1 use crate::cmp::{self, Ordering};
2 use crate::ops::{ChangeOutputType, ControlFlow, FromResidual, Residual, Try};
4 use super::super::TrustedRandomAccessNoCoerce;
5 use super::super::{Chain, Cloned, Copied, Cycle, Enumerate, Filter, FilterMap, Fuse};
6 use super::super::{FlatMap, Flatten};
7 use super::super::{FromIterator, Intersperse, IntersperseWith, Product, Sum, Zip};
9 Inspect, Map, MapWhile, Peekable, Rev, Scan, Skip, SkipWhile, StepBy, Take, TakeWhile,
12 fn _assert_is_object_safe(_: &dyn Iterator<Item = ()>) {}
14 /// An interface for dealing with iterators.
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].
20 /// [module-level documentation]: crate::iter
21 /// [impl]: crate::iter#implementing-iterator
22 #[stable(feature = "rust1", since = "1.0.0")]
23 #[rustc_on_unimplemented(
25 _Self = "std::ops::RangeTo<Idx>",
26 label = "if you meant to iterate until a value, add a starting value",
27 note = "`..end` is a `RangeTo`, which cannot be iterated on; you might have meant to have a \
28 bounded `Range`: `0..end`"
31 _Self = "std::ops::RangeToInclusive<Idx>",
32 label = "if you meant to iterate until a value (including it), add a starting value",
33 note = "`..=end` is a `RangeToInclusive`, which cannot be iterated on; you might have meant \
34 to have a bounded `RangeInclusive`: `0..=end`"
38 label = "`{Self}` is not an iterator; try calling `.chars()` or `.bytes()`"
41 _Self = "std::string::String",
42 label = "`{Self}` is not an iterator; try calling `.chars()` or `.bytes()`"
46 note = "if you want to iterate between `start` until a value `end`, use the exclusive range \
47 syntax `start..end` or the inclusive range syntax `start..=end`"
49 label = "`{Self}` is not an iterator",
50 message = "`{Self}` is not an iterator"
53 #[rustc_diagnostic_item = "Iterator"]
54 #[must_use = "iterators are lazy and do nothing unless consumed"]
56 /// The type of the elements being iterated over.
57 #[stable(feature = "rust1", since = "1.0.0")]
60 /// Advances the iterator and returns the next value.
62 /// Returns [`None`] when iteration is finished. Individual iterator
63 /// implementations may choose to resume iteration, and so calling `next()`
64 /// again may or may not eventually start returning [`Some(Item)`] again at some
67 /// [`Some(Item)`]: Some
74 /// let a = [1, 2, 3];
76 /// let mut iter = a.iter();
78 /// // A call to next() returns the next value...
79 /// assert_eq!(Some(&1), iter.next());
80 /// assert_eq!(Some(&2), iter.next());
81 /// assert_eq!(Some(&3), iter.next());
83 /// // ... and then None once it's over.
84 /// assert_eq!(None, iter.next());
86 /// // More calls may or may not return `None`. Here, they always will.
87 /// assert_eq!(None, iter.next());
88 /// assert_eq!(None, iter.next());
91 #[stable(feature = "rust1", since = "1.0.0")]
92 fn next(&mut self) -> Option<Self::Item>;
94 /// Returns the bounds on the remaining length of the iterator.
96 /// Specifically, `size_hint()` returns a tuple where the first element
97 /// is the lower bound, and the second element is the upper bound.
99 /// The second half of the tuple that is returned is an <code>[Option]<[usize]></code>.
100 /// A [`None`] here means that either there is no known upper bound, or the
101 /// upper bound is larger than [`usize`].
103 /// # Implementation notes
105 /// It is not enforced that an iterator implementation yields the declared
106 /// number of elements. A buggy iterator may yield less than the lower bound
107 /// or more than the upper bound of elements.
109 /// `size_hint()` is primarily intended to be used for optimizations such as
110 /// reserving space for the elements of the iterator, but must not be
111 /// trusted to e.g., omit bounds checks in unsafe code. An incorrect
112 /// implementation of `size_hint()` should not lead to memory safety
115 /// That said, the implementation should provide a correct estimation,
116 /// because otherwise it would be a violation of the trait's protocol.
118 /// The default implementation returns <code>(0, [None])</code> which is correct for any
126 /// let a = [1, 2, 3];
127 /// let iter = a.iter();
129 /// assert_eq!((3, Some(3)), iter.size_hint());
132 /// A more complex example:
135 /// // The even numbers in the range of zero to nine.
136 /// let iter = (0..10).filter(|x| x % 2 == 0);
138 /// // We might iterate from zero to ten times. Knowing that it's five
139 /// // exactly wouldn't be possible without executing filter().
140 /// assert_eq!((0, Some(10)), iter.size_hint());
142 /// // Let's add five more numbers with chain()
143 /// let iter = (0..10).filter(|x| x % 2 == 0).chain(15..20);
145 /// // now both bounds are increased by five
146 /// assert_eq!((5, Some(15)), iter.size_hint());
149 /// Returning `None` for an upper bound:
152 /// // an infinite iterator has no upper bound
153 /// // and the maximum possible lower bound
156 /// assert_eq!((usize::MAX, None), iter.size_hint());
159 #[stable(feature = "rust1", since = "1.0.0")]
160 fn size_hint(&self) -> (usize, Option<usize>) {
164 /// Consumes the iterator, counting the number of iterations and returning it.
166 /// This method will call [`next`] repeatedly until [`None`] is encountered,
167 /// returning the number of times it saw [`Some`]. Note that [`next`] has to be
168 /// called at least once even if the iterator does not have any elements.
170 /// [`next`]: Iterator::next
172 /// # Overflow Behavior
174 /// The method does no guarding against overflows, so counting elements of
175 /// an iterator with more than [`usize::MAX`] elements either produces the
176 /// wrong result or panics. If debug assertions are enabled, a panic is
181 /// This function might panic if the iterator has more than [`usize::MAX`]
189 /// let a = [1, 2, 3];
190 /// assert_eq!(a.iter().count(), 3);
192 /// let a = [1, 2, 3, 4, 5];
193 /// assert_eq!(a.iter().count(), 5);
196 #[stable(feature = "rust1", since = "1.0.0")]
197 fn count(self) -> usize
203 #[rustc_inherit_overflow_checks]
204 |count, _| count + 1,
208 /// Consumes the iterator, returning the last element.
210 /// This method will evaluate the iterator until it returns [`None`]. While
211 /// doing so, it keeps track of the current element. After [`None`] is
212 /// returned, `last()` will then return the last element it saw.
219 /// let a = [1, 2, 3];
220 /// assert_eq!(a.iter().last(), Some(&3));
222 /// let a = [1, 2, 3, 4, 5];
223 /// assert_eq!(a.iter().last(), Some(&5));
226 #[stable(feature = "rust1", since = "1.0.0")]
227 fn last(self) -> Option<Self::Item>
232 fn some<T>(_: Option<T>, x: T) -> Option<T> {
236 self.fold(None, some)
239 /// Advances the iterator by `n` elements.
241 /// This method will eagerly skip `n` elements by calling [`next`] up to `n`
242 /// times until [`None`] is encountered.
244 /// `advance_by(n)` will return [`Ok(())`][Ok] if the iterator successfully advances by
245 /// `n` elements, or [`Err(k)`][Err] if [`None`] is encountered, where `k` is the number
246 /// of elements the iterator is advanced by before running out of elements (i.e. the
247 /// length of the iterator). Note that `k` is always less than `n`.
249 /// Calling `advance_by(0)` can do meaningful work, for example [`Flatten`]
250 /// can advance its outer iterator until it finds an inner iterator that is not empty, which
251 /// then often allows it to return a more accurate `size_hint()` than in its initial state.
253 /// [`Flatten`]: crate::iter::Flatten
254 /// [`next`]: Iterator::next
261 /// #![feature(iter_advance_by)]
263 /// let a = [1, 2, 3, 4];
264 /// let mut iter = a.iter();
266 /// assert_eq!(iter.advance_by(2), Ok(()));
267 /// assert_eq!(iter.next(), Some(&3));
268 /// assert_eq!(iter.advance_by(0), Ok(()));
269 /// assert_eq!(iter.advance_by(100), Err(1)); // only `&4` was skipped
272 #[unstable(feature = "iter_advance_by", reason = "recently added", issue = "77404")]
273 fn advance_by(&mut self, n: usize) -> Result<(), usize> {
275 self.next().ok_or(i)?;
280 /// Returns the `n`th element of the iterator.
282 /// Like most indexing operations, the count starts from zero, so `nth(0)`
283 /// returns the first value, `nth(1)` the second, and so on.
285 /// Note that all preceding elements, as well as the returned element, will be
286 /// consumed from the iterator. That means that the preceding elements will be
287 /// discarded, and also that calling `nth(0)` multiple times on the same iterator
288 /// will return different elements.
290 /// `nth()` will return [`None`] if `n` is greater than or equal to the length of the
298 /// let a = [1, 2, 3];
299 /// assert_eq!(a.iter().nth(1), Some(&2));
302 /// Calling `nth()` multiple times doesn't rewind the iterator:
305 /// let a = [1, 2, 3];
307 /// let mut iter = a.iter();
309 /// assert_eq!(iter.nth(1), Some(&2));
310 /// assert_eq!(iter.nth(1), None);
313 /// Returning `None` if there are less than `n + 1` elements:
316 /// let a = [1, 2, 3];
317 /// assert_eq!(a.iter().nth(10), None);
320 #[stable(feature = "rust1", since = "1.0.0")]
321 fn nth(&mut self, n: usize) -> Option<Self::Item> {
322 self.advance_by(n).ok()?;
326 /// Creates an iterator starting at the same point, but stepping by
327 /// the given amount at each iteration.
329 /// Note 1: The first element of the iterator will always be returned,
330 /// regardless of the step given.
332 /// Note 2: The time at which ignored elements are pulled is not fixed.
333 /// `StepBy` behaves like the sequence `self.next()`, `self.nth(step-1)`,
334 /// `self.nth(step-1)`, …, but is also free to behave like the sequence
335 /// `advance_n_and_return_first(&mut self, step)`,
336 /// `advance_n_and_return_first(&mut self, step)`, …
337 /// Which way is used may change for some iterators for performance reasons.
338 /// The second way will advance the iterator earlier and may consume more items.
340 /// `advance_n_and_return_first` is the equivalent of:
342 /// fn advance_n_and_return_first<I>(iter: &mut I, n: usize) -> Option<I::Item>
346 /// let next = iter.next();
356 /// The method will panic if the given step is `0`.
363 /// let a = [0, 1, 2, 3, 4, 5];
364 /// let mut iter = a.iter().step_by(2);
366 /// assert_eq!(iter.next(), Some(&0));
367 /// assert_eq!(iter.next(), Some(&2));
368 /// assert_eq!(iter.next(), Some(&4));
369 /// assert_eq!(iter.next(), None);
372 #[stable(feature = "iterator_step_by", since = "1.28.0")]
373 fn step_by(self, step: usize) -> StepBy<Self>
377 StepBy::new(self, step)
380 /// Takes two iterators and creates a new iterator over both in sequence.
382 /// `chain()` will return a new iterator which will first iterate over
383 /// values from the first iterator and then over values from the second
386 /// In other words, it links two iterators together, in a chain. 🔗
388 /// [`once`] is commonly used to adapt a single value into a chain of
389 /// other kinds of iteration.
396 /// let a1 = [1, 2, 3];
397 /// let a2 = [4, 5, 6];
399 /// let mut iter = a1.iter().chain(a2.iter());
401 /// assert_eq!(iter.next(), Some(&1));
402 /// assert_eq!(iter.next(), Some(&2));
403 /// assert_eq!(iter.next(), Some(&3));
404 /// assert_eq!(iter.next(), Some(&4));
405 /// assert_eq!(iter.next(), Some(&5));
406 /// assert_eq!(iter.next(), Some(&6));
407 /// assert_eq!(iter.next(), None);
410 /// Since the argument to `chain()` uses [`IntoIterator`], we can pass
411 /// anything that can be converted into an [`Iterator`], not just an
412 /// [`Iterator`] itself. For example, slices (`&[T]`) implement
413 /// [`IntoIterator`], and so can be passed to `chain()` directly:
416 /// let s1 = &[1, 2, 3];
417 /// let s2 = &[4, 5, 6];
419 /// let mut iter = s1.iter().chain(s2);
421 /// assert_eq!(iter.next(), Some(&1));
422 /// assert_eq!(iter.next(), Some(&2));
423 /// assert_eq!(iter.next(), Some(&3));
424 /// assert_eq!(iter.next(), Some(&4));
425 /// assert_eq!(iter.next(), Some(&5));
426 /// assert_eq!(iter.next(), Some(&6));
427 /// assert_eq!(iter.next(), None);
430 /// If you work with Windows API, you may wish to convert [`OsStr`] to `Vec<u16>`:
434 /// fn os_str_to_utf16(s: &std::ffi::OsStr) -> Vec<u16> {
435 /// use std::os::windows::ffi::OsStrExt;
436 /// s.encode_wide().chain(std::iter::once(0)).collect()
440 /// [`once`]: crate::iter::once
441 /// [`OsStr`]: ../../std/ffi/struct.OsStr.html
443 #[stable(feature = "rust1", since = "1.0.0")]
444 fn chain<U>(self, other: U) -> Chain<Self, U::IntoIter>
447 U: IntoIterator<Item = Self::Item>,
449 Chain::new(self, other.into_iter())
452 /// 'Zips up' two iterators into a single iterator of pairs.
454 /// `zip()` returns a new iterator that will iterate over two other
455 /// iterators, returning a tuple where the first element comes from the
456 /// first iterator, and the second element comes from the second iterator.
458 /// In other words, it zips two iterators together, into a single one.
460 /// If either iterator returns [`None`], [`next`] from the zipped iterator
461 /// will return [`None`].
462 /// If the zipped iterator has no more elements to return then each further attempt to advance
463 /// it will first try to advance the first iterator at most one time and if it still yielded an item
464 /// try to advance the second iterator at most one time.
471 /// let a1 = [1, 2, 3];
472 /// let a2 = [4, 5, 6];
474 /// let mut iter = a1.iter().zip(a2.iter());
476 /// assert_eq!(iter.next(), Some((&1, &4)));
477 /// assert_eq!(iter.next(), Some((&2, &5)));
478 /// assert_eq!(iter.next(), Some((&3, &6)));
479 /// assert_eq!(iter.next(), None);
482 /// Since the argument to `zip()` uses [`IntoIterator`], we can pass
483 /// anything that can be converted into an [`Iterator`], not just an
484 /// [`Iterator`] itself. For example, slices (`&[T]`) implement
485 /// [`IntoIterator`], and so can be passed to `zip()` directly:
488 /// let s1 = &[1, 2, 3];
489 /// let s2 = &[4, 5, 6];
491 /// let mut iter = s1.iter().zip(s2);
493 /// assert_eq!(iter.next(), Some((&1, &4)));
494 /// assert_eq!(iter.next(), Some((&2, &5)));
495 /// assert_eq!(iter.next(), Some((&3, &6)));
496 /// assert_eq!(iter.next(), None);
499 /// `zip()` is often used to zip an infinite iterator to a finite one.
500 /// This works because the finite iterator will eventually return [`None`],
501 /// ending the zipper. Zipping with `(0..)` can look a lot like [`enumerate`]:
504 /// let enumerate: Vec<_> = "foo".chars().enumerate().collect();
506 /// let zipper: Vec<_> = (0..).zip("foo".chars()).collect();
508 /// assert_eq!((0, 'f'), enumerate[0]);
509 /// assert_eq!((0, 'f'), zipper[0]);
511 /// assert_eq!((1, 'o'), enumerate[1]);
512 /// assert_eq!((1, 'o'), zipper[1]);
514 /// assert_eq!((2, 'o'), enumerate[2]);
515 /// assert_eq!((2, 'o'), zipper[2]);
518 /// If both iterators have roughly equivalent syntax, it may be more readable to use [`zip`]:
521 /// use std::iter::zip;
523 /// let a = [1, 2, 3];
524 /// let b = [2, 3, 4];
526 /// let mut zipped = zip(
527 /// a.into_iter().map(|x| x * 2).skip(1),
528 /// b.into_iter().map(|x| x * 2).skip(1),
531 /// assert_eq!(zipped.next(), Some((4, 6)));
532 /// assert_eq!(zipped.next(), Some((6, 8)));
533 /// assert_eq!(zipped.next(), None);
539 /// # let a = [1, 2, 3];
540 /// # let b = [2, 3, 4];
542 /// let mut zipped = a
546 /// .zip(b.into_iter().map(|x| x * 2).skip(1));
548 /// # assert_eq!(zipped.next(), Some((4, 6)));
549 /// # assert_eq!(zipped.next(), Some((6, 8)));
550 /// # assert_eq!(zipped.next(), None);
553 /// [`enumerate`]: Iterator::enumerate
554 /// [`next`]: Iterator::next
555 /// [`zip`]: crate::iter::zip
557 #[stable(feature = "rust1", since = "1.0.0")]
558 fn zip<U>(self, other: U) -> Zip<Self, U::IntoIter>
563 Zip::new(self, other.into_iter())
566 /// Creates a new iterator which places a copy of `separator` between adjacent
567 /// items of the original iterator.
569 /// In case `separator` does not implement [`Clone`] or needs to be
570 /// computed every time, use [`intersperse_with`].
577 /// #![feature(iter_intersperse)]
579 /// let mut a = [0, 1, 2].iter().intersperse(&100);
580 /// assert_eq!(a.next(), Some(&0)); // The first element from `a`.
581 /// assert_eq!(a.next(), Some(&100)); // The separator.
582 /// assert_eq!(a.next(), Some(&1)); // The next element from `a`.
583 /// assert_eq!(a.next(), Some(&100)); // The separator.
584 /// assert_eq!(a.next(), Some(&2)); // The last element from `a`.
585 /// assert_eq!(a.next(), None); // The iterator is finished.
588 /// `intersperse` can be very useful to join an iterator's items using a common element:
590 /// #![feature(iter_intersperse)]
592 /// let hello = ["Hello", "World", "!"].iter().copied().intersperse(" ").collect::<String>();
593 /// assert_eq!(hello, "Hello World !");
596 /// [`Clone`]: crate::clone::Clone
597 /// [`intersperse_with`]: Iterator::intersperse_with
599 #[unstable(feature = "iter_intersperse", reason = "recently added", issue = "79524")]
600 fn intersperse(self, separator: Self::Item) -> Intersperse<Self>
605 Intersperse::new(self, separator)
608 /// Creates a new iterator which places an item generated by `separator`
609 /// between adjacent items of the original iterator.
611 /// The closure will be called exactly once each time an item is placed
612 /// between two adjacent items from the underlying iterator; specifically,
613 /// the closure is not called if the underlying iterator yields less than
614 /// two items and after the last item is yielded.
616 /// If the iterator's item implements [`Clone`], it may be easier to use
624 /// #![feature(iter_intersperse)]
626 /// #[derive(PartialEq, Debug)]
627 /// struct NotClone(usize);
629 /// let v = [NotClone(0), NotClone(1), NotClone(2)];
630 /// let mut it = v.into_iter().intersperse_with(|| NotClone(99));
632 /// assert_eq!(it.next(), Some(NotClone(0))); // The first element from `v`.
633 /// assert_eq!(it.next(), Some(NotClone(99))); // The separator.
634 /// assert_eq!(it.next(), Some(NotClone(1))); // The next element from `v`.
635 /// assert_eq!(it.next(), Some(NotClone(99))); // The separator.
636 /// assert_eq!(it.next(), Some(NotClone(2))); // The last element from from `v`.
637 /// assert_eq!(it.next(), None); // The iterator is finished.
640 /// `intersperse_with` can be used in situations where the separator needs
643 /// #![feature(iter_intersperse)]
645 /// let src = ["Hello", "to", "all", "people", "!!"].iter().copied();
647 /// // The closure mutably borrows its context to generate an item.
648 /// let mut happy_emojis = [" ❤️ ", " 😀 "].iter().copied();
649 /// let separator = || happy_emojis.next().unwrap_or(" 🦀 ");
651 /// let result = src.intersperse_with(separator).collect::<String>();
652 /// assert_eq!(result, "Hello ❤️ to 😀 all 🦀 people 🦀 !!");
654 /// [`Clone`]: crate::clone::Clone
655 /// [`intersperse`]: Iterator::intersperse
657 #[unstable(feature = "iter_intersperse", reason = "recently added", issue = "79524")]
658 fn intersperse_with<G>(self, separator: G) -> IntersperseWith<Self, G>
661 G: FnMut() -> Self::Item,
663 IntersperseWith::new(self, separator)
666 /// Takes a closure and creates an iterator which calls that closure on each
669 /// `map()` transforms one iterator into another, by means of its argument:
670 /// something that implements [`FnMut`]. It produces a new iterator which
671 /// calls this closure on each element of the original iterator.
673 /// If you are good at thinking in types, you can think of `map()` like this:
674 /// If you have an iterator that gives you elements of some type `A`, and
675 /// you want an iterator of some other type `B`, you can use `map()`,
676 /// passing a closure that takes an `A` and returns a `B`.
678 /// `map()` is conceptually similar to a [`for`] loop. However, as `map()` is
679 /// lazy, it is best used when you're already working with other iterators.
680 /// If you're doing some sort of looping for a side effect, it's considered
681 /// more idiomatic to use [`for`] than `map()`.
683 /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
684 /// [`FnMut`]: crate::ops::FnMut
691 /// let a = [1, 2, 3];
693 /// let mut iter = a.iter().map(|x| 2 * x);
695 /// assert_eq!(iter.next(), Some(2));
696 /// assert_eq!(iter.next(), Some(4));
697 /// assert_eq!(iter.next(), Some(6));
698 /// assert_eq!(iter.next(), None);
701 /// If you're doing some sort of side effect, prefer [`for`] to `map()`:
704 /// # #![allow(unused_must_use)]
705 /// // don't do this:
706 /// (0..5).map(|x| println!("{}", x));
708 /// // it won't even execute, as it is lazy. Rust will warn you about this.
710 /// // Instead, use for:
712 /// println!("{}", x);
716 #[stable(feature = "rust1", since = "1.0.0")]
717 fn map<B, F>(self, f: F) -> Map<Self, F>
720 F: FnMut(Self::Item) -> B,
725 /// Calls a closure on each element of an iterator.
727 /// This is equivalent to using a [`for`] loop on the iterator, although
728 /// `break` and `continue` are not possible from a closure. It's generally
729 /// more idiomatic to use a `for` loop, but `for_each` may be more legible
730 /// when processing items at the end of longer iterator chains. In some
731 /// cases `for_each` may also be faster than a loop, because it will use
732 /// internal iteration on adapters like `Chain`.
734 /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
741 /// use std::sync::mpsc::channel;
743 /// let (tx, rx) = channel();
744 /// (0..5).map(|x| x * 2 + 1)
745 /// .for_each(move |x| tx.send(x).unwrap());
747 /// let v: Vec<_> = rx.iter().collect();
748 /// assert_eq!(v, vec![1, 3, 5, 7, 9]);
751 /// For such a small example, a `for` loop may be cleaner, but `for_each`
752 /// might be preferable to keep a functional style with longer iterators:
755 /// (0..5).flat_map(|x| x * 100 .. x * 110)
757 /// .filter(|&(i, x)| (i + x) % 3 == 0)
758 /// .for_each(|(i, x)| println!("{}:{}", i, x));
761 #[stable(feature = "iterator_for_each", since = "1.21.0")]
762 fn for_each<F>(self, f: F)
765 F: FnMut(Self::Item),
768 fn call<T>(mut f: impl FnMut(T)) -> impl FnMut((), T) {
769 move |(), item| f(item)
772 self.fold((), call(f));
775 /// Creates an iterator which uses a closure to determine if an element
776 /// should be yielded.
778 /// Given an element the closure must return `true` or `false`. The returned
779 /// iterator will yield only the elements for which the closure returns
787 /// let a = [0i32, 1, 2];
789 /// let mut iter = a.iter().filter(|x| x.is_positive());
791 /// assert_eq!(iter.next(), Some(&1));
792 /// assert_eq!(iter.next(), Some(&2));
793 /// assert_eq!(iter.next(), None);
796 /// Because the closure passed to `filter()` takes a reference, and many
797 /// iterators iterate over references, this leads to a possibly confusing
798 /// situation, where the type of the closure is a double reference:
801 /// let a = [0, 1, 2];
803 /// let mut iter = a.iter().filter(|x| **x > 1); // need two *s!
805 /// assert_eq!(iter.next(), Some(&2));
806 /// assert_eq!(iter.next(), None);
809 /// It's common to instead use destructuring on the argument to strip away
813 /// let a = [0, 1, 2];
815 /// let mut iter = a.iter().filter(|&x| *x > 1); // both & and *
817 /// assert_eq!(iter.next(), Some(&2));
818 /// assert_eq!(iter.next(), None);
824 /// let a = [0, 1, 2];
826 /// let mut iter = a.iter().filter(|&&x| x > 1); // two &s
828 /// assert_eq!(iter.next(), Some(&2));
829 /// assert_eq!(iter.next(), None);
834 /// Note that `iter.filter(f).next()` is equivalent to `iter.find(f)`.
836 #[stable(feature = "rust1", since = "1.0.0")]
837 fn filter<P>(self, predicate: P) -> Filter<Self, P>
840 P: FnMut(&Self::Item) -> bool,
842 Filter::new(self, predicate)
845 /// Creates an iterator that both filters and maps.
847 /// The returned iterator yields only the `value`s for which the supplied
848 /// closure returns `Some(value)`.
850 /// `filter_map` can be used to make chains of [`filter`] and [`map`] more
851 /// concise. The example below shows how a `map().filter().map()` can be
852 /// shortened to a single call to `filter_map`.
854 /// [`filter`]: Iterator::filter
855 /// [`map`]: Iterator::map
862 /// let a = ["1", "two", "NaN", "four", "5"];
864 /// let mut iter = a.iter().filter_map(|s| s.parse().ok());
866 /// assert_eq!(iter.next(), Some(1));
867 /// assert_eq!(iter.next(), Some(5));
868 /// assert_eq!(iter.next(), None);
871 /// Here's the same example, but with [`filter`] and [`map`]:
874 /// let a = ["1", "two", "NaN", "four", "5"];
875 /// let mut iter = a.iter().map(|s| s.parse()).filter(|s| s.is_ok()).map(|s| s.unwrap());
876 /// assert_eq!(iter.next(), Some(1));
877 /// assert_eq!(iter.next(), Some(5));
878 /// assert_eq!(iter.next(), None);
881 #[stable(feature = "rust1", since = "1.0.0")]
882 fn filter_map<B, F>(self, f: F) -> FilterMap<Self, F>
885 F: FnMut(Self::Item) -> Option<B>,
887 FilterMap::new(self, f)
890 /// Creates an iterator which gives the current iteration count as well as
893 /// The iterator returned yields pairs `(i, val)`, where `i` is the
894 /// current index of iteration and `val` is the value returned by the
897 /// `enumerate()` keeps its count as a [`usize`]. If you want to count by a
898 /// different sized integer, the [`zip`] function provides similar
901 /// # Overflow Behavior
903 /// The method does no guarding against overflows, so enumerating more than
904 /// [`usize::MAX`] elements either produces the wrong result or panics. If
905 /// debug assertions are enabled, a panic is guaranteed.
909 /// The returned iterator might panic if the to-be-returned index would
910 /// overflow a [`usize`].
912 /// [`zip`]: Iterator::zip
917 /// let a = ['a', 'b', 'c'];
919 /// let mut iter = a.iter().enumerate();
921 /// assert_eq!(iter.next(), Some((0, &'a')));
922 /// assert_eq!(iter.next(), Some((1, &'b')));
923 /// assert_eq!(iter.next(), Some((2, &'c')));
924 /// assert_eq!(iter.next(), None);
927 #[stable(feature = "rust1", since = "1.0.0")]
928 fn enumerate(self) -> Enumerate<Self>
935 /// Creates an iterator which can use the [`peek`] and [`peek_mut`] methods
936 /// to look at the next element of the iterator without consuming it. See
937 /// their documentation for more information.
939 /// Note that the underlying iterator is still advanced when [`peek`] or
940 /// [`peek_mut`] are called for the first time: In order to retrieve the
941 /// next element, [`next`] is called on the underlying iterator, hence any
942 /// side effects (i.e. anything other than fetching the next value) of
943 /// the [`next`] method will occur.
951 /// let xs = [1, 2, 3];
953 /// let mut iter = xs.iter().peekable();
955 /// // peek() lets us see into the future
956 /// assert_eq!(iter.peek(), Some(&&1));
957 /// assert_eq!(iter.next(), Some(&1));
959 /// assert_eq!(iter.next(), Some(&2));
961 /// // we can peek() multiple times, the iterator won't advance
962 /// assert_eq!(iter.peek(), Some(&&3));
963 /// assert_eq!(iter.peek(), Some(&&3));
965 /// assert_eq!(iter.next(), Some(&3));
967 /// // after the iterator is finished, so is peek()
968 /// assert_eq!(iter.peek(), None);
969 /// assert_eq!(iter.next(), None);
972 /// Using [`peek_mut`] to mutate the next item without advancing the
976 /// let xs = [1, 2, 3];
978 /// let mut iter = xs.iter().peekable();
980 /// // `peek_mut()` lets us see into the future
981 /// assert_eq!(iter.peek_mut(), Some(&mut &1));
982 /// assert_eq!(iter.peek_mut(), Some(&mut &1));
983 /// assert_eq!(iter.next(), Some(&1));
985 /// if let Some(mut p) = iter.peek_mut() {
986 /// assert_eq!(*p, &2);
987 /// // put a value into the iterator
991 /// // The value reappears as the iterator continues
992 /// assert_eq!(iter.collect::<Vec<_>>(), vec![&1000, &3]);
994 /// [`peek`]: Peekable::peek
995 /// [`peek_mut`]: Peekable::peek_mut
996 /// [`next`]: Iterator::next
998 #[stable(feature = "rust1", since = "1.0.0")]
999 fn peekable(self) -> Peekable<Self>
1006 /// Creates an iterator that [`skip`]s elements based on a predicate.
1008 /// [`skip`]: Iterator::skip
1010 /// `skip_while()` takes a closure as an argument. It will call this
1011 /// closure on each element of the iterator, and ignore elements
1012 /// until it returns `false`.
1014 /// After `false` is returned, `skip_while()`'s job is over, and the
1015 /// rest of the elements are yielded.
1022 /// let a = [-1i32, 0, 1];
1024 /// let mut iter = a.iter().skip_while(|x| x.is_negative());
1026 /// assert_eq!(iter.next(), Some(&0));
1027 /// assert_eq!(iter.next(), Some(&1));
1028 /// assert_eq!(iter.next(), None);
1031 /// Because the closure passed to `skip_while()` takes a reference, and many
1032 /// iterators iterate over references, this leads to a possibly confusing
1033 /// situation, where the type of the closure argument is a double reference:
1036 /// let a = [-1, 0, 1];
1038 /// let mut iter = a.iter().skip_while(|x| **x < 0); // need two *s!
1040 /// assert_eq!(iter.next(), Some(&0));
1041 /// assert_eq!(iter.next(), Some(&1));
1042 /// assert_eq!(iter.next(), None);
1045 /// Stopping after an initial `false`:
1048 /// let a = [-1, 0, 1, -2];
1050 /// let mut iter = a.iter().skip_while(|x| **x < 0);
1052 /// assert_eq!(iter.next(), Some(&0));
1053 /// assert_eq!(iter.next(), Some(&1));
1055 /// // while this would have been false, since we already got a false,
1056 /// // skip_while() isn't used any more
1057 /// assert_eq!(iter.next(), Some(&-2));
1059 /// assert_eq!(iter.next(), None);
1062 #[doc(alias = "drop_while")]
1063 #[stable(feature = "rust1", since = "1.0.0")]
1064 fn skip_while<P>(self, predicate: P) -> SkipWhile<Self, P>
1067 P: FnMut(&Self::Item) -> bool,
1069 SkipWhile::new(self, predicate)
1072 /// Creates an iterator that yields elements based on a predicate.
1074 /// `take_while()` takes a closure as an argument. It will call this
1075 /// closure on each element of the iterator, and yield elements
1076 /// while it returns `true`.
1078 /// After `false` is returned, `take_while()`'s job is over, and the
1079 /// rest of the elements are ignored.
1086 /// let a = [-1i32, 0, 1];
1088 /// let mut iter = a.iter().take_while(|x| x.is_negative());
1090 /// assert_eq!(iter.next(), Some(&-1));
1091 /// assert_eq!(iter.next(), None);
1094 /// Because the closure passed to `take_while()` takes a reference, and many
1095 /// iterators iterate over references, this leads to a possibly confusing
1096 /// situation, where the type of the closure is a double reference:
1099 /// let a = [-1, 0, 1];
1101 /// let mut iter = a.iter().take_while(|x| **x < 0); // need two *s!
1103 /// assert_eq!(iter.next(), Some(&-1));
1104 /// assert_eq!(iter.next(), None);
1107 /// Stopping after an initial `false`:
1110 /// let a = [-1, 0, 1, -2];
1112 /// let mut iter = a.iter().take_while(|x| **x < 0);
1114 /// assert_eq!(iter.next(), Some(&-1));
1116 /// // We have more elements that are less than zero, but since we already
1117 /// // got a false, take_while() isn't used any more
1118 /// assert_eq!(iter.next(), None);
1121 /// Because `take_while()` needs to look at the value in order to see if it
1122 /// should be included or not, consuming iterators will see that it is
1126 /// let a = [1, 2, 3, 4];
1127 /// let mut iter = a.iter();
1129 /// let result: Vec<i32> = iter.by_ref()
1130 /// .take_while(|n| **n != 3)
1134 /// assert_eq!(result, &[1, 2]);
1136 /// let result: Vec<i32> = iter.cloned().collect();
1138 /// assert_eq!(result, &[4]);
1141 /// The `3` is no longer there, because it was consumed in order to see if
1142 /// the iteration should stop, but wasn't placed back into the iterator.
1144 #[stable(feature = "rust1", since = "1.0.0")]
1145 fn take_while<P>(self, predicate: P) -> TakeWhile<Self, P>
1148 P: FnMut(&Self::Item) -> bool,
1150 TakeWhile::new(self, predicate)
1153 /// Creates an iterator that both yields elements based on a predicate and maps.
1155 /// `map_while()` takes a closure as an argument. It will call this
1156 /// closure on each element of the iterator, and yield elements
1157 /// while it returns [`Some(_)`][`Some`].
1164 /// let a = [-1i32, 4, 0, 1];
1166 /// let mut iter = a.iter().map_while(|x| 16i32.checked_div(*x));
1168 /// assert_eq!(iter.next(), Some(-16));
1169 /// assert_eq!(iter.next(), Some(4));
1170 /// assert_eq!(iter.next(), None);
1173 /// Here's the same example, but with [`take_while`] and [`map`]:
1175 /// [`take_while`]: Iterator::take_while
1176 /// [`map`]: Iterator::map
1179 /// let a = [-1i32, 4, 0, 1];
1181 /// let mut iter = a.iter()
1182 /// .map(|x| 16i32.checked_div(*x))
1183 /// .take_while(|x| x.is_some())
1184 /// .map(|x| x.unwrap());
1186 /// assert_eq!(iter.next(), Some(-16));
1187 /// assert_eq!(iter.next(), Some(4));
1188 /// assert_eq!(iter.next(), None);
1191 /// Stopping after an initial [`None`]:
1194 /// let a = [0, 1, 2, -3, 4, 5, -6];
1196 /// let iter = a.iter().map_while(|x| u32::try_from(*x).ok());
1197 /// let vec = iter.collect::<Vec<_>>();
1199 /// // We have more elements which could fit in u32 (4, 5), but `map_while` returned `None` for `-3`
1200 /// // (as the `predicate` returned `None`) and `collect` stops at the first `None` encountered.
1201 /// assert_eq!(vec, vec![0, 1, 2]);
1204 /// Because `map_while()` needs to look at the value in order to see if it
1205 /// should be included or not, consuming iterators will see that it is
1209 /// let a = [1, 2, -3, 4];
1210 /// let mut iter = a.iter();
1212 /// let result: Vec<u32> = iter.by_ref()
1213 /// .map_while(|n| u32::try_from(*n).ok())
1216 /// assert_eq!(result, &[1, 2]);
1218 /// let result: Vec<i32> = iter.cloned().collect();
1220 /// assert_eq!(result, &[4]);
1223 /// The `-3` is no longer there, because it was consumed in order to see if
1224 /// the iteration should stop, but wasn't placed back into the iterator.
1226 /// Note that unlike [`take_while`] this iterator is **not** fused.
1227 /// It is also not specified what this iterator returns after the first [`None`] is returned.
1228 /// If you need fused iterator, use [`fuse`].
1230 /// [`fuse`]: Iterator::fuse
1232 #[stable(feature = "iter_map_while", since = "1.57.0")]
1233 fn map_while<B, P>(self, predicate: P) -> MapWhile<Self, P>
1236 P: FnMut(Self::Item) -> Option<B>,
1238 MapWhile::new(self, predicate)
1241 /// Creates an iterator that skips the first `n` elements.
1243 /// `skip(n)` skips elements until `n` elements are skipped or the end of the
1244 /// iterator is reached (whichever happens first). After that, all the remaining
1245 /// elements are yielded. In particular, if the original iterator is too short,
1246 /// then the returned iterator is empty.
1248 /// Rather than overriding this method directly, instead override the `nth` method.
1255 /// let a = [1, 2, 3];
1257 /// let mut iter = a.iter().skip(2);
1259 /// assert_eq!(iter.next(), Some(&3));
1260 /// assert_eq!(iter.next(), None);
1263 #[stable(feature = "rust1", since = "1.0.0")]
1264 fn skip(self, n: usize) -> Skip<Self>
1271 /// Creates an iterator that yields the first `n` elements, or fewer
1272 /// if the underlying iterator ends sooner.
1274 /// `take(n)` yields elements until `n` elements are yielded or the end of
1275 /// the iterator is reached (whichever happens first).
1276 /// The returned iterator is a prefix of length `n` if the original iterator
1277 /// contains at least `n` elements, otherwise it contains all of the
1278 /// (fewer than `n`) elements of the original iterator.
1285 /// let a = [1, 2, 3];
1287 /// let mut iter = a.iter().take(2);
1289 /// assert_eq!(iter.next(), Some(&1));
1290 /// assert_eq!(iter.next(), Some(&2));
1291 /// assert_eq!(iter.next(), None);
1294 /// `take()` is often used with an infinite iterator, to make it finite:
1297 /// let mut iter = (0..).take(3);
1299 /// assert_eq!(iter.next(), Some(0));
1300 /// assert_eq!(iter.next(), Some(1));
1301 /// assert_eq!(iter.next(), Some(2));
1302 /// assert_eq!(iter.next(), None);
1305 /// If less than `n` elements are available,
1306 /// `take` will limit itself to the size of the underlying iterator:
1310 /// let mut iter = v.into_iter().take(5);
1311 /// assert_eq!(iter.next(), Some(1));
1312 /// assert_eq!(iter.next(), Some(2));
1313 /// assert_eq!(iter.next(), None);
1316 #[stable(feature = "rust1", since = "1.0.0")]
1317 fn take(self, n: usize) -> Take<Self>
1324 /// An iterator adapter similar to [`fold`] that holds internal state and
1325 /// produces a new iterator.
1327 /// [`fold`]: Iterator::fold
1329 /// `scan()` takes two arguments: an initial value which seeds the internal
1330 /// state, and a closure with two arguments, the first being a mutable
1331 /// reference to the internal state and the second an iterator element.
1332 /// The closure can assign to the internal state to share state between
1335 /// On iteration, the closure will be applied to each element of the
1336 /// iterator and the return value from the closure, an [`Option`], is
1337 /// yielded by the iterator.
1344 /// let a = [1, 2, 3];
1346 /// let mut iter = a.iter().scan(1, |state, &x| {
1347 /// // each iteration, we'll multiply the state by the element
1348 /// *state = *state * x;
1350 /// // then, we'll yield the negation of the state
1354 /// assert_eq!(iter.next(), Some(-1));
1355 /// assert_eq!(iter.next(), Some(-2));
1356 /// assert_eq!(iter.next(), Some(-6));
1357 /// assert_eq!(iter.next(), None);
1360 #[stable(feature = "rust1", since = "1.0.0")]
1361 fn scan<St, B, F>(self, initial_state: St, f: F) -> Scan<Self, St, F>
1364 F: FnMut(&mut St, Self::Item) -> Option<B>,
1366 Scan::new(self, initial_state, f)
1369 /// Creates an iterator that works like map, but flattens nested structure.
1371 /// The [`map`] adapter is very useful, but only when the closure
1372 /// argument produces values. If it produces an iterator instead, there's
1373 /// an extra layer of indirection. `flat_map()` will remove this extra layer
1376 /// You can think of `flat_map(f)` as the semantic equivalent
1377 /// of [`map`]ping, and then [`flatten`]ing as in `map(f).flatten()`.
1379 /// Another way of thinking about `flat_map()`: [`map`]'s closure returns
1380 /// one item for each element, and `flat_map()`'s closure returns an
1381 /// iterator for each element.
1383 /// [`map`]: Iterator::map
1384 /// [`flatten`]: Iterator::flatten
1391 /// let words = ["alpha", "beta", "gamma"];
1393 /// // chars() returns an iterator
1394 /// let merged: String = words.iter()
1395 /// .flat_map(|s| s.chars())
1397 /// assert_eq!(merged, "alphabetagamma");
1400 #[stable(feature = "rust1", since = "1.0.0")]
1401 fn flat_map<U, F>(self, f: F) -> FlatMap<Self, U, F>
1405 F: FnMut(Self::Item) -> U,
1407 FlatMap::new(self, f)
1410 /// Creates an iterator that flattens nested structure.
1412 /// This is useful when you have an iterator of iterators or an iterator of
1413 /// things that can be turned into iterators and you want to remove one
1414 /// level of indirection.
1421 /// let data = vec![vec![1, 2, 3, 4], vec![5, 6]];
1422 /// let flattened = data.into_iter().flatten().collect::<Vec<u8>>();
1423 /// assert_eq!(flattened, &[1, 2, 3, 4, 5, 6]);
1426 /// Mapping and then flattening:
1429 /// let words = ["alpha", "beta", "gamma"];
1431 /// // chars() returns an iterator
1432 /// let merged: String = words.iter()
1433 /// .map(|s| s.chars())
1436 /// assert_eq!(merged, "alphabetagamma");
1439 /// You can also rewrite this in terms of [`flat_map()`], which is preferable
1440 /// in this case since it conveys intent more clearly:
1443 /// let words = ["alpha", "beta", "gamma"];
1445 /// // chars() returns an iterator
1446 /// let merged: String = words.iter()
1447 /// .flat_map(|s| s.chars())
1449 /// assert_eq!(merged, "alphabetagamma");
1452 /// Flattening only removes one level of nesting at a time:
1455 /// let d3 = [[[1, 2], [3, 4]], [[5, 6], [7, 8]]];
1457 /// let d2 = d3.iter().flatten().collect::<Vec<_>>();
1458 /// assert_eq!(d2, [&[1, 2], &[3, 4], &[5, 6], &[7, 8]]);
1460 /// let d1 = d3.iter().flatten().flatten().collect::<Vec<_>>();
1461 /// assert_eq!(d1, [&1, &2, &3, &4, &5, &6, &7, &8]);
1464 /// Here we see that `flatten()` does not perform a "deep" flatten.
1465 /// Instead, only one level of nesting is removed. That is, if you
1466 /// `flatten()` a three-dimensional array, the result will be
1467 /// two-dimensional and not one-dimensional. To get a one-dimensional
1468 /// structure, you have to `flatten()` again.
1470 /// [`flat_map()`]: Iterator::flat_map
1472 #[stable(feature = "iterator_flatten", since = "1.29.0")]
1473 fn flatten(self) -> Flatten<Self>
1476 Self::Item: IntoIterator,
1481 /// Creates an iterator which ends after the first [`None`].
1483 /// After an iterator returns [`None`], future calls may or may not yield
1484 /// [`Some(T)`] again. `fuse()` adapts an iterator, ensuring that after a
1485 /// [`None`] is given, it will always return [`None`] forever.
1487 /// Note that the [`Fuse`] wrapper is a no-op on iterators that implement
1488 /// the [`FusedIterator`] trait. `fuse()` may therefore behave incorrectly
1489 /// if the [`FusedIterator`] trait is improperly implemented.
1491 /// [`Some(T)`]: Some
1492 /// [`FusedIterator`]: crate::iter::FusedIterator
1499 /// // an iterator which alternates between Some and None
1500 /// struct Alternate {
1504 /// impl Iterator for Alternate {
1505 /// type Item = i32;
1507 /// fn next(&mut self) -> Option<i32> {
1508 /// let val = self.state;
1509 /// self.state = self.state + 1;
1511 /// // if it's even, Some(i32), else None
1512 /// if val % 2 == 0 {
1520 /// let mut iter = Alternate { state: 0 };
1522 /// // we can see our iterator going back and forth
1523 /// assert_eq!(iter.next(), Some(0));
1524 /// assert_eq!(iter.next(), None);
1525 /// assert_eq!(iter.next(), Some(2));
1526 /// assert_eq!(iter.next(), None);
1528 /// // however, once we fuse it...
1529 /// let mut iter = iter.fuse();
1531 /// assert_eq!(iter.next(), Some(4));
1532 /// assert_eq!(iter.next(), None);
1534 /// // it will always return `None` after the first time.
1535 /// assert_eq!(iter.next(), None);
1536 /// assert_eq!(iter.next(), None);
1537 /// assert_eq!(iter.next(), None);
1540 #[stable(feature = "rust1", since = "1.0.0")]
1541 fn fuse(self) -> Fuse<Self>
1548 /// Does something with each element of an iterator, passing the value on.
1550 /// When using iterators, you'll often chain several of them together.
1551 /// While working on such code, you might want to check out what's
1552 /// happening at various parts in the pipeline. To do that, insert
1553 /// a call to `inspect()`.
1555 /// It's more common for `inspect()` to be used as a debugging tool than to
1556 /// exist in your final code, but applications may find it useful in certain
1557 /// situations when errors need to be logged before being discarded.
1564 /// let a = [1, 4, 2, 3];
1566 /// // this iterator sequence is complex.
1567 /// let sum = a.iter()
1569 /// .filter(|x| x % 2 == 0)
1570 /// .fold(0, |sum, i| sum + i);
1572 /// println!("{}", sum);
1574 /// // let's add some inspect() calls to investigate what's happening
1575 /// let sum = a.iter()
1577 /// .inspect(|x| println!("about to filter: {}", x))
1578 /// .filter(|x| x % 2 == 0)
1579 /// .inspect(|x| println!("made it through filter: {}", x))
1580 /// .fold(0, |sum, i| sum + i);
1582 /// println!("{}", sum);
1585 /// This will print:
1589 /// about to filter: 1
1590 /// about to filter: 4
1591 /// made it through filter: 4
1592 /// about to filter: 2
1593 /// made it through filter: 2
1594 /// about to filter: 3
1598 /// Logging errors before discarding them:
1601 /// let lines = ["1", "2", "a"];
1603 /// let sum: i32 = lines
1605 /// .map(|line| line.parse::<i32>())
1606 /// .inspect(|num| {
1607 /// if let Err(ref e) = *num {
1608 /// println!("Parsing error: {}", e);
1611 /// .filter_map(Result::ok)
1614 /// println!("Sum: {}", sum);
1617 /// This will print:
1620 /// Parsing error: invalid digit found in string
1624 #[stable(feature = "rust1", since = "1.0.0")]
1625 fn inspect<F>(self, f: F) -> Inspect<Self, F>
1628 F: FnMut(&Self::Item),
1630 Inspect::new(self, f)
1633 /// Borrows an iterator, rather than consuming it.
1635 /// This is useful to allow applying iterator adapters while still
1636 /// retaining ownership of the original iterator.
1643 /// let mut words = ["hello", "world", "of", "Rust"].into_iter();
1645 /// // Take the first two words.
1646 /// let hello_world: Vec<_> = words.by_ref().take(2).collect();
1647 /// assert_eq!(hello_world, vec!["hello", "world"]);
1649 /// // Collect the rest of the words.
1650 /// // We can only do this because we used `by_ref` earlier.
1651 /// let of_rust: Vec<_> = words.collect();
1652 /// assert_eq!(of_rust, vec!["of", "Rust"]);
1654 #[stable(feature = "rust1", since = "1.0.0")]
1655 fn by_ref(&mut self) -> &mut Self
1662 /// Transforms an iterator into a collection.
1664 /// `collect()` can take anything iterable, and turn it into a relevant
1665 /// collection. This is one of the more powerful methods in the standard
1666 /// library, used in a variety of contexts.
1668 /// The most basic pattern in which `collect()` is used is to turn one
1669 /// collection into another. You take a collection, call [`iter`] on it,
1670 /// do a bunch of transformations, and then `collect()` at the end.
1672 /// `collect()` can also create instances of types that are not typical
1673 /// collections. For example, a [`String`] can be built from [`char`]s,
1674 /// and an iterator of [`Result<T, E>`][`Result`] items can be collected
1675 /// into `Result<Collection<T>, E>`. See the examples below for more.
1677 /// Because `collect()` is so general, it can cause problems with type
1678 /// inference. As such, `collect()` is one of the few times you'll see
1679 /// the syntax affectionately known as the 'turbofish': `::<>`. This
1680 /// helps the inference algorithm understand specifically which collection
1681 /// you're trying to collect into.
1688 /// let a = [1, 2, 3];
1690 /// let doubled: Vec<i32> = a.iter()
1691 /// .map(|&x| x * 2)
1694 /// assert_eq!(vec![2, 4, 6], doubled);
1697 /// Note that we needed the `: Vec<i32>` on the left-hand side. This is because
1698 /// we could collect into, for example, a [`VecDeque<T>`] instead:
1700 /// [`VecDeque<T>`]: ../../std/collections/struct.VecDeque.html
1703 /// use std::collections::VecDeque;
1705 /// let a = [1, 2, 3];
1707 /// let doubled: VecDeque<i32> = a.iter().map(|&x| x * 2).collect();
1709 /// assert_eq!(2, doubled[0]);
1710 /// assert_eq!(4, doubled[1]);
1711 /// assert_eq!(6, doubled[2]);
1714 /// Using the 'turbofish' instead of annotating `doubled`:
1717 /// let a = [1, 2, 3];
1719 /// let doubled = a.iter().map(|x| x * 2).collect::<Vec<i32>>();
1721 /// assert_eq!(vec![2, 4, 6], doubled);
1724 /// Because `collect()` only cares about what you're collecting into, you can
1725 /// still use a partial type hint, `_`, with the turbofish:
1728 /// let a = [1, 2, 3];
1730 /// let doubled = a.iter().map(|x| x * 2).collect::<Vec<_>>();
1732 /// assert_eq!(vec![2, 4, 6], doubled);
1735 /// Using `collect()` to make a [`String`]:
1738 /// let chars = ['g', 'd', 'k', 'k', 'n'];
1740 /// let hello: String = chars.iter()
1741 /// .map(|&x| x as u8)
1742 /// .map(|x| (x + 1) as char)
1745 /// assert_eq!("hello", hello);
1748 /// If you have a list of [`Result<T, E>`][`Result`]s, you can use `collect()` to
1749 /// see if any of them failed:
1752 /// let results = [Ok(1), Err("nope"), Ok(3), Err("bad")];
1754 /// let result: Result<Vec<_>, &str> = results.iter().cloned().collect();
1756 /// // gives us the first error
1757 /// assert_eq!(Err("nope"), result);
1759 /// let results = [Ok(1), Ok(3)];
1761 /// let result: Result<Vec<_>, &str> = results.iter().cloned().collect();
1763 /// // gives us the list of answers
1764 /// assert_eq!(Ok(vec![1, 3]), result);
1767 /// [`iter`]: Iterator::next
1768 /// [`String`]: ../../std/string/struct.String.html
1769 /// [`char`]: type@char
1771 #[stable(feature = "rust1", since = "1.0.0")]
1772 #[must_use = "if you really need to exhaust the iterator, consider `.for_each(drop)` instead"]
1773 fn collect<B: FromIterator<Self::Item>>(self) -> B
1777 FromIterator::from_iter(self)
1780 /// Consumes an iterator, creating two collections from it.
1782 /// The predicate passed to `partition()` can return `true`, or `false`.
1783 /// `partition()` returns a pair, all of the elements for which it returned
1784 /// `true`, and all of the elements for which it returned `false`.
1786 /// See also [`is_partitioned()`] and [`partition_in_place()`].
1788 /// [`is_partitioned()`]: Iterator::is_partitioned
1789 /// [`partition_in_place()`]: Iterator::partition_in_place
1796 /// let a = [1, 2, 3];
1798 /// let (even, odd): (Vec<i32>, Vec<i32>) = a
1800 /// .partition(|&n| n % 2 == 0);
1802 /// assert_eq!(even, vec![2]);
1803 /// assert_eq!(odd, vec![1, 3]);
1805 #[stable(feature = "rust1", since = "1.0.0")]
1806 fn partition<B, F>(self, f: F) -> (B, B)
1809 B: Default + Extend<Self::Item>,
1810 F: FnMut(&Self::Item) -> bool,
1813 fn extend<'a, T, B: Extend<T>>(
1814 mut f: impl FnMut(&T) -> bool + 'a,
1817 ) -> impl FnMut((), T) + 'a {
1822 right.extend_one(x);
1827 let mut left: B = Default::default();
1828 let mut right: B = Default::default();
1830 self.fold((), extend(f, &mut left, &mut right));
1835 /// Reorders the elements of this iterator *in-place* according to the given predicate,
1836 /// such that all those that return `true` precede all those that return `false`.
1837 /// Returns the number of `true` elements found.
1839 /// The relative order of partitioned items is not maintained.
1841 /// # Current implementation
1843 /// Current algorithms tries finding the first element for which the predicate evaluates
1844 /// to false, and the last element for which it evaluates to true and repeatedly swaps them.
1846 /// Time complexity: *O*(*n*)
1848 /// See also [`is_partitioned()`] and [`partition()`].
1850 /// [`is_partitioned()`]: Iterator::is_partitioned
1851 /// [`partition()`]: Iterator::partition
1856 /// #![feature(iter_partition_in_place)]
1858 /// let mut a = [1, 2, 3, 4, 5, 6, 7];
1860 /// // Partition in-place between evens and odds
1861 /// let i = a.iter_mut().partition_in_place(|&n| n % 2 == 0);
1863 /// assert_eq!(i, 3);
1864 /// assert!(a[..i].iter().all(|&n| n % 2 == 0)); // evens
1865 /// assert!(a[i..].iter().all(|&n| n % 2 == 1)); // odds
1867 #[unstable(feature = "iter_partition_in_place", reason = "new API", issue = "62543")]
1868 fn partition_in_place<'a, T: 'a, P>(mut self, ref mut predicate: P) -> usize
1870 Self: Sized + DoubleEndedIterator<Item = &'a mut T>,
1871 P: FnMut(&T) -> bool,
1873 // FIXME: should we worry about the count overflowing? The only way to have more than
1874 // `usize::MAX` mutable references is with ZSTs, which aren't useful to partition...
1876 // These closure "factory" functions exist to avoid genericity in `Self`.
1880 predicate: &'a mut impl FnMut(&T) -> bool,
1881 true_count: &'a mut usize,
1882 ) -> impl FnMut(&&mut T) -> bool + 'a {
1884 let p = predicate(&**x);
1885 *true_count += p as usize;
1891 fn is_true<T>(predicate: &mut impl FnMut(&T) -> bool) -> impl FnMut(&&mut T) -> bool + '_ {
1892 move |x| predicate(&**x)
1895 // Repeatedly find the first `false` and swap it with the last `true`.
1896 let mut true_count = 0;
1897 while let Some(head) = self.find(is_false(predicate, &mut true_count)) {
1898 if let Some(tail) = self.rfind(is_true(predicate)) {
1899 crate::mem::swap(head, tail);
1908 /// Checks if the elements of this iterator are partitioned according to the given predicate,
1909 /// such that all those that return `true` precede all those that return `false`.
1911 /// See also [`partition()`] and [`partition_in_place()`].
1913 /// [`partition()`]: Iterator::partition
1914 /// [`partition_in_place()`]: Iterator::partition_in_place
1919 /// #![feature(iter_is_partitioned)]
1921 /// assert!("Iterator".chars().is_partitioned(char::is_uppercase));
1922 /// assert!(!"IntoIterator".chars().is_partitioned(char::is_uppercase));
1924 #[unstable(feature = "iter_is_partitioned", reason = "new API", issue = "62544")]
1925 fn is_partitioned<P>(mut self, mut predicate: P) -> bool
1928 P: FnMut(Self::Item) -> bool,
1930 // Either all items test `true`, or the first clause stops at `false`
1931 // and we check that there are no more `true` items after that.
1932 self.all(&mut predicate) || !self.any(predicate)
1935 /// An iterator method that applies a function as long as it returns
1936 /// successfully, producing a single, final value.
1938 /// `try_fold()` takes two arguments: an initial value, and a closure with
1939 /// two arguments: an 'accumulator', and an element. The closure either
1940 /// returns successfully, with the value that the accumulator should have
1941 /// for the next iteration, or it returns failure, with an error value that
1942 /// is propagated back to the caller immediately (short-circuiting).
1944 /// The initial value is the value the accumulator will have on the first
1945 /// call. If applying the closure succeeded against every element of the
1946 /// iterator, `try_fold()` returns the final accumulator as success.
1948 /// Folding is useful whenever you have a collection of something, and want
1949 /// to produce a single value from it.
1951 /// # Note to Implementors
1953 /// Several of the other (forward) methods have default implementations in
1954 /// terms of this one, so try to implement this explicitly if it can
1955 /// do something better than the default `for` loop implementation.
1957 /// In particular, try to have this call `try_fold()` on the internal parts
1958 /// from which this iterator is composed. If multiple calls are needed,
1959 /// the `?` operator may be convenient for chaining the accumulator value
1960 /// along, but beware any invariants that need to be upheld before those
1961 /// early returns. This is a `&mut self` method, so iteration needs to be
1962 /// resumable after hitting an error here.
1969 /// let a = [1, 2, 3];
1971 /// // the checked sum of all of the elements of the array
1972 /// let sum = a.iter().try_fold(0i8, |acc, &x| acc.checked_add(x));
1974 /// assert_eq!(sum, Some(6));
1977 /// Short-circuiting:
1980 /// let a = [10, 20, 30, 100, 40, 50];
1981 /// let mut it = a.iter();
1983 /// // This sum overflows when adding the 100 element
1984 /// let sum = it.try_fold(0i8, |acc, &x| acc.checked_add(x));
1985 /// assert_eq!(sum, None);
1987 /// // Because it short-circuited, the remaining elements are still
1988 /// // available through the iterator.
1989 /// assert_eq!(it.len(), 2);
1990 /// assert_eq!(it.next(), Some(&40));
1993 /// While you cannot `break` from a closure, the [`ControlFlow`] type allows
1997 /// use std::ops::ControlFlow;
1999 /// let triangular = (1..30).try_fold(0_i8, |prev, x| {
2000 /// if let Some(next) = prev.checked_add(x) {
2001 /// ControlFlow::Continue(next)
2003 /// ControlFlow::Break(prev)
2006 /// assert_eq!(triangular, ControlFlow::Break(120));
2008 /// let triangular = (1..30).try_fold(0_u64, |prev, x| {
2009 /// if let Some(next) = prev.checked_add(x) {
2010 /// ControlFlow::Continue(next)
2012 /// ControlFlow::Break(prev)
2015 /// assert_eq!(triangular, ControlFlow::Continue(435));
2018 #[stable(feature = "iterator_try_fold", since = "1.27.0")]
2019 fn try_fold<B, F, R>(&mut self, init: B, mut f: F) -> R
2022 F: FnMut(B, Self::Item) -> R,
2025 let mut accum = init;
2026 while let Some(x) = self.next() {
2027 accum = f(accum, x)?;
2032 /// An iterator method that applies a fallible function to each item in the
2033 /// iterator, stopping at the first error and returning that error.
2035 /// This can also be thought of as the fallible form of [`for_each()`]
2036 /// or as the stateless version of [`try_fold()`].
2038 /// [`for_each()`]: Iterator::for_each
2039 /// [`try_fold()`]: Iterator::try_fold
2044 /// use std::fs::rename;
2045 /// use std::io::{stdout, Write};
2046 /// use std::path::Path;
2048 /// let data = ["no_tea.txt", "stale_bread.json", "torrential_rain.png"];
2050 /// let res = data.iter().try_for_each(|x| writeln!(stdout(), "{}", x));
2051 /// assert!(res.is_ok());
2053 /// let mut it = data.iter().cloned();
2054 /// let res = it.try_for_each(|x| rename(x, Path::new(x).with_extension("old")));
2055 /// assert!(res.is_err());
2056 /// // It short-circuited, so the remaining items are still in the iterator:
2057 /// assert_eq!(it.next(), Some("stale_bread.json"));
2060 /// The [`ControlFlow`] type can be used with this method for the situations
2061 /// in which you'd use `break` and `continue` in a normal loop:
2064 /// use std::ops::ControlFlow;
2066 /// let r = (2..100).try_for_each(|x| {
2067 /// if 323 % x == 0 {
2068 /// return ControlFlow::Break(x)
2071 /// ControlFlow::Continue(())
2073 /// assert_eq!(r, ControlFlow::Break(17));
2076 #[stable(feature = "iterator_try_fold", since = "1.27.0")]
2077 fn try_for_each<F, R>(&mut self, f: F) -> R
2080 F: FnMut(Self::Item) -> R,
2081 R: Try<Output = ()>,
2084 fn call<T, R>(mut f: impl FnMut(T) -> R) -> impl FnMut((), T) -> R {
2088 self.try_fold((), call(f))
2091 /// Folds every element into an accumulator by applying an operation,
2092 /// returning the final result.
2094 /// `fold()` takes two arguments: an initial value, and a closure with two
2095 /// arguments: an 'accumulator', and an element. The closure returns the value that
2096 /// the accumulator should have for the next iteration.
2098 /// The initial value is the value the accumulator will have on the first
2101 /// After applying this closure to every element of the iterator, `fold()`
2102 /// returns the accumulator.
2104 /// This operation is sometimes called 'reduce' or 'inject'.
2106 /// Folding is useful whenever you have a collection of something, and want
2107 /// to produce a single value from it.
2109 /// Note: `fold()`, and similar methods that traverse the entire iterator,
2110 /// might not terminate for infinite iterators, even on traits for which a
2111 /// result is determinable in finite time.
2113 /// Note: [`reduce()`] can be used to use the first element as the initial
2114 /// value, if the accumulator type and item type is the same.
2116 /// Note: `fold()` combines elements in a *left-associative* fashion. For associative
2117 /// operators like `+`, the order the elements are combined in is not important, but for non-associative
2118 /// operators like `-` the order will affect the final result.
2119 /// For a *right-associative* version of `fold()`, see [`DoubleEndedIterator::rfold()`].
2121 /// # Note to Implementors
2123 /// Several of the other (forward) methods have default implementations in
2124 /// terms of this one, so try to implement this explicitly if it can
2125 /// do something better than the default `for` loop implementation.
2127 /// In particular, try to have this call `fold()` on the internal parts
2128 /// from which this iterator is composed.
2135 /// let a = [1, 2, 3];
2137 /// // the sum of all of the elements of the array
2138 /// let sum = a.iter().fold(0, |acc, x| acc + x);
2140 /// assert_eq!(sum, 6);
2143 /// Let's walk through each step of the iteration here:
2145 /// | element | acc | x | result |
2146 /// |---------|-----|---|--------|
2148 /// | 1 | 0 | 1 | 1 |
2149 /// | 2 | 1 | 2 | 3 |
2150 /// | 3 | 3 | 3 | 6 |
2152 /// And so, our final result, `6`.
2154 /// This example demonstrates the left-associative nature of `fold()`:
2155 /// it builds a string, starting with an initial value
2156 /// and continuing with each element from the front until the back:
2159 /// let numbers = [1, 2, 3, 4, 5];
2161 /// let zero = "0".to_string();
2163 /// let result = numbers.iter().fold(zero, |acc, &x| {
2164 /// format!("({} + {})", acc, x)
2167 /// assert_eq!(result, "(((((0 + 1) + 2) + 3) + 4) + 5)");
2169 /// It's common for people who haven't used iterators a lot to
2170 /// use a `for` loop with a list of things to build up a result. Those
2171 /// can be turned into `fold()`s:
2173 /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
2176 /// let numbers = [1, 2, 3, 4, 5];
2178 /// let mut result = 0;
2181 /// for i in &numbers {
2182 /// result = result + i;
2186 /// let result2 = numbers.iter().fold(0, |acc, &x| acc + x);
2188 /// // they're the same
2189 /// assert_eq!(result, result2);
2192 /// [`reduce()`]: Iterator::reduce
2193 #[doc(alias = "inject", alias = "foldl")]
2195 #[stable(feature = "rust1", since = "1.0.0")]
2196 fn fold<B, F>(mut self, init: B, mut f: F) -> B
2199 F: FnMut(B, Self::Item) -> B,
2201 let mut accum = init;
2202 while let Some(x) = self.next() {
2203 accum = f(accum, x);
2208 /// Reduces the elements to a single one, by repeatedly applying a reducing
2211 /// If the iterator is empty, returns [`None`]; otherwise, returns the
2212 /// result of the reduction.
2214 /// The reducing function is a closure with two arguments: an 'accumulator', and an element.
2215 /// For iterators with at least one element, this is the same as [`fold()`]
2216 /// with the first element of the iterator as the initial accumulator value, folding
2217 /// every subsequent element into it.
2219 /// [`fold()`]: Iterator::fold
2223 /// Find the maximum value:
2226 /// fn find_max<I>(iter: I) -> Option<I::Item>
2227 /// where I: Iterator,
2230 /// iter.reduce(|accum, item| {
2231 /// if accum >= item { accum } else { item }
2234 /// let a = [10, 20, 5, -23, 0];
2235 /// let b: [u32; 0] = [];
2237 /// assert_eq!(find_max(a.iter()), Some(&20));
2238 /// assert_eq!(find_max(b.iter()), None);
2241 #[stable(feature = "iterator_fold_self", since = "1.51.0")]
2242 fn reduce<F>(mut self, f: F) -> Option<Self::Item>
2245 F: FnMut(Self::Item, Self::Item) -> Self::Item,
2247 let first = self.next()?;
2248 Some(self.fold(first, f))
2251 /// Reduces the elements to a single one by repeatedly applying a reducing operation. If the
2252 /// closure returns a failure, the failure is propagated back to the caller immediately.
2254 /// The return type of this method depends on the return type of the closure. If the closure
2255 /// returns `Result<Self::Item, E>`, then this function will return `Result<Option<Self::Item>,
2256 /// E>`. If the closure returns `Option<Self::Item>`, then this function will return
2257 /// `Option<Option<Self::Item>>`.
2259 /// When called on an empty iterator, this function will return either `Some(None)` or
2260 /// `Ok(None)` depending on the type of the provided closure.
2262 /// For iterators with at least one element, this is essentially the same as calling
2263 /// [`try_fold()`] with the first element of the iterator as the initial accumulator value.
2265 /// [`try_fold()`]: Iterator::try_fold
2269 /// Safely calculate the sum of a series of numbers:
2272 /// #![feature(iterator_try_reduce)]
2274 /// let numbers: Vec<usize> = vec![10, 20, 5, 23, 0];
2275 /// let sum = numbers.into_iter().try_reduce(|x, y| x.checked_add(y));
2276 /// assert_eq!(sum, Some(Some(58)));
2279 /// Determine when a reduction short circuited:
2282 /// #![feature(iterator_try_reduce)]
2284 /// let numbers = vec![1, 2, 3, usize::MAX, 4, 5];
2285 /// let sum = numbers.into_iter().try_reduce(|x, y| x.checked_add(y));
2286 /// assert_eq!(sum, None);
2289 /// Determine when a reduction was not performed because there are no elements:
2292 /// #![feature(iterator_try_reduce)]
2294 /// let numbers: Vec<usize> = Vec::new();
2295 /// let sum = numbers.into_iter().try_reduce(|x, y| x.checked_add(y));
2296 /// assert_eq!(sum, Some(None));
2299 /// Use a [`Result`] instead of an [`Option`]:
2302 /// #![feature(iterator_try_reduce)]
2304 /// let numbers = vec!["1", "2", "3", "4", "5"];
2305 /// let max: Result<Option<_>, <usize as std::str::FromStr>::Err> =
2306 /// numbers.into_iter().try_reduce(|x, y| {
2307 /// if x.parse::<usize>()? > y.parse::<usize>()? { Ok(x) } else { Ok(y) }
2309 /// assert_eq!(max, Ok(Some("5")));
2312 #[unstable(feature = "iterator_try_reduce", reason = "new API", issue = "87053")]
2313 fn try_reduce<F, R>(&mut self, f: F) -> ChangeOutputType<R, Option<R::Output>>
2316 F: FnMut(Self::Item, Self::Item) -> R,
2317 R: Try<Output = Self::Item>,
2318 R::Residual: Residual<Option<Self::Item>>,
2320 let first = match self.next() {
2322 None => return Try::from_output(None),
2325 match self.try_fold(first, f).branch() {
2326 ControlFlow::Break(r) => FromResidual::from_residual(r),
2327 ControlFlow::Continue(i) => Try::from_output(Some(i)),
2331 /// Tests if every element of the iterator matches a predicate.
2333 /// `all()` takes a closure that returns `true` or `false`. It applies
2334 /// this closure to each element of the iterator, and if they all return
2335 /// `true`, then so does `all()`. If any of them return `false`, it
2336 /// returns `false`.
2338 /// `all()` is short-circuiting; in other words, it will stop processing
2339 /// as soon as it finds a `false`, given that no matter what else happens,
2340 /// the result will also be `false`.
2342 /// An empty iterator returns `true`.
2349 /// let a = [1, 2, 3];
2351 /// assert!(a.iter().all(|&x| x > 0));
2353 /// assert!(!a.iter().all(|&x| x > 2));
2356 /// Stopping at the first `false`:
2359 /// let a = [1, 2, 3];
2361 /// let mut iter = a.iter();
2363 /// assert!(!iter.all(|&x| x != 2));
2365 /// // we can still use `iter`, as there are more elements.
2366 /// assert_eq!(iter.next(), Some(&3));
2369 #[stable(feature = "rust1", since = "1.0.0")]
2370 fn all<F>(&mut self, f: F) -> bool
2373 F: FnMut(Self::Item) -> bool,
2376 fn check<T>(mut f: impl FnMut(T) -> bool) -> impl FnMut((), T) -> ControlFlow<()> {
2378 if f(x) { ControlFlow::CONTINUE } else { ControlFlow::BREAK }
2381 self.try_fold((), check(f)) == ControlFlow::CONTINUE
2384 /// Tests if any element of the iterator matches a predicate.
2386 /// `any()` takes a closure that returns `true` or `false`. It applies
2387 /// this closure to each element of the iterator, and if any of them return
2388 /// `true`, then so does `any()`. If they all return `false`, it
2389 /// returns `false`.
2391 /// `any()` is short-circuiting; in other words, it will stop processing
2392 /// as soon as it finds a `true`, given that no matter what else happens,
2393 /// the result will also be `true`.
2395 /// An empty iterator returns `false`.
2402 /// let a = [1, 2, 3];
2404 /// assert!(a.iter().any(|&x| x > 0));
2406 /// assert!(!a.iter().any(|&x| x > 5));
2409 /// Stopping at the first `true`:
2412 /// let a = [1, 2, 3];
2414 /// let mut iter = a.iter();
2416 /// assert!(iter.any(|&x| x != 2));
2418 /// // we can still use `iter`, as there are more elements.
2419 /// assert_eq!(iter.next(), Some(&2));
2422 #[stable(feature = "rust1", since = "1.0.0")]
2423 fn any<F>(&mut self, f: F) -> bool
2426 F: FnMut(Self::Item) -> bool,
2429 fn check<T>(mut f: impl FnMut(T) -> bool) -> impl FnMut((), T) -> ControlFlow<()> {
2431 if f(x) { ControlFlow::BREAK } else { ControlFlow::CONTINUE }
2435 self.try_fold((), check(f)) == ControlFlow::BREAK
2438 /// Searches for an element of an iterator that satisfies a predicate.
2440 /// `find()` takes a closure that returns `true` or `false`. It applies
2441 /// this closure to each element of the iterator, and if any of them return
2442 /// `true`, then `find()` returns [`Some(element)`]. If they all return
2443 /// `false`, it returns [`None`].
2445 /// `find()` is short-circuiting; in other words, it will stop processing
2446 /// as soon as the closure returns `true`.
2448 /// Because `find()` takes a reference, and many iterators iterate over
2449 /// references, this leads to a possibly confusing situation where the
2450 /// argument is a double reference. You can see this effect in the
2451 /// examples below, with `&&x`.
2453 /// [`Some(element)`]: Some
2460 /// let a = [1, 2, 3];
2462 /// assert_eq!(a.iter().find(|&&x| x == 2), Some(&2));
2464 /// assert_eq!(a.iter().find(|&&x| x == 5), None);
2467 /// Stopping at the first `true`:
2470 /// let a = [1, 2, 3];
2472 /// let mut iter = a.iter();
2474 /// assert_eq!(iter.find(|&&x| x == 2), Some(&2));
2476 /// // we can still use `iter`, as there are more elements.
2477 /// assert_eq!(iter.next(), Some(&3));
2480 /// Note that `iter.find(f)` is equivalent to `iter.filter(f).next()`.
2482 #[stable(feature = "rust1", since = "1.0.0")]
2483 fn find<P>(&mut self, predicate: P) -> Option<Self::Item>
2486 P: FnMut(&Self::Item) -> bool,
2489 fn check<T>(mut predicate: impl FnMut(&T) -> bool) -> impl FnMut((), T) -> ControlFlow<T> {
2491 if predicate(&x) { ControlFlow::Break(x) } else { ControlFlow::CONTINUE }
2495 self.try_fold((), check(predicate)).break_value()
2498 /// Applies function to the elements of iterator and returns
2499 /// the first non-none result.
2501 /// `iter.find_map(f)` is equivalent to `iter.filter_map(f).next()`.
2506 /// let a = ["lol", "NaN", "2", "5"];
2508 /// let first_number = a.iter().find_map(|s| s.parse().ok());
2510 /// assert_eq!(first_number, Some(2));
2513 #[stable(feature = "iterator_find_map", since = "1.30.0")]
2514 fn find_map<B, F>(&mut self, f: F) -> Option<B>
2517 F: FnMut(Self::Item) -> Option<B>,
2520 fn check<T, B>(mut f: impl FnMut(T) -> Option<B>) -> impl FnMut((), T) -> ControlFlow<B> {
2521 move |(), x| match f(x) {
2522 Some(x) => ControlFlow::Break(x),
2523 None => ControlFlow::CONTINUE,
2527 self.try_fold((), check(f)).break_value()
2530 /// Applies function to the elements of iterator and returns
2531 /// the first true result or the first error.
2533 /// The return type of this method depends on the return type of the closure.
2534 /// If you return `Result<bool, E>` from the closure, you'll get a `Result<Option<Self::Item>; E>`.
2535 /// If you return `Option<bool>` from the closure, you'll get an `Option<Option<Self::Item>>`.
2540 /// #![feature(try_find)]
2542 /// let a = ["1", "2", "lol", "NaN", "5"];
2544 /// let is_my_num = |s: &str, search: i32| -> Result<bool, std::num::ParseIntError> {
2545 /// Ok(s.parse::<i32>()? == search)
2548 /// let result = a.iter().try_find(|&&s| is_my_num(s, 2));
2549 /// assert_eq!(result, Ok(Some(&"2")));
2551 /// let result = a.iter().try_find(|&&s| is_my_num(s, 5));
2552 /// assert!(result.is_err());
2555 /// This also supports other types which implement `Try`, not just `Result`.
2557 /// #![feature(try_find)]
2559 /// use std::num::NonZeroU32;
2560 /// let a = [3, 5, 7, 4, 9, 0, 11];
2561 /// let result = a.iter().try_find(|&&x| NonZeroU32::new(x).map(|y| y.is_power_of_two()));
2562 /// assert_eq!(result, Some(Some(&4)));
2563 /// let result = a.iter().take(3).try_find(|&&x| NonZeroU32::new(x).map(|y| y.is_power_of_two()));
2564 /// assert_eq!(result, Some(None));
2565 /// let result = a.iter().rev().try_find(|&&x| NonZeroU32::new(x).map(|y| y.is_power_of_two()));
2566 /// assert_eq!(result, None);
2569 #[unstable(feature = "try_find", reason = "new API", issue = "63178")]
2570 fn try_find<F, R>(&mut self, f: F) -> ChangeOutputType<R, Option<Self::Item>>
2573 F: FnMut(&Self::Item) -> R,
2574 R: Try<Output = bool>,
2575 R::Residual: Residual<Option<Self::Item>>,
2579 mut f: impl FnMut(&I) -> V,
2580 ) -> impl FnMut((), I) -> ControlFlow<R::TryType>
2582 V: Try<Output = bool, Residual = R>,
2583 R: Residual<Option<I>>,
2585 move |(), x| match f(&x).branch() {
2586 ControlFlow::Continue(false) => ControlFlow::CONTINUE,
2587 ControlFlow::Continue(true) => ControlFlow::Break(Try::from_output(Some(x))),
2588 ControlFlow::Break(r) => ControlFlow::Break(FromResidual::from_residual(r)),
2592 match self.try_fold((), check(f)) {
2593 ControlFlow::Break(x) => x,
2594 ControlFlow::Continue(()) => Try::from_output(None),
2598 /// Searches for an element in an iterator, returning its index.
2600 /// `position()` takes a closure that returns `true` or `false`. It applies
2601 /// this closure to each element of the iterator, and if one of them
2602 /// returns `true`, then `position()` returns [`Some(index)`]. If all of
2603 /// them return `false`, it returns [`None`].
2605 /// `position()` is short-circuiting; in other words, it will stop
2606 /// processing as soon as it finds a `true`.
2608 /// # Overflow Behavior
2610 /// The method does no guarding against overflows, so if there are more
2611 /// than [`usize::MAX`] non-matching elements, it either produces the wrong
2612 /// result or panics. If debug assertions are enabled, a panic is
2617 /// This function might panic if the iterator has more than `usize::MAX`
2618 /// non-matching elements.
2620 /// [`Some(index)`]: Some
2627 /// let a = [1, 2, 3];
2629 /// assert_eq!(a.iter().position(|&x| x == 2), Some(1));
2631 /// assert_eq!(a.iter().position(|&x| x == 5), None);
2634 /// Stopping at the first `true`:
2637 /// let a = [1, 2, 3, 4];
2639 /// let mut iter = a.iter();
2641 /// assert_eq!(iter.position(|&x| x >= 2), Some(1));
2643 /// // we can still use `iter`, as there are more elements.
2644 /// assert_eq!(iter.next(), Some(&3));
2646 /// // The returned index depends on iterator state
2647 /// assert_eq!(iter.position(|&x| x == 4), Some(0));
2651 #[stable(feature = "rust1", since = "1.0.0")]
2652 fn position<P>(&mut self, predicate: P) -> Option<usize>
2655 P: FnMut(Self::Item) -> bool,
2659 mut predicate: impl FnMut(T) -> bool,
2660 ) -> impl FnMut(usize, T) -> ControlFlow<usize, usize> {
2661 #[rustc_inherit_overflow_checks]
2663 if predicate(x) { ControlFlow::Break(i) } else { ControlFlow::Continue(i + 1) }
2667 self.try_fold(0, check(predicate)).break_value()
2670 /// Searches for an element in an iterator from the right, returning its
2673 /// `rposition()` takes a closure that returns `true` or `false`. It applies
2674 /// this closure to each element of the iterator, starting from the end,
2675 /// and if one of them returns `true`, then `rposition()` returns
2676 /// [`Some(index)`]. If all of them return `false`, it returns [`None`].
2678 /// `rposition()` is short-circuiting; in other words, it will stop
2679 /// processing as soon as it finds a `true`.
2681 /// [`Some(index)`]: Some
2688 /// let a = [1, 2, 3];
2690 /// assert_eq!(a.iter().rposition(|&x| x == 3), Some(2));
2692 /// assert_eq!(a.iter().rposition(|&x| x == 5), None);
2695 /// Stopping at the first `true`:
2698 /// let a = [1, 2, 3];
2700 /// let mut iter = a.iter();
2702 /// assert_eq!(iter.rposition(|&x| x == 2), Some(1));
2704 /// // we can still use `iter`, as there are more elements.
2705 /// assert_eq!(iter.next(), Some(&1));
2708 #[stable(feature = "rust1", since = "1.0.0")]
2709 fn rposition<P>(&mut self, predicate: P) -> Option<usize>
2711 P: FnMut(Self::Item) -> bool,
2712 Self: Sized + ExactSizeIterator + DoubleEndedIterator,
2714 // No need for an overflow check here, because `ExactSizeIterator`
2715 // implies that the number of elements fits into a `usize`.
2718 mut predicate: impl FnMut(T) -> bool,
2719 ) -> impl FnMut(usize, T) -> ControlFlow<usize, usize> {
2722 if predicate(x) { ControlFlow::Break(i) } else { ControlFlow::Continue(i) }
2727 self.try_rfold(n, check(predicate)).break_value()
2730 /// Returns the maximum element of an iterator.
2732 /// If several elements are equally maximum, the last element is
2733 /// returned. If the iterator is empty, [`None`] is returned.
2735 /// Note that [`f32`]/[`f64`] doesn't implement [`Ord`] due to NaN being
2736 /// incomparable. You can work around this by using [`Iterator::reduce`]:
2739 /// [2.4, f32::NAN, 1.3]
2741 /// .reduce(f32::max)
2752 /// let a = [1, 2, 3];
2753 /// let b: Vec<u32> = Vec::new();
2755 /// assert_eq!(a.iter().max(), Some(&3));
2756 /// assert_eq!(b.iter().max(), None);
2759 #[stable(feature = "rust1", since = "1.0.0")]
2760 fn max(self) -> Option<Self::Item>
2765 self.max_by(Ord::cmp)
2768 /// Returns the minimum element of an iterator.
2770 /// If several elements are equally minimum, the first element is returned.
2771 /// If the iterator is empty, [`None`] is returned.
2773 /// Note that [`f32`]/[`f64`] doesn't implement [`Ord`] due to NaN being
2774 /// incomparable. You can work around this by using [`Iterator::reduce`]:
2777 /// [2.4, f32::NAN, 1.3]
2779 /// .reduce(f32::min)
2790 /// let a = [1, 2, 3];
2791 /// let b: Vec<u32> = Vec::new();
2793 /// assert_eq!(a.iter().min(), Some(&1));
2794 /// assert_eq!(b.iter().min(), None);
2797 #[stable(feature = "rust1", since = "1.0.0")]
2798 fn min(self) -> Option<Self::Item>
2803 self.min_by(Ord::cmp)
2806 /// Returns the element that gives the maximum value from the
2807 /// specified function.
2809 /// If several elements are equally maximum, the last element is
2810 /// returned. If the iterator is empty, [`None`] is returned.
2815 /// let a = [-3_i32, 0, 1, 5, -10];
2816 /// assert_eq!(*a.iter().max_by_key(|x| x.abs()).unwrap(), -10);
2819 #[stable(feature = "iter_cmp_by_key", since = "1.6.0")]
2820 fn max_by_key<B: Ord, F>(self, f: F) -> Option<Self::Item>
2823 F: FnMut(&Self::Item) -> B,
2826 fn key<T, B>(mut f: impl FnMut(&T) -> B) -> impl FnMut(T) -> (B, T) {
2831 fn compare<T, B: Ord>((x_p, _): &(B, T), (y_p, _): &(B, T)) -> Ordering {
2835 let (_, x) = self.map(key(f)).max_by(compare)?;
2839 /// Returns the element that gives the maximum value with respect to the
2840 /// specified comparison function.
2842 /// If several elements are equally maximum, the last element is
2843 /// returned. If the iterator is empty, [`None`] is returned.
2848 /// let a = [-3_i32, 0, 1, 5, -10];
2849 /// assert_eq!(*a.iter().max_by(|x, y| x.cmp(y)).unwrap(), 5);
2852 #[stable(feature = "iter_max_by", since = "1.15.0")]
2853 fn max_by<F>(self, compare: F) -> Option<Self::Item>
2856 F: FnMut(&Self::Item, &Self::Item) -> Ordering,
2859 fn fold<T>(mut compare: impl FnMut(&T, &T) -> Ordering) -> impl FnMut(T, T) -> T {
2860 move |x, y| cmp::max_by(x, y, &mut compare)
2863 self.reduce(fold(compare))
2866 /// Returns the element that gives the minimum value from the
2867 /// specified function.
2869 /// If several elements are equally minimum, the first element is
2870 /// returned. If the iterator is empty, [`None`] is returned.
2875 /// let a = [-3_i32, 0, 1, 5, -10];
2876 /// assert_eq!(*a.iter().min_by_key(|x| x.abs()).unwrap(), 0);
2879 #[stable(feature = "iter_cmp_by_key", since = "1.6.0")]
2880 fn min_by_key<B: Ord, F>(self, f: F) -> Option<Self::Item>
2883 F: FnMut(&Self::Item) -> B,
2886 fn key<T, B>(mut f: impl FnMut(&T) -> B) -> impl FnMut(T) -> (B, T) {
2891 fn compare<T, B: Ord>((x_p, _): &(B, T), (y_p, _): &(B, T)) -> Ordering {
2895 let (_, x) = self.map(key(f)).min_by(compare)?;
2899 /// Returns the element that gives the minimum value with respect to the
2900 /// specified comparison function.
2902 /// If several elements are equally minimum, the first element is
2903 /// returned. If the iterator is empty, [`None`] is returned.
2908 /// let a = [-3_i32, 0, 1, 5, -10];
2909 /// assert_eq!(*a.iter().min_by(|x, y| x.cmp(y)).unwrap(), -10);
2912 #[stable(feature = "iter_min_by", since = "1.15.0")]
2913 fn min_by<F>(self, compare: F) -> Option<Self::Item>
2916 F: FnMut(&Self::Item, &Self::Item) -> Ordering,
2919 fn fold<T>(mut compare: impl FnMut(&T, &T) -> Ordering) -> impl FnMut(T, T) -> T {
2920 move |x, y| cmp::min_by(x, y, &mut compare)
2923 self.reduce(fold(compare))
2926 /// Reverses an iterator's direction.
2928 /// Usually, iterators iterate from left to right. After using `rev()`,
2929 /// an iterator will instead iterate from right to left.
2931 /// This is only possible if the iterator has an end, so `rev()` only
2932 /// works on [`DoubleEndedIterator`]s.
2937 /// let a = [1, 2, 3];
2939 /// let mut iter = a.iter().rev();
2941 /// assert_eq!(iter.next(), Some(&3));
2942 /// assert_eq!(iter.next(), Some(&2));
2943 /// assert_eq!(iter.next(), Some(&1));
2945 /// assert_eq!(iter.next(), None);
2948 #[doc(alias = "reverse")]
2949 #[stable(feature = "rust1", since = "1.0.0")]
2950 fn rev(self) -> Rev<Self>
2952 Self: Sized + DoubleEndedIterator,
2957 /// Converts an iterator of pairs into a pair of containers.
2959 /// `unzip()` consumes an entire iterator of pairs, producing two
2960 /// collections: one from the left elements of the pairs, and one
2961 /// from the right elements.
2963 /// This function is, in some sense, the opposite of [`zip`].
2965 /// [`zip`]: Iterator::zip
2972 /// let a = [(1, 2), (3, 4), (5, 6)];
2974 /// let (left, right): (Vec<_>, Vec<_>) = a.iter().cloned().unzip();
2976 /// assert_eq!(left, [1, 3, 5]);
2977 /// assert_eq!(right, [2, 4, 6]);
2979 /// // you can also unzip multiple nested tuples at once
2980 /// let a = [(1, (2, 3)), (4, (5, 6))];
2982 /// let (x, (y, z)): (Vec<_>, (Vec<_>, Vec<_>)) = a.iter().cloned().unzip();
2983 /// assert_eq!(x, [1, 4]);
2984 /// assert_eq!(y, [2, 5]);
2985 /// assert_eq!(z, [3, 6]);
2987 #[stable(feature = "rust1", since = "1.0.0")]
2988 fn unzip<A, B, FromA, FromB>(self) -> (FromA, FromB)
2990 FromA: Default + Extend<A>,
2991 FromB: Default + Extend<B>,
2992 Self: Sized + Iterator<Item = (A, B)>,
2994 let mut unzipped: (FromA, FromB) = Default::default();
2995 unzipped.extend(self);
2999 /// Creates an iterator which copies all of its elements.
3001 /// This is useful when you have an iterator over `&T`, but you need an
3002 /// iterator over `T`.
3009 /// let a = [1, 2, 3];
3011 /// let v_copied: Vec<_> = a.iter().copied().collect();
3013 /// // copied is the same as .map(|&x| x)
3014 /// let v_map: Vec<_> = a.iter().map(|&x| x).collect();
3016 /// assert_eq!(v_copied, vec![1, 2, 3]);
3017 /// assert_eq!(v_map, vec![1, 2, 3]);
3019 #[stable(feature = "iter_copied", since = "1.36.0")]
3020 fn copied<'a, T: 'a>(self) -> Copied<Self>
3022 Self: Sized + Iterator<Item = &'a T>,
3028 /// Creates an iterator which [`clone`]s all of its elements.
3030 /// This is useful when you have an iterator over `&T`, but you need an
3031 /// iterator over `T`.
3033 /// [`clone`]: Clone::clone
3040 /// let a = [1, 2, 3];
3042 /// let v_cloned: Vec<_> = a.iter().cloned().collect();
3044 /// // cloned is the same as .map(|&x| x), for integers
3045 /// let v_map: Vec<_> = a.iter().map(|&x| x).collect();
3047 /// assert_eq!(v_cloned, vec![1, 2, 3]);
3048 /// assert_eq!(v_map, vec![1, 2, 3]);
3050 #[stable(feature = "rust1", since = "1.0.0")]
3051 fn cloned<'a, T: 'a>(self) -> Cloned<Self>
3053 Self: Sized + Iterator<Item = &'a T>,
3059 /// Repeats an iterator endlessly.
3061 /// Instead of stopping at [`None`], the iterator will instead start again,
3062 /// from the beginning. After iterating again, it will start at the
3063 /// beginning again. And again. And again. Forever. Note that in case the
3064 /// original iterator is empty, the resulting iterator will also be empty.
3071 /// let a = [1, 2, 3];
3073 /// let mut it = a.iter().cycle();
3075 /// assert_eq!(it.next(), Some(&1));
3076 /// assert_eq!(it.next(), Some(&2));
3077 /// assert_eq!(it.next(), Some(&3));
3078 /// assert_eq!(it.next(), Some(&1));
3079 /// assert_eq!(it.next(), Some(&2));
3080 /// assert_eq!(it.next(), Some(&3));
3081 /// assert_eq!(it.next(), Some(&1));
3083 #[stable(feature = "rust1", since = "1.0.0")]
3085 fn cycle(self) -> Cycle<Self>
3087 Self: Sized + Clone,
3092 /// Sums the elements of an iterator.
3094 /// Takes each element, adds them together, and returns the result.
3096 /// An empty iterator returns the zero value of the type.
3100 /// When calling `sum()` and a primitive integer type is being returned, this
3101 /// method will panic if the computation overflows and debug assertions are
3109 /// let a = [1, 2, 3];
3110 /// let sum: i32 = a.iter().sum();
3112 /// assert_eq!(sum, 6);
3114 #[stable(feature = "iter_arith", since = "1.11.0")]
3115 fn sum<S>(self) -> S
3123 /// Iterates over the entire iterator, multiplying all the elements
3125 /// An empty iterator returns the one value of the type.
3129 /// When calling `product()` and a primitive integer type is being returned,
3130 /// method will panic if the computation overflows and debug assertions are
3136 /// fn factorial(n: u32) -> u32 {
3137 /// (1..=n).product()
3139 /// assert_eq!(factorial(0), 1);
3140 /// assert_eq!(factorial(1), 1);
3141 /// assert_eq!(factorial(5), 120);
3143 #[stable(feature = "iter_arith", since = "1.11.0")]
3144 fn product<P>(self) -> P
3147 P: Product<Self::Item>,
3149 Product::product(self)
3152 /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
3158 /// use std::cmp::Ordering;
3160 /// assert_eq!([1].iter().cmp([1].iter()), Ordering::Equal);
3161 /// assert_eq!([1].iter().cmp([1, 2].iter()), Ordering::Less);
3162 /// assert_eq!([1, 2].iter().cmp([1].iter()), Ordering::Greater);
3164 #[stable(feature = "iter_order", since = "1.5.0")]
3165 fn cmp<I>(self, other: I) -> Ordering
3167 I: IntoIterator<Item = Self::Item>,
3171 self.cmp_by(other, |x, y| x.cmp(&y))
3174 /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
3175 /// of another with respect to the specified comparison function.
3182 /// #![feature(iter_order_by)]
3184 /// use std::cmp::Ordering;
3186 /// let xs = [1, 2, 3, 4];
3187 /// let ys = [1, 4, 9, 16];
3189 /// assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| x.cmp(&y)), Ordering::Less);
3190 /// assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| (x * x).cmp(&y)), Ordering::Equal);
3191 /// assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| (2 * x).cmp(&y)), Ordering::Greater);
3193 #[unstable(feature = "iter_order_by", issue = "64295")]
3194 fn cmp_by<I, F>(mut self, other: I, mut cmp: F) -> Ordering
3198 F: FnMut(Self::Item, I::Item) -> Ordering,
3200 let mut other = other.into_iter();
3203 let x = match self.next() {
3205 if other.next().is_none() {
3206 return Ordering::Equal;
3208 return Ordering::Less;
3214 let y = match other.next() {
3215 None => return Ordering::Greater,
3220 Ordering::Equal => (),
3221 non_eq => return non_eq,
3226 /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
3232 /// use std::cmp::Ordering;
3234 /// assert_eq!([1.].iter().partial_cmp([1.].iter()), Some(Ordering::Equal));
3235 /// assert_eq!([1.].iter().partial_cmp([1., 2.].iter()), Some(Ordering::Less));
3236 /// assert_eq!([1., 2.].iter().partial_cmp([1.].iter()), Some(Ordering::Greater));
3238 /// assert_eq!([f64::NAN].iter().partial_cmp([1.].iter()), None);
3240 #[stable(feature = "iter_order", since = "1.5.0")]
3241 fn partial_cmp<I>(self, other: I) -> Option<Ordering>
3244 Self::Item: PartialOrd<I::Item>,
3247 self.partial_cmp_by(other, |x, y| x.partial_cmp(&y))
3250 /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
3251 /// of another with respect to the specified comparison function.
3258 /// #![feature(iter_order_by)]
3260 /// use std::cmp::Ordering;
3262 /// let xs = [1.0, 2.0, 3.0, 4.0];
3263 /// let ys = [1.0, 4.0, 9.0, 16.0];
3266 /// xs.iter().partial_cmp_by(&ys, |&x, &y| x.partial_cmp(&y)),
3267 /// Some(Ordering::Less)
3270 /// xs.iter().partial_cmp_by(&ys, |&x, &y| (x * x).partial_cmp(&y)),
3271 /// Some(Ordering::Equal)
3274 /// xs.iter().partial_cmp_by(&ys, |&x, &y| (2.0 * x).partial_cmp(&y)),
3275 /// Some(Ordering::Greater)
3278 #[unstable(feature = "iter_order_by", issue = "64295")]
3279 fn partial_cmp_by<I, F>(mut self, other: I, mut partial_cmp: F) -> Option<Ordering>
3283 F: FnMut(Self::Item, I::Item) -> Option<Ordering>,
3285 let mut other = other.into_iter();
3288 let x = match self.next() {
3290 if other.next().is_none() {
3291 return Some(Ordering::Equal);
3293 return Some(Ordering::Less);
3299 let y = match other.next() {
3300 None => return Some(Ordering::Greater),
3304 match partial_cmp(x, y) {
3305 Some(Ordering::Equal) => (),
3306 non_eq => return non_eq,
3311 /// Determines if the elements of this [`Iterator`] are equal to those of
3317 /// assert_eq!([1].iter().eq([1].iter()), true);
3318 /// assert_eq!([1].iter().eq([1, 2].iter()), false);
3320 #[stable(feature = "iter_order", since = "1.5.0")]
3321 fn eq<I>(self, other: I) -> bool
3324 Self::Item: PartialEq<I::Item>,
3327 self.eq_by(other, |x, y| x == y)
3330 /// Determines if the elements of this [`Iterator`] are equal to those of
3331 /// another with respect to the specified equality function.
3338 /// #![feature(iter_order_by)]
3340 /// let xs = [1, 2, 3, 4];
3341 /// let ys = [1, 4, 9, 16];
3343 /// assert!(xs.iter().eq_by(&ys, |&x, &y| x * x == y));
3345 #[unstable(feature = "iter_order_by", issue = "64295")]
3346 fn eq_by<I, F>(mut self, other: I, mut eq: F) -> bool
3350 F: FnMut(Self::Item, I::Item) -> bool,
3352 let mut other = other.into_iter();
3355 let x = match self.next() {
3356 None => return other.next().is_none(),
3360 let y = match other.next() {
3361 None => return false,
3371 /// Determines if the elements of this [`Iterator`] are unequal to those of
3377 /// assert_eq!([1].iter().ne([1].iter()), false);
3378 /// assert_eq!([1].iter().ne([1, 2].iter()), true);
3380 #[stable(feature = "iter_order", since = "1.5.0")]
3381 fn ne<I>(self, other: I) -> bool
3384 Self::Item: PartialEq<I::Item>,
3390 /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
3391 /// less than those of another.
3396 /// assert_eq!([1].iter().lt([1].iter()), false);
3397 /// assert_eq!([1].iter().lt([1, 2].iter()), true);
3398 /// assert_eq!([1, 2].iter().lt([1].iter()), false);
3399 /// assert_eq!([1, 2].iter().lt([1, 2].iter()), false);
3401 #[stable(feature = "iter_order", since = "1.5.0")]
3402 fn lt<I>(self, other: I) -> bool
3405 Self::Item: PartialOrd<I::Item>,
3408 self.partial_cmp(other) == Some(Ordering::Less)
3411 /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
3412 /// less or equal to those of another.
3417 /// assert_eq!([1].iter().le([1].iter()), true);
3418 /// assert_eq!([1].iter().le([1, 2].iter()), true);
3419 /// assert_eq!([1, 2].iter().le([1].iter()), false);
3420 /// assert_eq!([1, 2].iter().le([1, 2].iter()), true);
3422 #[stable(feature = "iter_order", since = "1.5.0")]
3423 fn le<I>(self, other: I) -> bool
3426 Self::Item: PartialOrd<I::Item>,
3429 matches!(self.partial_cmp(other), Some(Ordering::Less | Ordering::Equal))
3432 /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
3433 /// greater than those of another.
3438 /// assert_eq!([1].iter().gt([1].iter()), false);
3439 /// assert_eq!([1].iter().gt([1, 2].iter()), false);
3440 /// assert_eq!([1, 2].iter().gt([1].iter()), true);
3441 /// assert_eq!([1, 2].iter().gt([1, 2].iter()), false);
3443 #[stable(feature = "iter_order", since = "1.5.0")]
3444 fn gt<I>(self, other: I) -> bool
3447 Self::Item: PartialOrd<I::Item>,
3450 self.partial_cmp(other) == Some(Ordering::Greater)
3453 /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
3454 /// greater than or equal to those of another.
3459 /// assert_eq!([1].iter().ge([1].iter()), true);
3460 /// assert_eq!([1].iter().ge([1, 2].iter()), false);
3461 /// assert_eq!([1, 2].iter().ge([1].iter()), true);
3462 /// assert_eq!([1, 2].iter().ge([1, 2].iter()), true);
3464 #[stable(feature = "iter_order", since = "1.5.0")]
3465 fn ge<I>(self, other: I) -> bool
3468 Self::Item: PartialOrd<I::Item>,
3471 matches!(self.partial_cmp(other), Some(Ordering::Greater | Ordering::Equal))
3474 /// Checks if the elements of this iterator are sorted.
3476 /// That is, for each element `a` and its following element `b`, `a <= b` must hold. If the
3477 /// iterator yields exactly zero or one element, `true` is returned.
3479 /// Note that if `Self::Item` is only `PartialOrd`, but not `Ord`, the above definition
3480 /// implies that this function returns `false` if any two consecutive items are not
3486 /// #![feature(is_sorted)]
3488 /// assert!([1, 2, 2, 9].iter().is_sorted());
3489 /// assert!(![1, 3, 2, 4].iter().is_sorted());
3490 /// assert!([0].iter().is_sorted());
3491 /// assert!(std::iter::empty::<i32>().is_sorted());
3492 /// assert!(![0.0, 1.0, f32::NAN].iter().is_sorted());
3495 #[unstable(feature = "is_sorted", reason = "new API", issue = "53485")]
3496 fn is_sorted(self) -> bool
3499 Self::Item: PartialOrd,
3501 self.is_sorted_by(PartialOrd::partial_cmp)
3504 /// Checks if the elements of this iterator are sorted using the given comparator function.
3506 /// Instead of using `PartialOrd::partial_cmp`, this function uses the given `compare`
3507 /// function to determine the ordering of two elements. Apart from that, it's equivalent to
3508 /// [`is_sorted`]; see its documentation for more information.
3513 /// #![feature(is_sorted)]
3515 /// assert!([1, 2, 2, 9].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
3516 /// assert!(![1, 3, 2, 4].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
3517 /// assert!([0].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
3518 /// assert!(std::iter::empty::<i32>().is_sorted_by(|a, b| a.partial_cmp(b)));
3519 /// assert!(![0.0, 1.0, f32::NAN].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
3522 /// [`is_sorted`]: Iterator::is_sorted
3523 #[unstable(feature = "is_sorted", reason = "new API", issue = "53485")]
3524 fn is_sorted_by<F>(mut self, compare: F) -> bool
3527 F: FnMut(&Self::Item, &Self::Item) -> Option<Ordering>,
3532 mut compare: impl FnMut(&T, &T) -> Option<Ordering> + 'a,
3533 ) -> impl FnMut(T) -> bool + 'a {
3535 if let Some(Ordering::Greater) | None = compare(&last, &curr) {
3543 let mut last = match self.next() {
3545 None => return true,
3548 self.all(check(&mut last, compare))
3551 /// Checks if the elements of this iterator are sorted using the given key extraction
3554 /// Instead of comparing the iterator's elements directly, this function compares the keys of
3555 /// the elements, as determined by `f`. Apart from that, it's equivalent to [`is_sorted`]; see
3556 /// its documentation for more information.
3558 /// [`is_sorted`]: Iterator::is_sorted
3563 /// #![feature(is_sorted)]
3565 /// assert!(["c", "bb", "aaa"].iter().is_sorted_by_key(|s| s.len()));
3566 /// assert!(![-2i32, -1, 0, 3].iter().is_sorted_by_key(|n| n.abs()));
3569 #[unstable(feature = "is_sorted", reason = "new API", issue = "53485")]
3570 fn is_sorted_by_key<F, K>(self, f: F) -> bool
3573 F: FnMut(Self::Item) -> K,
3576 self.map(f).is_sorted()
3579 /// See [TrustedRandomAccess][super::super::TrustedRandomAccess]
3580 // The unusual name is to avoid name collisions in method resolution
3584 #[unstable(feature = "trusted_random_access", issue = "none")]
3585 unsafe fn __iterator_get_unchecked(&mut self, _idx: usize) -> Self::Item
3587 Self: TrustedRandomAccessNoCoerce,
3589 unreachable!("Always specialized");
3593 #[stable(feature = "rust1", since = "1.0.0")]
3594 impl<I: Iterator + ?Sized> Iterator for &mut I {
3595 type Item = I::Item;
3597 fn next(&mut self) -> Option<I::Item> {
3600 fn size_hint(&self) -> (usize, Option<usize>) {
3601 (**self).size_hint()
3603 fn advance_by(&mut self, n: usize) -> Result<(), usize> {
3604 (**self).advance_by(n)
3606 fn nth(&mut self, n: usize) -> Option<Self::Item> {