate.
2729 ///
2730 /// `any()` takes a closure that returns `true` or `false`. It applies
2731 /// this closure to each element of the iterator, and if any of them return
2732 /// `true`, then so does `any()`. If they all return `false`, it
2733 /// returns `false`.
2734 ///
2735 /// `any()` is short-circuiting; in other words, it will stop processing
2736 /// as soon as it finds a `true`, given that no matter what else happens,
2737 /// the result will also be `true`.
2738 ///
2739 /// An empty iterator returns `false`.
2740 ///
2741 /// # Examples
2742 ///
2743 /// Basic usage:
2744 ///
2745 /// ```
2746 /// let a = [1, 2, 3];
2747 ///
2748 /// assert!(a.iter().any(|&x| x > 0));
2749 ///
2750 /// assert!(!a.iter().any(|&x| x > 5));
2751 /// ```
2752 ///
2753 /// Stopping at the first `true`:
2754 ///
2755 /// ```
2756 /// let a = [1, 2, 3];
2757 ///
2758 /// let mut iter = a.iter();
2759 ///
2760 /// assert!(iter.any(|&x| x != 2));
2761 ///
2762 /// // we can still use `iter`, as there are more elements.
2763 /// assert_eq!(iter.next(), Some(&2));
2764 /// ```
2765 #[inline]
2766 #[stable(feature = "rust1", since = "1.0.0")]
2767 fn any<F>(&mut self, f: F) -> bool
2768 where
2769 Self: Sized,
2770 F: FnMut(Self::Item) -> bool,
2771 {
2772 #[inline]
2773 fn check<T>(mut f: impl FnMut(T) -> bool) -> impl FnMut((), T) -> ControlFlow<()> {
2774 move |(), x| {
2775 if f(x) { ControlFlow::Break(()) } else { ControlFlow::Continue(()) }
2776 }
2777 }
2778
2779 self.try_fold((), check(f)) == ControlFlow::Break(())
2780 }
2781
2782 /// Searches for an element of an iterator that satisfies a predicate.
2783 ///
2784 /// `find()` takes a closure that returns `true` or `false`. It applies
2785 /// this closure to each element of the iterator, and if any of them return
2786 /// `true`, then `find()` returns [`Some(element)`]. If they all return
2787 /// `false`, it returns [`None`].
2788 ///
2789 /// `find()` is short-circuiting; in other words, it will stop processing
2790 /// as soon as the closure returns `true`.
2791 ///
2792 /// Because `find()` takes a reference, and many iterators iterate over
2793 /// references, this leads to a possibly confusing situation where the
2794 /// argument is a double reference. You can see this effect in the
2795 /// examples below, with `&&x`.
2796 ///
2797 /// If you need the index of the element, see [`position()`].
2798 ///
2799 /// [`Some(element)`]: Some
2800 /// [`position()`]: Iterator::position
2801 ///
2802 /// # Examples
2803 ///
2804 /// Basic usage:
2805 ///
2806 /// ```
2807 /// let a = [1, 2, 3];
2808 ///
2809 /// assert_eq!(a.iter().find(|&&x| x == 2), Some(&2));
2810 ///
2811 /// assert_eq!(a.iter().find(|&&x| x == 5), None);
2812 /// ```
2813 ///
2814 /// Stopping at the first `true`:
2815 ///
2816 /// ```
2817 /// let a = [1, 2, 3];
2818 ///
2819 /// let mut iter = a.iter();
2820 ///
2821 /// assert_eq!(iter.find(|&&x| x == 2), Some(&2));
2822 ///
2823 /// // we can still use `iter`, as there are more elements.
2824 /// assert_eq!(iter.next(), Some(&3));
2825 /// ```
2826 ///
2827 /// Note that `iter.find(f)` is equivalent to `iter.filter(f).next()`.
2828 #[inline]
2829 #[stable(feature = "rust1", since = "1.0.0")]
2830 fn find<P>(&mut self, predicate: P) -> Option<Self::Item>
2831 where
2832 Self: Sized,
2833 P: FnMut(&Self::Item) -> bool,
2834 {
2835 #[inline]
2836 fn check<T>(mut predicate: impl FnMut(&T) -> bool) -> impl FnMut((), T) -> ControlFlow<T> {
2837 move |(), x| {
2838 if predicate(&x) { ControlFlow::Break(x) } else { ControlFlow::Continue(()) }
2839 }
2840 }
2841
2842 self.try_fold((), check(predicate)).break_value()
2843 }
2844
2845 /// Applies function to the elements of iterator and returns
2846 /// the first non-none result.
2847 ///
2848 /// `iter.find_map(f)` is equivalent to `iter.filter_map(f).next()`.
2849 ///
2850 /// # Examples
2851 ///
2852 /// ```
2853 /// let a = ["lol", "NaN", "2", "5"];
2854 ///
2855 /// let first_number = a.iter().find_map(|s| s.parse().ok());
2856 ///
2857 /// assert_eq!(first_number, Some(2));
2858 /// ```
2859 #[inline]
2860 #[stable(feature = "iterator_find_map", since = "1.30.0")]
2861 fn find_map<B, F>(&mut self, f: F) -> Option<B>
2862 where
2863 Self: Sized,
2864 F: FnMut(Self::Item) -> Option<B>,
2865 {
2866 #[inline]
2867 fn check<T, B>(mut f: impl FnMut(T) -> Option<B>) -> impl FnMut((), T) -> ControlFlow<B> {
2868 move |(), x| match f(x) {
2869 Some(x) => ControlFlow::Break(x),
2870 None => ControlFlow::Continue(()),
2871 }
2872 }
2873
2874 self.try_fold((), check(f)).break_value()
2875 }
2876
2877 /// Applies function to the elements of iterator and returns
2878 /// the first true result or the first error.
2879 ///
2880 /// The return type of this method depends on the return type of the closure.
2881 /// If you return `Result<bool, E>` from the closure, you'll get a `Result<Option<Self::Item>, E>`.
2882 /// If you return `Option<bool>` from the closure, you'll get an `Option<Option<Self::Item>>`.
2883 ///
2884 /// # Examples
2885 ///
2886 /// ```
2887 /// #![feature(try_find)]
2888 ///
2889 /// let a = ["1", "2", "lol", "NaN", "5"];
2890 ///
2891 /// let is_my_num = |s: &str, search: i32| -> Result<bool, std::num::ParseIntError> {
2892 /// Ok(s.parse::<i32>()? == search)
2893 /// };
2894 ///
2895 /// let result = a.iter().try_find(|&&s| is_my_num(s, 2));
2896 /// assert_eq!(result, Ok(Some(&"2")));
2897 ///
2898 /// let result = a.iter().try_find(|&&s| is_my_num(s, 5));
2899 /// assert!(result.is_err());
2900 /// ```
2901 ///
2902 /// This also supports other types which implement [`Try`], not just [`Result`].
2903 ///
2904 /// ```
2905 /// #![feature(try_find)]
2906 ///
2907 /// use std::num::NonZero;
2908 ///
2909 /// let a = [3, 5, 7, 4, 9, 0, 11u32];
2910 /// let result = a.iter().try_find(|&&x| NonZero::new(x).map(|y| y.is_power_of_two()));
2911 /// assert_eq!(result, Some(Some(&4)));
2912 /// let result = a.iter().take(3).try_find(|&&x| NonZero::new(x).map(|y| y.is_power_of_two()));
2913 /// assert_eq!(result, Some(None));
2914 /// let result = a.iter().rev().try_find(|&&x| NonZero::new(x).map(|y| y.is_power_of_two()));
2915 /// assert_eq!(result, None);
2916 /// ```
2917 #[inline]
2918 #[unstable(feature = "try_find", reason = "new API", issue = "63178")]
2919 fn try_find<R>(
2920 &mut self,
2921 f: impl FnMut(&Self::Item) -> R,
2922 ) -> ChangeOutputType<R, Option<Self::Item>>
2923 where
2924 Self: Sized,
2925 R: Try<Output = bool, Residual: Residual<Option<Self::Item>>>,
2926 {
2927 #[inline]
2928 fn check<I, V, R>(
2929 mut f: impl FnMut(&I) -> V,
2930 ) -> impl FnMut((), I) -> ControlFlow<R::TryType>
2931 where
2932 V: Try<Output = bool, Residual = R>,
2933 R: Residual<Option<I>>,
2934 {
2935 move |(), x| match f(&x).branch() {
2936 ControlFlow::Continue(false) => ControlFlow::Continue(()),
2937 ControlFlow::Continue(true) => ControlFlow::Break(Try::from_output(Some(x))),
2938 ControlFlow::Break(r) => ControlFlow::Break(FromResidual::from_residual(r)),
2939 }
2940 }
2941
2942 match self.try_fold((), check(f)) {
2943 ControlFlow::Break(x) => x,
2944 ControlFlow::Continue(()) => Try::from_output(None),
2945 }
2946 }
2947
2948 /// Searches for an element in an iterator, returning its index.
2949 ///
2950 /// `position()` takes a closure that returns `true` or `false`. It applies
2951 /// this closure to each element of the iterator, and if one of them
2952 /// returns `true`, then `position()` returns [`Some(index)`]. If all of
2953 /// them return `false`, it returns [`None`].
2954 ///
2955 /// `position()` is short-circuiting; in other words, it will stop
2956 /// processing as soon as it finds a `true`.
2957 ///
2958 /// # Overflow Behavior
2959 ///
2960 /// The method does no guarding against overflows, so if there are more
2961 /// than [`usize::MAX`] non-matching elements, it either produces the wrong
2962 /// result or panics. If debug assertions are enabled, a panic is
2963 /// guaranteed.
2964 ///
2965 /// # Panics
2966 ///
2967 /// This function might panic if the iterator has more than `usize::MAX`
2968 /// non-matching elements.
2969 ///
2970 /// [`Some(index)`]: Some
2971 ///
2972 /// # Examples
2973 ///
2974 /// Basic usage:
2975 ///
2976 /// ```
2977 /// let a = [1, 2, 3];
2978 ///
2979 /// assert_eq!(a.iter().position(|&x| x == 2), Some(1));
2980 ///
2981 /// assert_eq!(a.iter().position(|&x| x == 5), None);
2982 /// ```
2983 ///
2984 /// Stopping at the first `true`:
2985 ///
2986 /// ```
2987 /// let a = [1, 2, 3, 4];
2988 ///
2989 /// let mut iter = a.iter();
2990 ///
2991 /// assert_eq!(iter.position(|&x| x >= 2), Some(1));
2992 ///
2993 /// // we can still use `iter`, as there are more elements.
2994 /// assert_eq!(iter.next(), Some(&3));
2995 ///
2996 /// // The returned index depends on iterator state
2997 /// assert_eq!(iter.position(|&x| x == 4), Some(0));
2998 ///
2999 /// ```
3000 #[inline]
3001 #[stable(feature = "rust1", since = "1.0.0")]
3002 fn position<P>(&mut self, predicate: P) -> Option<usize>
3003 where
3004 Self: Sized,
3005 P: FnMut(Self::Item) -> bool,
3006 {
3007 #[inline]
3008 fn check<'a, T>(
3009 mut predicate: impl FnMut(T) -> bool + 'a,
3010 acc: &'a mut usize,
3011 ) -> impl FnMut((), T) -> ControlFlow<usize, ()> + 'a {
3012 #[rustc_inherit_overflow_checks]
3013 move |_, x| {
3014 if predicate(x) {
3015 ControlFlow::Break(*acc)
3016 } else {
3017 *acc += 1;
3018 ControlFlow::Continue(())
3019 }
3020 }
3021 }
3022
3023 let mut acc = 0;
3024 self.try_fold((), check(predicate, &mut acc)).break_value()
3025 }
3026
3027 /// Searches for an element in an iterator from the right, returning its
3028 /// index.
3029 ///
3030 /// `rposition()` takes a closure that returns `true` or `false`. It applies
3031 /// this closure to each element of the iterator, starting from the end,
3032 /// and if one of them returns `true`, then `rposition()` returns
3033 /// [`Some(index)`]. If all of them return `false`, it returns [`None`].
3034 ///
3035 /// `rposition()` is short-circuiting; in other words, it will stop
3036 /// processing as soon as it finds a `true`.
3037 ///
3038 /// [`Some(index)`]: Some
3039 ///
3040 /// # Examples
3041 ///
3042 /// Basic usage:
3043 ///
3044 /// ```
3045 /// let a = [1, 2, 3];
3046 ///
3047 /// assert_eq!(a.iter().rposition(|&x| x == 3), Some(2));
3048 ///
3049 /// assert_eq!(a.iter().rposition(|&x| x == 5), None);
3050 /// ```
3051 ///
3052 /// Stopping at the first `true`:
3053 ///
3054 /// ```
3055 /// let a = [-1, 2, 3, 4];
3056 ///
3057 /// let mut iter = a.iter();
3058 ///
3059 /// assert_eq!(iter.rposition(|&x| x >= 2), Some(3));
3060 ///
3061 /// // we can still use `iter`, as there are more elements.
3062 /// assert_eq!(iter.next(), Some(&-1));
3063 /// assert_eq!(iter.next_back(), Some(&3));
3064 /// ```
3065 #[inline]
3066 #[stable(feature = "rust1", since = "1.0.0")]
3067 fn rposition<P>(&mut self, predicate: P) -> Option<usize>
3068 where
3069 P: FnMut(Self::Item) -> bool,
3070 Self: Sized + ExactSizeIterator + DoubleEndedIterator,
3071 {
3072 // No need for an overflow check here, because `ExactSizeIterator`
3073 // implies that the number of elements fits into a `usize`.
3074 #[inline]
3075 fn check<T>(
3076 mut predicate: impl FnMut(T) -> bool,
3077 ) -> impl FnMut(usize, T) -> ControlFlow<usize, usize> {
3078 move |i, x| {
3079 let i = i - 1;
3080 if predicate(x) { ControlFlow::Break(i) } else { ControlFlow::Continue(i) }
3081 }
3082 }
3083
3084 let n = self.len();
3085 self.try_rfold(n, check(predicate)).break_value()
3086 }
3087
3088 /// Returns the maximum element of an iterator.
3089 ///
3090 /// If several elements are equally maximum, the last element is
3091 /// returned. If the iterator is empty, [`None`] is returned.
3092 ///
3093 /// Note that [`f32`]/[`f64`] doesn't implement [`Ord`] due to NaN being
3094 /// incomparable. You can work around this by using [`Iterator::reduce`]:
3095 /// ```
3096 /// assert_eq!(
3097 /// [2.4, f32::NAN, 1.3]
3098 /// .into_iter()
3099 /// .reduce(f32::max)
3100 /// .unwrap_or(0.),
3101 /// 2.4
3102 /// );
3103 /// ```
3104 ///
3105 /// # Examples
3106 ///
3107 /// ```
3108 /// let a = [1, 2, 3];
3109 /// let b: Vec<u32> = Vec::new();
3110 ///
3111 /// assert_eq!(a.iter().max(), Some(&3));
3112 /// assert_eq!(b.iter().max(), None);
3113 /// ```
3114 #[inline]
3115 #[stable(feature = "rust1", since = "1.0.0")]
3116 fn max(self) -> Option<Self::Item>
3117 where
3118 Self: Sized,
3119 Self::Item: Ord,
3120 {
3121 self.max_by(Ord::cmp)
3122 }
3123
3124 /// Returns the minimum element of an iterator.
3125 ///
3126 /// If several elements are equally minimum, the first element is returned.
3127 /// If the iterator is empty, [`None`] is returned.
3128 ///
3129 /// Note that [`f32`]/[`f64`] doesn't implement [`Ord`] due to NaN being
3130 /// incomparable. You can work around this by using [`Iterator::reduce`]:
3131 /// ```
3132 /// assert_eq!(
3133 /// [2.4, f32::NAN, 1.3]
3134 /// .into_iter()
3135 /// .reduce(f32::min)
3136 /// .unwrap_or(0.),
3137 /// 1.3
3138 /// );
3139 /// ```
3140 ///
3141 /// # Examples
3142 ///
3143 /// ```
3144 /// let a = [1, 2, 3];
3145 /// let b: Vec<u32> = Vec::new();
3146 ///
3147 /// assert_eq!(a.iter().min(), Some(&1));
3148 /// assert_eq!(b.iter().min(), None);
3149 /// ```
3150 #[inline]
3151 #[stable(feature = "rust1", since = "1.0.0")]
3152 fn min(self) -> Option<Self::Item>
3153 where
3154 Self: Sized,
3155 Self::Item: Ord,
3156 {
3157 self.min_by(Ord::cmp)
3158 }
3159
3160 /// Returns the element that gives the maximum value from the
3161 /// specified function.
3162 ///
3163 /// If several elements are equally maximum, the last element is
3164 /// returned. If the iterator is empty, [`None`] is returned.
3165 ///
3166 /// # Examples
3167 ///
3168 /// ```
3169 /// let a = [-3_i32, 0, 1, 5, -10];
3170 /// assert_eq!(*a.iter().max_by_key(|x| x.abs()).unwrap(), -10);
3171 /// ```
3172 #[inline]
3173 #[stable(feature = "iter_cmp_by_key", since = "1.6.0")]
3174 fn max_by_key<B: Ord, F>(self, f: F) -> Option<Self::Item>
3175 where
3176 Self: Sized,
3177 F: FnMut(&Self::Item) -> B,
3178 {
3179 #[inline]
3180 fn key<T, B>(mut f: impl FnMut(&T) -> B) -> impl FnMut(T) -> (B, T) {
3181 move |x| (f(&x), x)
3182 }
3183
3184 #[inline]
3185 fn compare<T, B: Ord>((x_p, _): &(B, T), (y_p, _): &(B, T)) -> Ordering {
3186 x_p.cmp(y_p)
3187 }
3188
3189 let (_, x) = self.map(key(f)).max_by(compare)?;
3190 Some(x)
3191 }
3192
3193 /// Returns the element that gives the maximum value with respect to the
3194 /// specified comparison function.
3195 ///
3196 /// If several elements are equally maximum, the last element is
3197 /// returned. If the iterator is empty, [`None`] is returned.
3198 ///
3199 /// # Examples
3200 ///
3201 /// ```
3202 /// let a = [-3_i32, 0, 1, 5, -10];
3203 /// assert_eq!(*a.iter().max_by(|x, y| x.cmp(y)).unwrap(), 5);
3204 /// ```
3205 #[inline]
3206 #[stable(feature = "iter_max_by", since = "1.15.0")]
3207 fn max_by<F>(self, compare: F) -> Option<Self::Item>
3208 where
3209 Self: Sized,
3210 F: FnMut(&Self::Item, &Self::Item) -> Ordering,
3211 {
3212 #[inline]
3213 fn fold<T>(mut compare: impl FnMut(&T, &T) -> Ordering) -> impl FnMut(T, T) -> T {
3214 move |x, y| cmp::max_by(x, y, &mut compare)
3215 }
3216
3217 self.reduce(fold(compare))
3218 }
3219
3220 /// Returns the element that gives the minimum value from the
3221 /// specified function.
3222 ///
3223 /// If several elements are equally minimum, the first element is
3224 /// returned. If the iterator is empty, [`None`] is returned.
3225 ///
3226 /// # Examples
3227 ///
3228 /// ```
3229 /// let a = [-3_i32, 0, 1, 5, -10];
3230 /// assert_eq!(*a.iter().min_by_key(|x| x.abs()).unwrap(), 0);
3231 /// ```
3232 #[inline]
3233 #[stable(feature = "iter_cmp_by_key", since = "1.6.0")]
3234 fn min_by_key<B: Ord, F>(self, f: F) -> Option<Self::Item>
3235 where
3236 Self: Sized,
3237 F: FnMut(&Self::Item) -> B,
3238 {
3239 #[inline]
3240 fn key<T, B>(mut f: impl FnMut(&T) -> B) -> impl FnMut(T) -> (B, T) {
3241 move |x| (f(&x), x)
3242 }
3243
3244 #[inline]
3245 fn compare<T, B: Ord>((x_p, _): &(B, T), (y_p, _): &(B, T)) -> Ordering {
3246 x_p.cmp(y_p)
3247 }
3248
3249 let (_, x) = self.map(key(f)).min_by(compare)?;
3250 Some(x)
3251 }
3252
3253 /// Returns the element that gives the minimum value with respect to the
3254 /// specified comparison function.
3255 ///
3256 /// If several elements are equally minimum, the first element is
3257 /// returned. If the iterator is empty, [`None`] is returned.
3258 ///
3259 /// # Examples
3260 ///
3261 /// ```
3262 /// let a = [-3_i32, 0, 1, 5, -10];
3263 /// assert_eq!(*a.iter().min_by(|x, y| x.cmp(y)).unwrap(), -10);
3264 /// ```
3265 #[inline]
3266 #[stable(feature = "iter_min_by", since = "1.15.0")]
3267 fn min_by<F>(self, compare: F) -> Option<Self::Item>
3268 where
3269 Self: Sized,
3270 F: FnMut(&Self::Item, &Self::Item) -> Ordering,
3271 {
3272 #[inline]
3273 fn fold<T>(mut compare: impl FnMut(&T, &T) -> Ordering) -> impl FnMut(T, T) -> T {
3274 move |x, y| cmp::min_by(x, y, &mut compare)
3275 }
3276
3277 self.reduce(fold(compare))
3278 }
3279
3280 /// Reverses an iterator's direction.
3281 ///
3282 /// Usually, iterators iterate from left to right. After using `rev()`,
3283 /// an iterator will instead iterate from right to left.
3284 ///
3285 /// This is only possible if the iterator has an end, so `rev()` only
3286 /// works on [`DoubleEndedIterator`]s.
3287 ///
3288 /// # Examples
3289 ///
3290 /// ```
3291 /// let a = [1, 2, 3];
3292 ///
3293 /// let mut iter = a.iter().rev();
3294 ///
3295 /// assert_eq!(iter.next(), Some(&3));
3296 /// assert_eq!(iter.next(), Some(&2));
3297 /// assert_eq!(iter.next(), Some(&1));
3298 ///
3299 /// assert_eq!(iter.next(), None);
3300 /// ```
3301 #[inline]
3302 #[doc(alias = "reverse")]
3303 #[stable(feature = "rust1", since = "1.0.0")]
3304 fn rev(self) -> Rev<Self>
3305 where
3306 Self: Sized + DoubleEndedIterator,
3307 {
3308 Rev::new(self)
3309 }
3310
3311 /// Converts an iterator of pairs into a pair of containers.
3312 ///
3313 /// `unzip()` consumes an entire iterator of pairs, producing two
3314 /// collections: one from the left elements of the pairs, and one
3315 /// from the right elements.
3316 ///
3317 /// This function is, in some sense, the opposite of [`zip`].
3318 ///
3319 /// [`zip`]: Iterator::zip
3320 ///
3321 /// # Examples
3322 ///
3323 /// ```
3324 /// let a = [(1, 2), (3, 4), (5, 6)];
3325 ///
3326 /// let (left, right): (Vec<_>, Vec<_>) = a.iter().cloned().unzip();
3327 ///
3328 /// assert_eq!(left, [1, 3, 5]);
3329 /// assert_eq!(right, [2, 4, 6]);
3330 ///
3331 /// // you can also unzip multiple nested tuples at once
3332 /// let a = [(1, (2, 3)), (4, (5, 6))];
3333 ///
3334 /// let (x, (y, z)): (Vec<_>, (Vec<_>, Vec<_>)) = a.iter().cloned().unzip();
3335 /// assert_eq!(x, [1, 4]);
3336 /// assert_eq!(y, [2, 5]);
3337 /// assert_eq!(z, [3, 6]);
3338 /// ```
3339 #[stable(feature = "rust1", since = "1.0.0")]
3340 fn unzip<A, B, FromA, FromB>(self) -> (FromA, FromB)
3341 where
3342 FromA: Default + Extend<A>,
3343 FromB: Default + Extend<B>,
3344 Self: Sized + Iterator<Item = (A, B)>,
3345 {
3346 let mut unzipped: (FromA, FromB) = Default::default();
3347 unzipped.extend(self);
3348 unzipped
3349 }
3350
3351 /// Creates an iterator which copies all of its elements.
3352 ///
3353 /// This is useful when you have an iterator over `&T`, but you need an
3354 /// iterator over `T`.
3355 ///
3356 /// # Examples
3357 ///
3358 /// ```
3359 /// let a = [1, 2, 3];
3360 ///
3361 /// let v_copied: Vec<_> = a.iter().copied().collect();
3362 ///
3363 /// // copied is the same as .map(|&x| x)
3364 /// let v_map: Vec<_> = a.iter().map(|&x| x).collect();
3365 ///
3366 /// assert_eq!(v_copied, vec![1, 2, 3]);
3367 /// assert_eq!(v_map, vec![1, 2, 3]);
3368 /// ```
3369 #[stable(feature = "iter_copied", since = "1.36.0")]
3370 #[cfg_attr(not(test), rustc_diagnostic_item = "iter_copied")]
3371 fn copied<'a, T: 'a>(self) -> Copied<Self>
3372 where
3373 Self: Sized + Iterator<Item = &'a T>,
3374 T: Copy,
3375 {
3376 Copied::new(self)
3377 }
3378
3379 /// Creates an iterator which [`clone`]s all of its elements.
3380 ///
3381 /// This is useful when you have an iterator over `&T`, but you need an
3382 /// iterator over `T`.
3383 ///
3384 /// There is no guarantee whatsoever about the `clone` method actually
3385 /// being called *or* optimized away. So code should not depend on
3386 /// either.
3387 ///
3388 /// [`clone`]: Clone::clone
3389 ///
3390 /// # Examples
3391 ///
3392 /// Basic usage:
3393 ///
3394 /// ```
3395 /// let a = [1, 2, 3];
3396 ///
3397 /// let v_cloned: Vec<_> = a.iter().cloned().collect();
3398 ///
3399 /// // cloned is the same as .map(|&x| x), for integers
3400 /// let v_map: Vec<_> = a.iter().map(|&x| x).collect();
3401 ///
3402 /// assert_eq!(v_cloned, vec![1, 2, 3]);
3403 /// assert_eq!(v_map, vec![1, 2, 3]);
3404 /// ```
3405 ///
3406 /// To get the best performance, try to clone late:
3407 ///
3408 /// ```
3409 /// let a = [vec![0_u8, 1, 2], vec![3, 4], vec![23]];
3410 /// // don't do this:
3411 /// let slower: Vec<_> = a.iter().cloned().filter(|s| s.len() == 1).collect();
3412 /// assert_eq!(&[vec![23]], &slower[..]);
3413 /// // instead call `cloned` late
3414 /// let faster: Vec<_> = a.iter().filter(|s| s.len() == 1).cloned().collect();
3415 /// assert_eq!(&[vec![23]], &faster[..]);
3416 /// ```
3417 #[stable(feature = "rust1", since = "1.0.0")]
3418 #[cfg_attr(not(test), rustc_diagnostic_item = "iter_cloned")]
3419 fn cloned<'a, T: 'a>(self) -> Cloned<Self>
3420 where
3421 Self: Sized + Iterator<Item = &'a T>,
3422 T: Clone,
3423 {
3424 Cloned::new(self)
3425 }
3426
3427 /// Repeats an iterator endlessly.
3428 ///
3429 /// Instead of stopping at [`None`], the iterator will instead start again,
3430 /// from the beginning. After iterating again, it will start at the
3431 /// beginning again. And again. And again. Forever. Note that in case the
3432 /// original iterator is empty, the resulting iterator will also be empty.
3433 ///
3434 /// # Examples
3435 ///
3436 /// ```
3437 /// let a = [1, 2, 3];
3438 ///
3439 /// let mut it = a.iter().cycle();
3440 ///
3441 /// assert_eq!(it.next(), Some(&1));
3442 /// assert_eq!(it.next(), Some(&2));
3443 /// assert_eq!(it.next(), Some(&3));
3444 /// assert_eq!(it.next(), Some(&1));
3445 /// assert_eq!(it.next(), Some(&2));
3446 /// assert_eq!(it.next(), Some(&3));
3447 /// assert_eq!(it.next(), Some(&1));
3448 /// ```
3449 #[stable(feature = "rust1", since = "1.0.0")]
3450 #[inline]
3451 fn cycle(self) -> Cycle<Self>
3452 where
3453 Self: Sized + Clone,
3454 {
3455 Cycle::new(self)
3456 }
3457
3458 /// Returns an iterator over `N` elements of the iterator at a time.
3459 ///
3460 /// The chunks do not overlap. If `N` does not divide the length of the
3461 /// iterator, then the last up to `N-1` elements will be omitted and can be
3462 /// retrieved from the [`.into_remainder()`][ArrayChunks::into_remainder]
3463 /// function of the iterator.
3464 ///
3465 /// # Panics
3466 ///
3467 /// Panics if `N` is zero.
3468 ///
3469 /// # Examples
3470 ///
3471 /// Basic usage:
3472 ///
3473 /// ```
3474 /// #![feature(iter_array_chunks)]
3475 ///
3476 /// let mut iter = "lorem".chars().array_chunks();
3477 /// assert_eq!(iter.next(), Some(['l', 'o']));
3478 /// assert_eq!(iter.next(), Some(['r', 'e']));
3479 /// assert_eq!(iter.next(), None);
3480 /// assert_eq!(iter.into_remainder().unwrap().as_slice(), &['m']);
3481 /// ```
3482 ///
3483 /// ```
3484 /// #![feature(iter_array_chunks)]
3485 ///
3486 /// let data = [1, 1, 2, -2, 6, 0, 3, 1];
3487 /// // ^-----^ ^------^
3488 /// for [x, y, z] in data.iter().array_chunks() {
3489 /// assert_eq!(x + y + z, 4);
3490 /// }
3491 /// ```
3492 #[track_caller]
3493 #[unstable(feature = "iter_array_chunks", reason = "recently added", issue = "100450")]
3494 fn array_chunks<const N: usize>(self) -> ArrayChunks<Self, N>
3495 where
3496 Self: Sized,
3497 {
3498 ArrayChunks::new(self)
3499 }
3500
3501 /// Sums the elements of an iterator.
3502 ///
3503 /// Takes each element, adds them together, and returns the result.
3504 ///
3505 /// An empty iterator returns the *additive identity* ("zero") of the type,
3506 /// which is `0` for integers and `-0.0` for floats.
3507 ///
3508 /// `sum()` can be used to sum any type implementing [`Sum`][`core::iter::Sum`],
3509 /// including [`Option`][`Option::sum`] and [`Result`][`Result::sum`].
3510 ///
3511 /// # Panics
3512 ///
3513 /// When calling `sum()` and a primitive integer type is being returned, this
3514 /// method will panic if the computation overflows and debug assertions are
3515 /// enabled.
3516 ///
3517 /// # Examples
3518 ///
3519 /// ```
3520 /// let a = [1, 2, 3];
3521 /// let sum: i32 = a.iter().sum();
3522 ///
3523 /// assert_eq!(sum, 6);
3524 ///
3525 /// let b: Vec<f32> = vec![];
3526 /// let sum: f32 = b.iter().sum();
3527 /// assert_eq!(sum, -0.0_f32);
3528 /// ```
3529 #[stable(feature = "iter_arith", since = "1.11.0")]
3530 fn sum<S>(self) -> S
3531 where
3532 Self: Sized,
3533 S: Sum<Self::Item>,
3534 {
3535 Sum::sum(self)
3536 }
3537
3538 /// Iterates over the entire iterator, multiplying all the elements
3539 ///
3540 /// An empty iterator returns the one value of the type.
3541 ///
3542 /// `product()` can be used to multiply any type implementing [`Product`][`core::iter::Product`],
3543 /// including [`Option`][`Option::product`] and [`Result`][`Result::product`].
3544 ///
3545 /// # Panics
3546 ///
3547 /// When calling `product()` and a primitive integer type is being returned,
3548 /// method will panic if the computation overflows and debug assertions are
3549 /// enabled.
3550 ///
3551 /// # Examples
3552 ///
3553 /// ```
3554 /// fn factorial(n: u32) -> u32 {
3555 /// (1..=n).product()
3556 /// }
3557 /// assert_eq!(factorial(0), 1);
3558 /// assert_eq!(factorial(1), 1);
3559 /// assert_eq!(factorial(5), 120);
3560 /// ```
3561 #[stable(feature = "iter_arith", since = "1.11.0")]
3562 fn product<P>(self) -> P
3563 where
3564 Self: Sized,
3565 P: Product<Self::Item>,
3566 {
3567 Product::product(self)
3568 }
3569
3570 /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
3571 /// of another.
3572 ///
3573 /// # Examples
3574 ///
3575 /// ```
3576 /// use std::cmp::Ordering;
3577 ///
3578 /// assert_eq!([1].iter().cmp([1].iter()), Ordering::Equal);
3579 /// assert_eq!([1].iter().cmp([1, 2].iter()), Ordering::Less);
3580 /// assert_eq!([1, 2].iter().cmp([1].iter()), Ordering::Greater);
3581 /// ```
3582 #[stable(feature = "iter_order", since = "1.5.0")]
3583 fn cmp<I>(self, other: I) -> Ordering
3584 where
3585 I: IntoIterator<Item = Self::Item>,
3586 Self::Item: Ord,
3587 Self: Sized,
3588 {
3589 self.cmp_by(other, |x, y| x.cmp(&y))
3590 }
3591
3592 /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
3593 /// of another with respect to the specified comparison function.
3594 ///
3595 /// # Examples
3596 ///
3597 /// ```
3598 /// #![feature(iter_order_by)]
3599 ///
3600 /// use std::cmp::Ordering;
3601 ///
3602 /// let xs = [1, 2, 3, 4];
3603 /// let ys = [1, 4, 9, 16];
3604 ///
3605 /// assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| x.cmp(&y)), Ordering::Less);
3606 /// assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| (x * x).cmp(&y)), Ordering::Equal);
3607 /// assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| (2 * x).cmp(&y)), Ordering::Greater);
3608 /// ```
3609 #[unstable(feature = "iter_order_by", issue = "64295")]
3610 fn cmp_by<I, F>(self, other: I, cmp: F) -> Ordering
3611 where
3612 Self: Sized,
3613 I: IntoIterator,
3614 F: FnMut(Self::Item, I::Item) -> Ordering,
3615 {
3616 #[inline]
3617 fn compare<X, Y, F>(mut cmp: F) -> impl FnMut(X, Y) -> ControlFlow<Ordering>
3618 where
3619 F: FnMut(X, Y) -> Ordering,
3620 {
3621 move |x, y| match cmp(x, y) {
3622 Ordering::Equal => ControlFlow::Continue(()),
3623 non_eq => ControlFlow::Break(non_eq),
3624 }
3625 }
3626
3627 match iter_compare(self, other.into_iter(), compare(cmp)) {
3628 ControlFlow::Continue(ord) => ord,
3629 ControlFlow::Break(ord) => ord,
3630 }
3631 }
3632
3633 /// [Lexicographically](Ord#lexicographical-comparison) compares the [`PartialOrd`] elements of
3634 /// this [`Iterator`] with those of another. The comparison works like short-circuit
3635 /// evaluation, returning a result without comparing the remaining elements.
3636 /// As soon as an order can be determined, the evaluation stops and a result is returned.
3637 ///
3638 /// # Examples
3639 ///
3640 /// ```
3641 /// use std::cmp::Ordering;
3642 ///
3643 /// assert_eq!([1.].iter().partial_cmp([1.].iter()), Some(Ordering::Equal));
3644 /// assert_eq!([1.].iter().partial_cmp([1., 2.].iter()), Some(Ordering::Less));
3645 /// assert_eq!([1., 2.].iter().partial_cmp([1.].iter()), Some(Ordering::Greater));
3646 /// ```
3647 ///
3648 /// For floating-point numbers, NaN does not have a total order and will result
3649 /// in `None` when compared:
3650 ///
3651 /// ```
3652 /// assert_eq!([f64::NAN].iter().partial_cmp([1.].iter()), None);
3653 /// ```
3654 ///
3655 /// The results are determined by the order of evaluation.
3656 ///
3657 /// ```
3658 /// use std::cmp::Ordering;
3659 ///
3660 /// assert_eq!([1.0, f64::NAN].iter().partial_cmp([2.0, f64::NAN].iter()), Some(Ordering::Less));
3661 /// assert_eq!([2.0, f64::NAN].iter().partial_cmp([1.0, f64::NAN].iter()), Some(Ordering::Greater));
3662 /// assert_eq!([f64::NAN, 1.0].iter().partial_cmp([f64::NAN, 2.0].iter()), None);
3663 /// ```
3664 ///
3665 #[stable(feature = "iter_order", since = "1.5.0")]
3666 fn partial_cmp<I>(self, other: I) -> Option<Ordering>
3667 where
3668 I: IntoIterator,
3669 Self::Item: PartialOrd<I::Item>,
3670 Self: Sized,
3671 {
3672 self.partial_cmp_by(other, |x, y| x.partial_cmp(&y))
3673 }
3674
3675 /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
3676 /// of another with respect to the specified comparison function.
3677 ///
3678 /// # Examples
3679 ///
3680 /// ```
3681 /// #![feature(iter_order_by)]
3682 ///
3683 /// use std::cmp::Ordering;
3684 ///
3685 /// let xs = [1.0, 2.0, 3.0, 4.0];
3686 /// let ys = [1.0, 4.0, 9.0, 16.0];
3687 ///
3688 /// assert_eq!(
3689 /// xs.iter().partial_cmp_by(&ys, |&x, &y| x.partial_cmp(&y)),
3690 /// Some(Ordering::Less)
3691 /// );
3692 /// assert_eq!(
3693 /// xs.iter().partial_cmp_by(&ys, |&x, &y| (x * x).partial_cmp(&y)),
3694 /// Some(Ordering::Equal)
3695 /// );
3696 /// assert_eq!(
3697 /// xs.iter().partial_cmp_by(&ys, |&x, &y| (2.0 * x).partial_cmp(&y)),
3698 /// Some(Ordering::Greater)
3699 /// );
3700 /// ```
3701 #[unstable(feature = "iter_order_by", issue = "64295")]
3702 fn partial_cmp_by<I, F>(self, other: I, partial_cmp: F) -> Option<Ordering>
3703 where
3704 Self: Sized,
3705 I: IntoIterator,
3706 F: FnMut(Self::Item, I::Item) -> Option<Ordering>,
3707 {
3708 #[inline]
3709 fn compare<X, Y, F>(mut partial_cmp: F) -> impl FnMut(X, Y) -> ControlFlow<Option<Ordering>>
3710 where
3711 F: FnMut(X, Y) -> Option<Ordering>,
3712 {
3713 move |x, y| match partial_cmp(x, y) {
3714 Some(Ordering::Equal) => ControlFlow::Continue(()),
3715 non_eq => ControlFlow::Break(non_eq),
3716 }
3717 }
3718
3719 match iter_compare(self, other.into_iter(), compare(partial_cmp)) {
3720 ControlFlow::Continue(ord) => Some(ord),
3721 ControlFlow::Break(ord) => ord,
3722 }
3723 }
3724
3725 /// Determines if the elements of this [`Iterator`] are equal to those of
3726 /// another.
3727 ///
3728 /// # Examples
3729 ///
3730 /// ```
3731 /// assert_eq!([1].iter().eq([1].iter()), true);
3732 /// assert_eq!([1].iter().eq([1, 2].iter()), false);
3733 /// ```
3734 #[stable(feature = "iter_order", since = "1.5.0")]
3735 fn eq<I>(self, other: I) -> bool
3736 where
3737 I: IntoIterator,
3738 Self::Item: PartialEq<I::Item>,
3739 Self: Sized,
3740 {
3741 self.eq_by(other, |x, y| x == y)
3742 }
3743
3744 /// Determines if the elements of this [`Iterator`] are equal to those of
3745 /// another with respect to the specified equality function.
3746 ///
3747 /// # Examples
3748 ///
3749 /// ```
3750 /// #![feature(iter_order_by)]
3751 ///
3752 /// let xs = [1, 2, 3, 4];
3753 /// let ys = [1, 4, 9, 16];
3754 ///
3755 /// assert!(xs.iter().eq_by(&ys, |&x, &y| x * x == y));
3756 /// ```
3757 #[unstable(feature = "iter_order_by", issue = "64295")]
3758 fn eq_by<I, F>(self, other: I, eq: F) -> bool
3759 where
3760 Self: Sized,
3761 I: IntoIterator,
3762 F: FnMut(Self::Item, I::Item) -> bool,
3763 {
3764 #[inline]
3765 fn compare<X, Y, F>(mut eq: F) -> impl FnMut(X, Y) -> ControlFlow<()>
3766 where
3767 F: FnMut(X, Y) -> bool,
3768 {
3769 move |x, y| {
3770 if eq(x, y) { ControlFlow::Continue(()) } else { ControlFlow::Break(()) }
3771 }
3772 }
3773
3774 match iter_compare(self, other.into_iter(), compare(eq)) {
3775 ControlFlow::Continue(ord) => ord == Ordering::Equal,
3776 ControlFlow::Break(()) => false,
3777 }
3778 }
3779
3780 /// Determines if the elements of this [`Iterator`] are not equal to those of
3781 /// another.
3782 ///
3783 /// # Examples
3784 ///
3785 /// ```
3786 /// assert_eq!([1].iter().ne([1].iter()), false);
3787 /// assert_eq!([1].iter().ne([1, 2].iter()), true);
3788 /// ```
3789 #[stable(feature = "iter_order", since = "1.5.0")]
3790 fn ne<I>(self, other: I) -> bool
3791 where
3792 I: IntoIterator,
3793 Self::Item: PartialEq<I::Item>,
3794 Self: Sized,
3795 {
3796 !self.eq(other)
3797 }
3798
3799 /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
3800 /// less than those of another.
3801 ///
3802 /// # Examples
3803 ///
3804 /// ```
3805 /// assert_eq!([1].iter().lt([1].iter()), false);
3806 /// assert_eq!([1].iter().lt([1, 2].iter()), true);
3807 /// assert_eq!([1, 2].iter().lt([1].iter()), false);
3808 /// assert_eq!([1, 2].iter().lt([1, 2].iter()), false);
3809 /// ```
3810 #[stable(feature = "iter_order", since = "1.5.0")]
3811 fn lt<I>(self, other: I) -> bool
3812 where
3813 I: IntoIterator,
3814 Self::Item: PartialOrd<I::Item>,
3815 Self: Sized,
3816 {
3817 self.partial_cmp(other) == Some(Ordering::Less)
3818 }
3819
3820 /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
3821 /// less or equal to those of another.
3822 ///
3823 /// # Examples
3824 ///
3825 /// ```
3826 /// assert_eq!([1].iter().le([1].iter()), true);
3827 /// assert_eq!([1].iter().le([1, 2].iter()), true);
3828 /// assert_eq!([1, 2].iter().le([1].iter()), false);
3829 /// assert_eq!([1, 2].iter().le([1, 2].iter()), true);
3830 /// ```
3831 #[stable(feature = "iter_order", since = "1.5.0")]
3832 fn le<I>(self, other: I) -> bool
3833 where
3834 I: IntoIterator,
3835 Self::Item: PartialOrd<I::Item>,
3836 Self: Sized,
3837 {
3838 matches!(self.partial_cmp(other), Some(Ordering::Less | Ordering::Equal))
3839 }
3840
3841 /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
3842 /// greater than those of another.
3843 ///
3844 /// # Examples
3845 ///
3846 /// ```
3847 /// assert_eq!([1].iter().gt([1].iter()), false);
3848 /// assert_eq!([1].iter().gt([1, 2].iter()), false);
3849 /// assert_eq!([1, 2].iter().gt([1].iter()), true);
3850 /// assert_eq!([1, 2].iter().gt([1, 2].iter()), false);
3851 /// ```
3852 #[stable(feature = "iter_order", since = "1.5.0")]
3853 fn gt<I>(self, other: I) -> bool
3854 where
3855 I: IntoIterator,
3856 Self::Item: PartialOrd<I::Item>,
3857 Self: Sized,
3858 {
3859 self.partial_cmp(other) == Some(Ordering::Greater)
3860 }
3861
3862 /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
3863 /// greater than or equal to those of another.
3864 ///
3865 /// # Examples
3866 ///
3867 /// ```
3868 /// assert_eq!([1].iter().ge([1].iter()), true);
3869 /// assert_eq!([1].iter().ge([1, 2].iter()), false);
3870 /// assert_eq!([1, 2].iter().ge([1].iter()), true);
3871 /// assert_eq!([1, 2].iter().ge([1, 2].iter()), true);
3872 /// ```
3873 #[stable(feature = "iter_order", since = "1.5.0")]
3874 fn ge<I>(self, other: I) -> bool
3875 where
3876 I: IntoIterator,
3877 Self::Item: PartialOrd<I::Item>,
3878 Self: Sized,
3879 {
3880 matches!(self.partial_cmp(other), Some(Ordering::Greater | Ordering::Equal))
3881 }
3882
3883 /// Checks if the elements of this iterator are sorted.
3884 ///
3885 /// That is, for each element `a` and its following element `b`, `a <= b` must hold. If the
3886 /// iterator yields exactly zero or one element, `true` is returned.
3887 ///
3888 /// Note that if `Self::Item` is only `PartialOrd`, but not `Ord`, the above definition
3889 /// implies that this function returns `false` if any two consecutive items are not
3890 /// comparable.
3891 ///
3892 /// # Examples
3893 ///
3894 /// ```
3895 /// assert!([1, 2, 2, 9].iter().is_sorted());
3896 /// assert!(![1, 3, 2, 4].iter().is_sorted());
3897 /// assert!([0].iter().is_sorted());
3898 /// assert!(std::iter::empty::<i32>().is_sorted());
3899 /// assert!(![0.0, 1.0, f32::NAN].iter().is_sorted());
3900 /// ```
3901 #[inline]
3902 #[stable(feature = "is_sorted", since = "1.82.0")]
3903 fn is_sorted(self) -> bool
3904 where
3905 Self: Sized,
3906 Self::Item: PartialOrd,
3907 {
3908 self.is_sorted_by(|a, b| a <= b)
3909 }
3910
3911 /// Checks if the elements of this iterator are sorted using the given comparator function.
3912 ///
3913 /// Instead of using `PartialOrd::partial_cmp`, this function uses the given `compare`
3914 /// function to determine whether two elements are to be considered in sorted order.
3915 ///
3916 /// # Examples
3917 ///
3918 /// ```
3919 /// assert!([1, 2, 2, 9].iter().is_sorted_by(|a, b| a <= b));
3920 /// assert!(![1, 2, 2, 9].iter().is_sorted_by(|a, b| a < b));
3921 ///
3922 /// assert!([0].iter().is_sorted_by(|a, b| true));
3923 /// assert!([0].iter().is_sorted_by(|a, b| false));
3924 ///
3925 /// assert!(std::iter::empty::<i32>().is_sorted_by(|a, b| false));
3926 /// assert!(std::iter::empty::<i32>().is_sorted_by(|a, b| true));
3927 /// ```
3928 #[stable(feature = "is_sorted", since = "1.82.0")]
3929 fn is_sorted_by<F>(mut self, compare: F) -> bool
3930 where
3931 Self: Sized,
3932 F: FnMut(&Self::Item, &Self::Item) -> bool,
3933 {
3934 #[inline]
3935 fn check<'a, T>(
3936 last: &'a mut T,
3937 mut compare: impl FnMut(&T, &T) -> bool + 'a,
3938 ) -> impl FnMut(T) -> bool + 'a {
3939 move |curr| {
3940 if !compare(&last, &curr) {
3941 return false;
3942 }
3943 *last = curr;
3944 true
3945 }
3946 }
3947
3948 let mut last = match self.next() {
3949 Some(e) => e,
3950 None => return true,
3951 };
3952
3953 self.all(check(&mut last, compare))
3954 }
3955
3956 /// Checks if the elements of this iterator are sorted using the given key extraction
3957 /// function.
3958 ///
3959 /// Instead of comparing the iterator's elements directly, this function compares the keys of
3960 /// the elements, as determined by `f`. Apart from that, it's equivalent to [`is_sorted`]; see
3961 /// its documentation for more information.
3962 ///
3963 /// [`is_sorted`]: Iterator::is_sorted
3964 ///
3965 /// # Examples
3966 ///
3967 /// ```
3968 /// assert!(["c", "bb", "aaa"].iter().is_sorted_by_key(|s| s.len()));
3969 /// assert!(![-2i32, -1, 0, 3].iter().is_sorted_by_key(|n| n.abs()));
3970 /// ```
3971 #[inline]
3972 #[stable(feature = "is_sorted", since = "1.82.0")]
3973 fn is_sorted_by_key<F, K>(self, f: F) -> bool
3974 where
3975 Self: Sized,
3976 F: FnMut(Self::Item) -> K,
3977 K: PartialOrd,
3978 {
3979 self.map(f).is_sorted()
3980 }
3981
3982 /// See [TrustedRandomAccess][super::super::TrustedRandomAccess]
3983 // The unusual name is to avoid name collisions in method resolution
3984 // see #76479.
3985 #[inline]
3986 #[doc(hidden)]
3987 #[unstable(feature = "trusted_random_access", issue = "none")]
3988 unsafe fn __iterator_get_unchecked(&mut self, _idx: usize) -> Self::Item
3989 where
3990 Self: TrustedRandomAccessNoCoerce,
3991 {
3992 unreachable!("Always specialized");
3993 }
3994}
3995
3996/// Compares two iterators element-wise using the given function.
3997///
3998/// If `ControlFlow::Continue(())` is returned from the function, the comparison moves on to the next
3999/// elements of both iterators. Returning `ControlFlow::Break(x)` short-circuits the iteration and
4000/// returns `ControlFlow::Break(x)`. If one of the iterators runs out of elements,
4001/// `ControlFlow::Continue(ord)` is returned where `ord` is the result of comparing the lengths of
4002/// the iterators.
4003///
4004/// Isolates the logic shared by ['cmp_by'](Iterator::cmp_by),
4005/// ['partial_cmp_by'](Iterator::partial_cmp_by), and ['eq_by'](Iterator::eq_by).
4006#[inline]
4007fn iter_compare<A, B, F, T>(mut a: A, mut b: B, f: F) -> ControlFlow<T, Ordering>
4008where
4009 A: Iterator,
4010 B: Iterator,
4011 F: FnMut(A::Item, B::Item) -> ControlFlow<T>,
4012{
4013 #[inline]
4014 fn compare<'a, B, X, T>(
4015 b: &'a mut B,
4016 mut f: impl FnMut(X, B::Item) -> ControlFlow<T> + 'a,
4017 ) -> impl FnMut(X) -> ControlFlow<ControlFlow<T, Ordering>> + 'a
4018 where
4019 B: Iterator,
4020 {
4021 move |x| match b.next() {
4022 None => ControlFlow::Break(ControlFlow::Continue(Ordering::Greater)),
4023 Some(y) => f(x, y).map_break(ControlFlow::Break),
4024 }
4025 }
4026
4027 match a.try_for_each(compare(&mut b, f)) {
4028 ControlFlow::Continue(()) => ControlFlow::Continue(match b.next() {
4029 None => Ordering::Equal,
4030 Some(_) => Ordering::Less,
4031 }),
4032 ControlFlow::Break(x) => x,
4033 }
4034}
4035
4036/// Implements `Iterator` for mutable references to iterators, such as those produced by [`Iterator::by_ref`].
4037///
4038/// This implementation passes all method calls on to the original iterator.
4039#[stable(feature = "rust1", since = "1.0.0")]
4040impl<I: Iterator + ?Sized> Iterator for &mut I {
4041 type Item = I::Item;
4042 #[inline]
4043 fn next(&mut self) -> Option<I::Item> {
4044 (**self).next()
4045 }
4046 fn size_hint(&self) -> (usize, Option<usize>) {
4047 (**self).size_hint()
4048 }
4049 fn advance_by(&mut self, n: usize) -> Result<(), NonZero<usize>> {
4050 (**self).advance_by(n)
4051 }
4052 fn nth(&mut self, n: usize) -> Option<Self::Item> {
4053 (**self).nth(n)
4054 }
4055 fn fold<B, F>(self, init: B, f: F) -> B
4056 where
4057 F: FnMut(B, Self::Item) -> B,
4058 {
4059 self.spec_fold(init, f)
4060 }
4061 fn try_fold<B, F, R>(&mut self, init: B, f: F) -> R
4062 where
4063 F: FnMut(B, Self::Item) -> R,
4064 R: Try<Output = B>,
4065 {
4066 self.spec_try_fold(init, f)
4067 }
4068}
4069
4070/// Helper trait to specialize `fold` and `try_fold` for `&mut I where I: Sized`
4071trait IteratorRefSpec: Iterator {
4072 fn spec_fold<B, F>(self, init: B, f: F) -> B
4073 where
4074 F: FnMut(B, Self::Item) -> B;
4075
4076 fn spec_try_fold<B, F, R>(&mut self, init: B, f: F) -> R
4077 where
4078 F: FnMut(B, Self::Item) -> R,
4079 R: Try<Output = B>;
4080}
4081
4082impl<I: Iterator + ?Sized> IteratorRefSpec for &mut I {
4083 default fn spec_fold<B, F>(self, init: B, mut f: F) -> B
4084 where
4085 F: FnMut(B, Self::Item) -> B,
4086 {
4087 let mut accum = init;
4088 while let Some(x) = self.next() {
4089 accum = f(accum, x);
4090 }
4091 accum
4092 }
4093
4094 default fn spec_try_fold<B, F, R>(&mut self, init: B, mut f: F) -> R
4095 where
4096 F: FnMut(B, Self::Item) -> R,
4097 R: Try<Output = B>,
4098 {
4099 let mut accum = init;
4100 while let Some(x) = self.next() {
4101 accum = f(accum, x)?;
4102 }
4103 try { accum }
4104 }
4105}
4106
4107impl<I: Iterator> IteratorRefSpec for &mut I {
4108 impl_fold_via_try_fold! { spec_fold -> spec_try_fold }
4109
4110 fn spec_try_fold<B, F, R>(&mut self, init: B, f: F) -> R
4111 where
4112 F: FnMut(B, Self::Item) -> R,
4113 R: Try<Output = B>,
4114 {
4115 (**self).try_fold(init, f)
4116 }
4117}