1 use crate::cmp::{self, Ordering};
2 use crate::ops::{ChangeOutputType, ControlFlow, FromResidual, Residual, Try};
4 use super::super::try_process;
5 use super::super::TrustedRandomAccessNoCoerce;
6 use super::super::{Chain, Cloned, Copied, Cycle, Enumerate, Filter, FilterMap, Fuse};
7 use super::super::{FlatMap, Flatten};
8 use super::super::{FromIterator, Intersperse, IntersperseWith, Product, Sum, Zip};
10 Inspect, Map, MapWhile, Peekable, Rev, Scan, Skip, SkipWhile, StepBy, Take, TakeWhile,
13 fn _assert_is_object_safe(_: &dyn Iterator<Item = ()>) {}
15 /// An interface for dealing with iterators.
17 /// This is the main iterator trait. For more about the concept of iterators
18 /// generally, please see the [module-level documentation]. In particular, you
19 /// may want to know how to [implement `Iterator`][impl].
21 /// [module-level documentation]: crate::iter
22 /// [impl]: crate::iter#implementing-iterator
23 #[stable(feature = "rust1", since = "1.0.0")]
24 #[rustc_on_unimplemented(
26 _Self = "std::ops::RangeTo<Idx>",
27 label = "if you meant to iterate until a value, add a starting value",
28 note = "`..end` is a `RangeTo`, which cannot be iterated on; you might have meant to have a \
29 bounded `Range`: `0..end`"
32 _Self = "std::ops::RangeToInclusive<Idx>",
33 label = "if you meant to iterate until a value (including it), add a starting value",
34 note = "`..=end` is a `RangeToInclusive`, which cannot be iterated on; you might have meant \
35 to have a bounded `RangeInclusive`: `0..=end`"
39 label = "`{Self}` is not an iterator; try calling `.chars()` or `.bytes()`"
42 _Self = "std::string::String",
43 label = "`{Self}` is not an iterator; try calling `.chars()` or `.bytes()`"
47 note = "if you want to iterate between `start` until a value `end`, use the exclusive range \
48 syntax `start..end` or the inclusive range syntax `start..=end`"
50 label = "`{Self}` is not an iterator",
51 message = "`{Self}` is not an iterator"
54 #[rustc_diagnostic_item = "Iterator"]
55 #[must_use = "iterators are lazy and do nothing unless consumed"]
57 /// The type of the elements being iterated over.
58 #[stable(feature = "rust1", since = "1.0.0")]
61 /// Advances the iterator and returns the next value.
63 /// Returns [`None`] when iteration is finished. Individual iterator
64 /// implementations may choose to resume iteration, and so calling `next()`
65 /// again may or may not eventually start returning [`Some(Item)`] again at some
68 /// [`Some(Item)`]: Some
75 /// let a = [1, 2, 3];
77 /// let mut iter = a.iter();
79 /// // A call to next() returns the next value...
80 /// assert_eq!(Some(&1), iter.next());
81 /// assert_eq!(Some(&2), iter.next());
82 /// assert_eq!(Some(&3), iter.next());
84 /// // ... and then None once it's over.
85 /// assert_eq!(None, iter.next());
87 /// // More calls may or may not return `None`. Here, they always will.
88 /// assert_eq!(None, iter.next());
89 /// assert_eq!(None, iter.next());
92 #[stable(feature = "rust1", since = "1.0.0")]
93 fn next(&mut self) -> Option<Self::Item>;
95 /// Returns the bounds on the remaining length of the iterator.
97 /// Specifically, `size_hint()` returns a tuple where the first element
98 /// is the lower bound, and the second element is the upper bound.
100 /// The second half of the tuple that is returned is an <code>[Option]<[usize]></code>.
101 /// A [`None`] here means that either there is no known upper bound, or the
102 /// upper bound is larger than [`usize`].
104 /// # Implementation notes
106 /// It is not enforced that an iterator implementation yields the declared
107 /// number of elements. A buggy iterator may yield less than the lower bound
108 /// or more than the upper bound of elements.
110 /// `size_hint()` is primarily intended to be used for optimizations such as
111 /// reserving space for the elements of the iterator, but must not be
112 /// trusted to e.g., omit bounds checks in unsafe code. An incorrect
113 /// implementation of `size_hint()` should not lead to memory safety
116 /// That said, the implementation should provide a correct estimation,
117 /// because otherwise it would be a violation of the trait's protocol.
119 /// The default implementation returns <code>(0, [None])</code> which is correct for any
127 /// let a = [1, 2, 3];
128 /// let iter = a.iter();
130 /// assert_eq!((3, Some(3)), iter.size_hint());
133 /// A more complex example:
136 /// // The even numbers in the range of zero to nine.
137 /// let iter = (0..10).filter(|x| x % 2 == 0);
139 /// // We might iterate from zero to ten times. Knowing that it's five
140 /// // exactly wouldn't be possible without executing filter().
141 /// assert_eq!((0, Some(10)), iter.size_hint());
143 /// // Let's add five more numbers with chain()
144 /// let iter = (0..10).filter(|x| x % 2 == 0).chain(15..20);
146 /// // now both bounds are increased by five
147 /// assert_eq!((5, Some(15)), iter.size_hint());
150 /// Returning `None` for an upper bound:
153 /// // an infinite iterator has no upper bound
154 /// // and the maximum possible lower bound
157 /// assert_eq!((usize::MAX, None), iter.size_hint());
160 #[stable(feature = "rust1", since = "1.0.0")]
161 fn size_hint(&self) -> (usize, Option<usize>) {
165 /// Consumes the iterator, counting the number of iterations and returning it.
167 /// This method will call [`next`] repeatedly until [`None`] is encountered,
168 /// returning the number of times it saw [`Some`]. Note that [`next`] has to be
169 /// called at least once even if the iterator does not have any elements.
171 /// [`next`]: Iterator::next
173 /// # Overflow Behavior
175 /// The method does no guarding against overflows, so counting elements of
176 /// an iterator with more than [`usize::MAX`] elements either produces the
177 /// wrong result or panics. If debug assertions are enabled, a panic is
182 /// This function might panic if the iterator has more than [`usize::MAX`]
190 /// let a = [1, 2, 3];
191 /// assert_eq!(a.iter().count(), 3);
193 /// let a = [1, 2, 3, 4, 5];
194 /// assert_eq!(a.iter().count(), 5);
197 #[stable(feature = "rust1", since = "1.0.0")]
198 fn count(self) -> usize
204 #[rustc_inherit_overflow_checks]
205 |count, _| count + 1,
209 /// Consumes the iterator, returning the last element.
211 /// This method will evaluate the iterator until it returns [`None`]. While
212 /// doing so, it keeps track of the current element. After [`None`] is
213 /// returned, `last()` will then return the last element it saw.
220 /// let a = [1, 2, 3];
221 /// assert_eq!(a.iter().last(), Some(&3));
223 /// let a = [1, 2, 3, 4, 5];
224 /// assert_eq!(a.iter().last(), Some(&5));
227 #[stable(feature = "rust1", since = "1.0.0")]
228 fn last(self) -> Option<Self::Item>
233 fn some<T>(_: Option<T>, x: T) -> Option<T> {
237 self.fold(None, some)
240 /// Advances the iterator by `n` elements.
242 /// This method will eagerly skip `n` elements by calling [`next`] up to `n`
243 /// times until [`None`] is encountered.
245 /// `advance_by(n)` will return [`Ok(())`][Ok] if the iterator successfully advances by
246 /// `n` elements, or [`Err(k)`][Err] if [`None`] is encountered, where `k` is the number
247 /// of elements the iterator is advanced by before running out of elements (i.e. the
248 /// length of the iterator). Note that `k` is always less than `n`.
250 /// Calling `advance_by(0)` can do meaningful work, for example [`Flatten`]
251 /// can advance its outer iterator until it finds an inner iterator that is not empty, which
252 /// then often allows it to return a more accurate `size_hint()` than in its initial state.
254 /// [`Flatten`]: crate::iter::Flatten
255 /// [`next`]: Iterator::next
262 /// #![feature(iter_advance_by)]
264 /// let a = [1, 2, 3, 4];
265 /// let mut iter = a.iter();
267 /// assert_eq!(iter.advance_by(2), Ok(()));
268 /// assert_eq!(iter.next(), Some(&3));
269 /// assert_eq!(iter.advance_by(0), Ok(()));
270 /// assert_eq!(iter.advance_by(100), Err(1)); // only `&4` was skipped
273 #[unstable(feature = "iter_advance_by", reason = "recently added", issue = "77404")]
274 fn advance_by(&mut self, n: usize) -> Result<(), usize> {
276 self.next().ok_or(i)?;
281 /// Returns the `n`th element of the iterator.
283 /// Like most indexing operations, the count starts from zero, so `nth(0)`
284 /// returns the first value, `nth(1)` the second, and so on.
286 /// Note that all preceding elements, as well as the returned element, will be
287 /// consumed from the iterator. That means that the preceding elements will be
288 /// discarded, and also that calling `nth(0)` multiple times on the same iterator
289 /// will return different elements.
291 /// `nth()` will return [`None`] if `n` is greater than or equal to the length of the
299 /// let a = [1, 2, 3];
300 /// assert_eq!(a.iter().nth(1), Some(&2));
303 /// Calling `nth()` multiple times doesn't rewind the iterator:
306 /// let a = [1, 2, 3];
308 /// let mut iter = a.iter();
310 /// assert_eq!(iter.nth(1), Some(&2));
311 /// assert_eq!(iter.nth(1), None);
314 /// Returning `None` if there are less than `n + 1` elements:
317 /// let a = [1, 2, 3];
318 /// assert_eq!(a.iter().nth(10), None);
321 #[stable(feature = "rust1", since = "1.0.0")]
322 fn nth(&mut self, n: usize) -> Option<Self::Item> {
323 self.advance_by(n).ok()?;
327 /// Creates an iterator starting at the same point, but stepping by
328 /// the given amount at each iteration.
330 /// Note 1: The first element of the iterator will always be returned,
331 /// regardless of the step given.
333 /// Note 2: The time at which ignored elements are pulled is not fixed.
334 /// `StepBy` behaves like the sequence `self.next()`, `self.nth(step-1)`,
335 /// `self.nth(step-1)`, …, but is also free to behave like the sequence
336 /// `advance_n_and_return_first(&mut self, step)`,
337 /// `advance_n_and_return_first(&mut self, step)`, …
338 /// Which way is used may change for some iterators for performance reasons.
339 /// The second way will advance the iterator earlier and may consume more items.
341 /// `advance_n_and_return_first` is the equivalent of:
343 /// fn advance_n_and_return_first<I>(iter: &mut I, n: usize) -> Option<I::Item>
347 /// let next = iter.next();
357 /// The method will panic if the given step is `0`.
364 /// let a = [0, 1, 2, 3, 4, 5];
365 /// let mut iter = a.iter().step_by(2);
367 /// assert_eq!(iter.next(), Some(&0));
368 /// assert_eq!(iter.next(), Some(&2));
369 /// assert_eq!(iter.next(), Some(&4));
370 /// assert_eq!(iter.next(), None);
373 #[stable(feature = "iterator_step_by", since = "1.28.0")]
374 fn step_by(self, step: usize) -> StepBy<Self>
378 StepBy::new(self, step)
381 /// Takes two iterators and creates a new iterator over both in sequence.
383 /// `chain()` will return a new iterator which will first iterate over
384 /// values from the first iterator and then over values from the second
387 /// In other words, it links two iterators together, in a chain. 🔗
389 /// [`once`] is commonly used to adapt a single value into a chain of
390 /// other kinds of iteration.
397 /// let a1 = [1, 2, 3];
398 /// let a2 = [4, 5, 6];
400 /// let mut iter = a1.iter().chain(a2.iter());
402 /// assert_eq!(iter.next(), Some(&1));
403 /// assert_eq!(iter.next(), Some(&2));
404 /// assert_eq!(iter.next(), Some(&3));
405 /// assert_eq!(iter.next(), Some(&4));
406 /// assert_eq!(iter.next(), Some(&5));
407 /// assert_eq!(iter.next(), Some(&6));
408 /// assert_eq!(iter.next(), None);
411 /// Since the argument to `chain()` uses [`IntoIterator`], we can pass
412 /// anything that can be converted into an [`Iterator`], not just an
413 /// [`Iterator`] itself. For example, slices (`&[T]`) implement
414 /// [`IntoIterator`], and so can be passed to `chain()` directly:
417 /// let s1 = &[1, 2, 3];
418 /// let s2 = &[4, 5, 6];
420 /// let mut iter = s1.iter().chain(s2);
422 /// assert_eq!(iter.next(), Some(&1));
423 /// assert_eq!(iter.next(), Some(&2));
424 /// assert_eq!(iter.next(), Some(&3));
425 /// assert_eq!(iter.next(), Some(&4));
426 /// assert_eq!(iter.next(), Some(&5));
427 /// assert_eq!(iter.next(), Some(&6));
428 /// assert_eq!(iter.next(), None);
431 /// If you work with Windows API, you may wish to convert [`OsStr`] to `Vec<u16>`:
435 /// fn os_str_to_utf16(s: &std::ffi::OsStr) -> Vec<u16> {
436 /// use std::os::windows::ffi::OsStrExt;
437 /// s.encode_wide().chain(std::iter::once(0)).collect()
441 /// [`once`]: crate::iter::once
442 /// [`OsStr`]: ../../std/ffi/struct.OsStr.html
444 #[stable(feature = "rust1", since = "1.0.0")]
445 fn chain<U>(self, other: U) -> Chain<Self, U::IntoIter>
448 U: IntoIterator<Item = Self::Item>,
450 Chain::new(self, other.into_iter())
453 /// 'Zips up' two iterators into a single iterator of pairs.
455 /// `zip()` returns a new iterator that will iterate over two other
456 /// iterators, returning a tuple where the first element comes from the
457 /// first iterator, and the second element comes from the second iterator.
459 /// In other words, it zips two iterators together, into a single one.
461 /// If either iterator returns [`None`], [`next`] from the zipped iterator
462 /// will return [`None`].
463 /// If the zipped iterator has no more elements to return then each further attempt to advance
464 /// it will first try to advance the first iterator at most one time and if it still yielded an item
465 /// try to advance the second iterator at most one time.
472 /// let a1 = [1, 2, 3];
473 /// let a2 = [4, 5, 6];
475 /// let mut iter = a1.iter().zip(a2.iter());
477 /// assert_eq!(iter.next(), Some((&1, &4)));
478 /// assert_eq!(iter.next(), Some((&2, &5)));
479 /// assert_eq!(iter.next(), Some((&3, &6)));
480 /// assert_eq!(iter.next(), None);
483 /// Since the argument to `zip()` uses [`IntoIterator`], we can pass
484 /// anything that can be converted into an [`Iterator`], not just an
485 /// [`Iterator`] itself. For example, slices (`&[T]`) implement
486 /// [`IntoIterator`], and so can be passed to `zip()` directly:
489 /// let s1 = &[1, 2, 3];
490 /// let s2 = &[4, 5, 6];
492 /// let mut iter = s1.iter().zip(s2);
494 /// assert_eq!(iter.next(), Some((&1, &4)));
495 /// assert_eq!(iter.next(), Some((&2, &5)));
496 /// assert_eq!(iter.next(), Some((&3, &6)));
497 /// assert_eq!(iter.next(), None);
500 /// `zip()` is often used to zip an infinite iterator to a finite one.
501 /// This works because the finite iterator will eventually return [`None`],
502 /// ending the zipper. Zipping with `(0..)` can look a lot like [`enumerate`]:
505 /// let enumerate: Vec<_> = "foo".chars().enumerate().collect();
507 /// let zipper: Vec<_> = (0..).zip("foo".chars()).collect();
509 /// assert_eq!((0, 'f'), enumerate[0]);
510 /// assert_eq!((0, 'f'), zipper[0]);
512 /// assert_eq!((1, 'o'), enumerate[1]);
513 /// assert_eq!((1, 'o'), zipper[1]);
515 /// assert_eq!((2, 'o'), enumerate[2]);
516 /// assert_eq!((2, 'o'), zipper[2]);
519 /// If both iterators have roughly equivalent syntax, it may be more readable to use [`zip`]:
522 /// use std::iter::zip;
524 /// let a = [1, 2, 3];
525 /// let b = [2, 3, 4];
527 /// let mut zipped = zip(
528 /// a.into_iter().map(|x| x * 2).skip(1),
529 /// b.into_iter().map(|x| x * 2).skip(1),
532 /// assert_eq!(zipped.next(), Some((4, 6)));
533 /// assert_eq!(zipped.next(), Some((6, 8)));
534 /// assert_eq!(zipped.next(), None);
540 /// # let a = [1, 2, 3];
541 /// # let b = [2, 3, 4];
543 /// let mut zipped = a
547 /// .zip(b.into_iter().map(|x| x * 2).skip(1));
549 /// # assert_eq!(zipped.next(), Some((4, 6)));
550 /// # assert_eq!(zipped.next(), Some((6, 8)));
551 /// # assert_eq!(zipped.next(), None);
554 /// [`enumerate`]: Iterator::enumerate
555 /// [`next`]: Iterator::next
556 /// [`zip`]: crate::iter::zip
558 #[stable(feature = "rust1", since = "1.0.0")]
559 fn zip<U>(self, other: U) -> Zip<Self, U::IntoIter>
564 Zip::new(self, other.into_iter())
567 /// Creates a new iterator which places a copy of `separator` between adjacent
568 /// items of the original iterator.
570 /// In case `separator` does not implement [`Clone`] or needs to be
571 /// computed every time, use [`intersperse_with`].
578 /// #![feature(iter_intersperse)]
580 /// let mut a = [0, 1, 2].iter().intersperse(&100);
581 /// assert_eq!(a.next(), Some(&0)); // The first element from `a`.
582 /// assert_eq!(a.next(), Some(&100)); // The separator.
583 /// assert_eq!(a.next(), Some(&1)); // The next element from `a`.
584 /// assert_eq!(a.next(), Some(&100)); // The separator.
585 /// assert_eq!(a.next(), Some(&2)); // The last element from `a`.
586 /// assert_eq!(a.next(), None); // The iterator is finished.
589 /// `intersperse` can be very useful to join an iterator's items using a common element:
591 /// #![feature(iter_intersperse)]
593 /// let hello = ["Hello", "World", "!"].iter().copied().intersperse(" ").collect::<String>();
594 /// assert_eq!(hello, "Hello World !");
597 /// [`Clone`]: crate::clone::Clone
598 /// [`intersperse_with`]: Iterator::intersperse_with
600 #[unstable(feature = "iter_intersperse", reason = "recently added", issue = "79524")]
601 fn intersperse(self, separator: Self::Item) -> Intersperse<Self>
606 Intersperse::new(self, separator)
609 /// Creates a new iterator which places an item generated by `separator`
610 /// between adjacent items of the original iterator.
612 /// The closure will be called exactly once each time an item is placed
613 /// between two adjacent items from the underlying iterator; specifically,
614 /// the closure is not called if the underlying iterator yields less than
615 /// two items and after the last item is yielded.
617 /// If the iterator's item implements [`Clone`], it may be easier to use
625 /// #![feature(iter_intersperse)]
627 /// #[derive(PartialEq, Debug)]
628 /// struct NotClone(usize);
630 /// let v = [NotClone(0), NotClone(1), NotClone(2)];
631 /// let mut it = v.into_iter().intersperse_with(|| NotClone(99));
633 /// assert_eq!(it.next(), Some(NotClone(0))); // The first element from `v`.
634 /// assert_eq!(it.next(), Some(NotClone(99))); // The separator.
635 /// assert_eq!(it.next(), Some(NotClone(1))); // The next element from `v`.
636 /// assert_eq!(it.next(), Some(NotClone(99))); // The separator.
637 /// assert_eq!(it.next(), Some(NotClone(2))); // The last element from from `v`.
638 /// assert_eq!(it.next(), None); // The iterator is finished.
641 /// `intersperse_with` can be used in situations where the separator needs
644 /// #![feature(iter_intersperse)]
646 /// let src = ["Hello", "to", "all", "people", "!!"].iter().copied();
648 /// // The closure mutably borrows its context to generate an item.
649 /// let mut happy_emojis = [" ❤️ ", " 😀 "].iter().copied();
650 /// let separator = || happy_emojis.next().unwrap_or(" 🦀 ");
652 /// let result = src.intersperse_with(separator).collect::<String>();
653 /// assert_eq!(result, "Hello ❤️ to 😀 all 🦀 people 🦀 !!");
655 /// [`Clone`]: crate::clone::Clone
656 /// [`intersperse`]: Iterator::intersperse
658 #[unstable(feature = "iter_intersperse", reason = "recently added", issue = "79524")]
659 fn intersperse_with<G>(self, separator: G) -> IntersperseWith<Self, G>
662 G: FnMut() -> Self::Item,
664 IntersperseWith::new(self, separator)
667 /// Takes a closure and creates an iterator which calls that closure on each
670 /// `map()` transforms one iterator into another, by means of its argument:
671 /// something that implements [`FnMut`]. It produces a new iterator which
672 /// calls this closure on each element of the original iterator.
674 /// If you are good at thinking in types, you can think of `map()` like this:
675 /// If you have an iterator that gives you elements of some type `A`, and
676 /// you want an iterator of some other type `B`, you can use `map()`,
677 /// passing a closure that takes an `A` and returns a `B`.
679 /// `map()` is conceptually similar to a [`for`] loop. However, as `map()` is
680 /// lazy, it is best used when you're already working with other iterators.
681 /// If you're doing some sort of looping for a side effect, it's considered
682 /// more idiomatic to use [`for`] than `map()`.
684 /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
685 /// [`FnMut`]: crate::ops::FnMut
692 /// let a = [1, 2, 3];
694 /// let mut iter = a.iter().map(|x| 2 * x);
696 /// assert_eq!(iter.next(), Some(2));
697 /// assert_eq!(iter.next(), Some(4));
698 /// assert_eq!(iter.next(), Some(6));
699 /// assert_eq!(iter.next(), None);
702 /// If you're doing some sort of side effect, prefer [`for`] to `map()`:
705 /// # #![allow(unused_must_use)]
706 /// // don't do this:
707 /// (0..5).map(|x| println!("{}", x));
709 /// // it won't even execute, as it is lazy. Rust will warn you about this.
711 /// // Instead, use for:
713 /// println!("{}", x);
717 #[stable(feature = "rust1", since = "1.0.0")]
718 fn map<B, F>(self, f: F) -> Map<Self, F>
721 F: FnMut(Self::Item) -> B,
726 /// Calls a closure on each element of an iterator.
728 /// This is equivalent to using a [`for`] loop on the iterator, although
729 /// `break` and `continue` are not possible from a closure. It's generally
730 /// more idiomatic to use a `for` loop, but `for_each` may be more legible
731 /// when processing items at the end of longer iterator chains. In some
732 /// cases `for_each` may also be faster than a loop, because it will use
733 /// internal iteration on adapters like `Chain`.
735 /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
742 /// use std::sync::mpsc::channel;
744 /// let (tx, rx) = channel();
745 /// (0..5).map(|x| x * 2 + 1)
746 /// .for_each(move |x| tx.send(x).unwrap());
748 /// let v: Vec<_> = rx.iter().collect();
749 /// assert_eq!(v, vec![1, 3, 5, 7, 9]);
752 /// For such a small example, a `for` loop may be cleaner, but `for_each`
753 /// might be preferable to keep a functional style with longer iterators:
756 /// (0..5).flat_map(|x| x * 100 .. x * 110)
758 /// .filter(|&(i, x)| (i + x) % 3 == 0)
759 /// .for_each(|(i, x)| println!("{}:{}", i, x));
762 #[stable(feature = "iterator_for_each", since = "1.21.0")]
763 fn for_each<F>(self, f: F)
766 F: FnMut(Self::Item),
769 fn call<T>(mut f: impl FnMut(T)) -> impl FnMut((), T) {
770 move |(), item| f(item)
773 self.fold((), call(f));
776 /// Creates an iterator which uses a closure to determine if an element
777 /// should be yielded.
779 /// Given an element the closure must return `true` or `false`. The returned
780 /// iterator will yield only the elements for which the closure returns
788 /// let a = [0i32, 1, 2];
790 /// let mut iter = a.iter().filter(|x| x.is_positive());
792 /// assert_eq!(iter.next(), Some(&1));
793 /// assert_eq!(iter.next(), Some(&2));
794 /// assert_eq!(iter.next(), None);
797 /// Because the closure passed to `filter()` takes a reference, and many
798 /// iterators iterate over references, this leads to a possibly confusing
799 /// situation, where the type of the closure is a double reference:
802 /// let a = [0, 1, 2];
804 /// let mut iter = a.iter().filter(|x| **x > 1); // need two *s!
806 /// assert_eq!(iter.next(), Some(&2));
807 /// assert_eq!(iter.next(), None);
810 /// It's common to instead use destructuring on the argument to strip away
814 /// let a = [0, 1, 2];
816 /// let mut iter = a.iter().filter(|&x| *x > 1); // both & and *
818 /// assert_eq!(iter.next(), Some(&2));
819 /// assert_eq!(iter.next(), None);
825 /// let a = [0, 1, 2];
827 /// let mut iter = a.iter().filter(|&&x| x > 1); // two &s
829 /// assert_eq!(iter.next(), Some(&2));
830 /// assert_eq!(iter.next(), None);
835 /// Note that `iter.filter(f).next()` is equivalent to `iter.find(f)`.
837 #[stable(feature = "rust1", since = "1.0.0")]
838 fn filter<P>(self, predicate: P) -> Filter<Self, P>
841 P: FnMut(&Self::Item) -> bool,
843 Filter::new(self, predicate)
846 /// Creates an iterator that both filters and maps.
848 /// The returned iterator yields only the `value`s for which the supplied
849 /// closure returns `Some(value)`.
851 /// `filter_map` can be used to make chains of [`filter`] and [`map`] more
852 /// concise. The example below shows how a `map().filter().map()` can be
853 /// shortened to a single call to `filter_map`.
855 /// [`filter`]: Iterator::filter
856 /// [`map`]: Iterator::map
863 /// let a = ["1", "two", "NaN", "four", "5"];
865 /// let mut iter = a.iter().filter_map(|s| s.parse().ok());
867 /// assert_eq!(iter.next(), Some(1));
868 /// assert_eq!(iter.next(), Some(5));
869 /// assert_eq!(iter.next(), None);
872 /// Here's the same example, but with [`filter`] and [`map`]:
875 /// let a = ["1", "two", "NaN", "four", "5"];
876 /// let mut iter = a.iter().map(|s| s.parse()).filter(|s| s.is_ok()).map(|s| s.unwrap());
877 /// assert_eq!(iter.next(), Some(1));
878 /// assert_eq!(iter.next(), Some(5));
879 /// assert_eq!(iter.next(), None);
882 #[stable(feature = "rust1", since = "1.0.0")]
883 fn filter_map<B, F>(self, f: F) -> FilterMap<Self, F>
886 F: FnMut(Self::Item) -> Option<B>,
888 FilterMap::new(self, f)
891 /// Creates an iterator which gives the current iteration count as well as
894 /// The iterator returned yields pairs `(i, val)`, where `i` is the
895 /// current index of iteration and `val` is the value returned by the
898 /// `enumerate()` keeps its count as a [`usize`]. If you want to count by a
899 /// different sized integer, the [`zip`] function provides similar
902 /// # Overflow Behavior
904 /// The method does no guarding against overflows, so enumerating more than
905 /// [`usize::MAX`] elements either produces the wrong result or panics. If
906 /// debug assertions are enabled, a panic is guaranteed.
910 /// The returned iterator might panic if the to-be-returned index would
911 /// overflow a [`usize`].
913 /// [`zip`]: Iterator::zip
918 /// let a = ['a', 'b', 'c'];
920 /// let mut iter = a.iter().enumerate();
922 /// assert_eq!(iter.next(), Some((0, &'a')));
923 /// assert_eq!(iter.next(), Some((1, &'b')));
924 /// assert_eq!(iter.next(), Some((2, &'c')));
925 /// assert_eq!(iter.next(), None);
928 #[stable(feature = "rust1", since = "1.0.0")]
929 fn enumerate(self) -> Enumerate<Self>
936 /// Creates an iterator which can use the [`peek`] and [`peek_mut`] methods
937 /// to look at the next element of the iterator without consuming it. See
938 /// their documentation for more information.
940 /// Note that the underlying iterator is still advanced when [`peek`] or
941 /// [`peek_mut`] are called for the first time: In order to retrieve the
942 /// next element, [`next`] is called on the underlying iterator, hence any
943 /// side effects (i.e. anything other than fetching the next value) of
944 /// the [`next`] method will occur.
952 /// let xs = [1, 2, 3];
954 /// let mut iter = xs.iter().peekable();
956 /// // peek() lets us see into the future
957 /// assert_eq!(iter.peek(), Some(&&1));
958 /// assert_eq!(iter.next(), Some(&1));
960 /// assert_eq!(iter.next(), Some(&2));
962 /// // we can peek() multiple times, the iterator won't advance
963 /// assert_eq!(iter.peek(), Some(&&3));
964 /// assert_eq!(iter.peek(), Some(&&3));
966 /// assert_eq!(iter.next(), Some(&3));
968 /// // after the iterator is finished, so is peek()
969 /// assert_eq!(iter.peek(), None);
970 /// assert_eq!(iter.next(), None);
973 /// Using [`peek_mut`] to mutate the next item without advancing the
977 /// let xs = [1, 2, 3];
979 /// let mut iter = xs.iter().peekable();
981 /// // `peek_mut()` lets us see into the future
982 /// assert_eq!(iter.peek_mut(), Some(&mut &1));
983 /// assert_eq!(iter.peek_mut(), Some(&mut &1));
984 /// assert_eq!(iter.next(), Some(&1));
986 /// if let Some(mut p) = iter.peek_mut() {
987 /// assert_eq!(*p, &2);
988 /// // put a value into the iterator
992 /// // The value reappears as the iterator continues
993 /// assert_eq!(iter.collect::<Vec<_>>(), vec![&1000, &3]);
995 /// [`peek`]: Peekable::peek
996 /// [`peek_mut`]: Peekable::peek_mut
997 /// [`next`]: Iterator::next
999 #[stable(feature = "rust1", since = "1.0.0")]
1000 fn peekable(self) -> Peekable<Self>
1007 /// Creates an iterator that [`skip`]s elements based on a predicate.
1009 /// [`skip`]: Iterator::skip
1011 /// `skip_while()` takes a closure as an argument. It will call this
1012 /// closure on each element of the iterator, and ignore elements
1013 /// until it returns `false`.
1015 /// After `false` is returned, `skip_while()`'s job is over, and the
1016 /// rest of the elements are yielded.
1023 /// let a = [-1i32, 0, 1];
1025 /// let mut iter = a.iter().skip_while(|x| x.is_negative());
1027 /// assert_eq!(iter.next(), Some(&0));
1028 /// assert_eq!(iter.next(), Some(&1));
1029 /// assert_eq!(iter.next(), None);
1032 /// Because the closure passed to `skip_while()` takes a reference, and many
1033 /// iterators iterate over references, this leads to a possibly confusing
1034 /// situation, where the type of the closure argument is a double reference:
1037 /// let a = [-1, 0, 1];
1039 /// let mut iter = a.iter().skip_while(|x| **x < 0); // need two *s!
1041 /// assert_eq!(iter.next(), Some(&0));
1042 /// assert_eq!(iter.next(), Some(&1));
1043 /// assert_eq!(iter.next(), None);
1046 /// Stopping after an initial `false`:
1049 /// let a = [-1, 0, 1, -2];
1051 /// let mut iter = a.iter().skip_while(|x| **x < 0);
1053 /// assert_eq!(iter.next(), Some(&0));
1054 /// assert_eq!(iter.next(), Some(&1));
1056 /// // while this would have been false, since we already got a false,
1057 /// // skip_while() isn't used any more
1058 /// assert_eq!(iter.next(), Some(&-2));
1060 /// assert_eq!(iter.next(), None);
1063 #[doc(alias = "drop_while")]
1064 #[stable(feature = "rust1", since = "1.0.0")]
1065 fn skip_while<P>(self, predicate: P) -> SkipWhile<Self, P>
1068 P: FnMut(&Self::Item) -> bool,
1070 SkipWhile::new(self, predicate)
1073 /// Creates an iterator that yields elements based on a predicate.
1075 /// `take_while()` takes a closure as an argument. It will call this
1076 /// closure on each element of the iterator, and yield elements
1077 /// while it returns `true`.
1079 /// After `false` is returned, `take_while()`'s job is over, and the
1080 /// rest of the elements are ignored.
1087 /// let a = [-1i32, 0, 1];
1089 /// let mut iter = a.iter().take_while(|x| x.is_negative());
1091 /// assert_eq!(iter.next(), Some(&-1));
1092 /// assert_eq!(iter.next(), None);
1095 /// Because the closure passed to `take_while()` takes a reference, and many
1096 /// iterators iterate over references, this leads to a possibly confusing
1097 /// situation, where the type of the closure is a double reference:
1100 /// let a = [-1, 0, 1];
1102 /// let mut iter = a.iter().take_while(|x| **x < 0); // need two *s!
1104 /// assert_eq!(iter.next(), Some(&-1));
1105 /// assert_eq!(iter.next(), None);
1108 /// Stopping after an initial `false`:
1111 /// let a = [-1, 0, 1, -2];
1113 /// let mut iter = a.iter().take_while(|x| **x < 0);
1115 /// assert_eq!(iter.next(), Some(&-1));
1117 /// // We have more elements that are less than zero, but since we already
1118 /// // got a false, take_while() isn't used any more
1119 /// assert_eq!(iter.next(), None);
1122 /// Because `take_while()` needs to look at the value in order to see if it
1123 /// should be included or not, consuming iterators will see that it is
1127 /// let a = [1, 2, 3, 4];
1128 /// let mut iter = a.iter();
1130 /// let result: Vec<i32> = iter.by_ref()
1131 /// .take_while(|n| **n != 3)
1135 /// assert_eq!(result, &[1, 2]);
1137 /// let result: Vec<i32> = iter.cloned().collect();
1139 /// assert_eq!(result, &[4]);
1142 /// The `3` is no longer there, because it was consumed in order to see if
1143 /// the iteration should stop, but wasn't placed back into the iterator.
1145 #[stable(feature = "rust1", since = "1.0.0")]
1146 fn take_while<P>(self, predicate: P) -> TakeWhile<Self, P>
1149 P: FnMut(&Self::Item) -> bool,
1151 TakeWhile::new(self, predicate)
1154 /// Creates an iterator that both yields elements based on a predicate and maps.
1156 /// `map_while()` takes a closure as an argument. It will call this
1157 /// closure on each element of the iterator, and yield elements
1158 /// while it returns [`Some(_)`][`Some`].
1165 /// let a = [-1i32, 4, 0, 1];
1167 /// let mut iter = a.iter().map_while(|x| 16i32.checked_div(*x));
1169 /// assert_eq!(iter.next(), Some(-16));
1170 /// assert_eq!(iter.next(), Some(4));
1171 /// assert_eq!(iter.next(), None);
1174 /// Here's the same example, but with [`take_while`] and [`map`]:
1176 /// [`take_while`]: Iterator::take_while
1177 /// [`map`]: Iterator::map
1180 /// let a = [-1i32, 4, 0, 1];
1182 /// let mut iter = a.iter()
1183 /// .map(|x| 16i32.checked_div(*x))
1184 /// .take_while(|x| x.is_some())
1185 /// .map(|x| x.unwrap());
1187 /// assert_eq!(iter.next(), Some(-16));
1188 /// assert_eq!(iter.next(), Some(4));
1189 /// assert_eq!(iter.next(), None);
1192 /// Stopping after an initial [`None`]:
1195 /// let a = [0, 1, 2, -3, 4, 5, -6];
1197 /// let iter = a.iter().map_while(|x| u32::try_from(*x).ok());
1198 /// let vec = iter.collect::<Vec<_>>();
1200 /// // We have more elements which could fit in u32 (4, 5), but `map_while` returned `None` for `-3`
1201 /// // (as the `predicate` returned `None`) and `collect` stops at the first `None` encountered.
1202 /// assert_eq!(vec, vec![0, 1, 2]);
1205 /// Because `map_while()` needs to look at the value in order to see if it
1206 /// should be included or not, consuming iterators will see that it is
1210 /// let a = [1, 2, -3, 4];
1211 /// let mut iter = a.iter();
1213 /// let result: Vec<u32> = iter.by_ref()
1214 /// .map_while(|n| u32::try_from(*n).ok())
1217 /// assert_eq!(result, &[1, 2]);
1219 /// let result: Vec<i32> = iter.cloned().collect();
1221 /// assert_eq!(result, &[4]);
1224 /// The `-3` is no longer there, because it was consumed in order to see if
1225 /// the iteration should stop, but wasn't placed back into the iterator.
1227 /// Note that unlike [`take_while`] this iterator is **not** fused.
1228 /// It is also not specified what this iterator returns after the first [`None`] is returned.
1229 /// If you need fused iterator, use [`fuse`].
1231 /// [`fuse`]: Iterator::fuse
1233 #[stable(feature = "iter_map_while", since = "1.57.0")]
1234 fn map_while<B, P>(self, predicate: P) -> MapWhile<Self, P>
1237 P: FnMut(Self::Item) -> Option<B>,
1239 MapWhile::new(self, predicate)
1242 /// Creates an iterator that skips the first `n` elements.
1244 /// `skip(n)` skips elements until `n` elements are skipped or the end of the
1245 /// iterator is reached (whichever happens first). After that, all the remaining
1246 /// elements are yielded. In particular, if the original iterator is too short,
1247 /// then the returned iterator is empty.
1249 /// Rather than overriding this method directly, instead override the `nth` method.
1256 /// let a = [1, 2, 3];
1258 /// let mut iter = a.iter().skip(2);
1260 /// assert_eq!(iter.next(), Some(&3));
1261 /// assert_eq!(iter.next(), None);
1264 #[stable(feature = "rust1", since = "1.0.0")]
1265 fn skip(self, n: usize) -> Skip<Self>
1272 /// Creates an iterator that yields the first `n` elements, or fewer
1273 /// if the underlying iterator ends sooner.
1275 /// `take(n)` yields elements until `n` elements are yielded or the end of
1276 /// the iterator is reached (whichever happens first).
1277 /// The returned iterator is a prefix of length `n` if the original iterator
1278 /// contains at least `n` elements, otherwise it contains all of the
1279 /// (fewer than `n`) elements of the original iterator.
1286 /// let a = [1, 2, 3];
1288 /// let mut iter = a.iter().take(2);
1290 /// assert_eq!(iter.next(), Some(&1));
1291 /// assert_eq!(iter.next(), Some(&2));
1292 /// assert_eq!(iter.next(), None);
1295 /// `take()` is often used with an infinite iterator, to make it finite:
1298 /// let mut iter = (0..).take(3);
1300 /// assert_eq!(iter.next(), Some(0));
1301 /// assert_eq!(iter.next(), Some(1));
1302 /// assert_eq!(iter.next(), Some(2));
1303 /// assert_eq!(iter.next(), None);
1306 /// If less than `n` elements are available,
1307 /// `take` will limit itself to the size of the underlying iterator:
1311 /// let mut iter = v.into_iter().take(5);
1312 /// assert_eq!(iter.next(), Some(1));
1313 /// assert_eq!(iter.next(), Some(2));
1314 /// assert_eq!(iter.next(), None);
1317 #[stable(feature = "rust1", since = "1.0.0")]
1318 fn take(self, n: usize) -> Take<Self>
1325 /// An iterator adapter similar to [`fold`] that holds internal state and
1326 /// produces a new iterator.
1328 /// [`fold`]: Iterator::fold
1330 /// `scan()` takes two arguments: an initial value which seeds the internal
1331 /// state, and a closure with two arguments, the first being a mutable
1332 /// reference to the internal state and the second an iterator element.
1333 /// The closure can assign to the internal state to share state between
1336 /// On iteration, the closure will be applied to each element of the
1337 /// iterator and the return value from the closure, an [`Option`], is
1338 /// yielded by the iterator.
1345 /// let a = [1, 2, 3];
1347 /// let mut iter = a.iter().scan(1, |state, &x| {
1348 /// // each iteration, we'll multiply the state by the element
1349 /// *state = *state * x;
1351 /// // then, we'll yield the negation of the state
1355 /// assert_eq!(iter.next(), Some(-1));
1356 /// assert_eq!(iter.next(), Some(-2));
1357 /// assert_eq!(iter.next(), Some(-6));
1358 /// assert_eq!(iter.next(), None);
1361 #[stable(feature = "rust1", since = "1.0.0")]
1362 fn scan<St, B, F>(self, initial_state: St, f: F) -> Scan<Self, St, F>
1365 F: FnMut(&mut St, Self::Item) -> Option<B>,
1367 Scan::new(self, initial_state, f)
1370 /// Creates an iterator that works like map, but flattens nested structure.
1372 /// The [`map`] adapter is very useful, but only when the closure
1373 /// argument produces values. If it produces an iterator instead, there's
1374 /// an extra layer of indirection. `flat_map()` will remove this extra layer
1377 /// You can think of `flat_map(f)` as the semantic equivalent
1378 /// of [`map`]ping, and then [`flatten`]ing as in `map(f).flatten()`.
1380 /// Another way of thinking about `flat_map()`: [`map`]'s closure returns
1381 /// one item for each element, and `flat_map()`'s closure returns an
1382 /// iterator for each element.
1384 /// [`map`]: Iterator::map
1385 /// [`flatten`]: Iterator::flatten
1392 /// let words = ["alpha", "beta", "gamma"];
1394 /// // chars() returns an iterator
1395 /// let merged: String = words.iter()
1396 /// .flat_map(|s| s.chars())
1398 /// assert_eq!(merged, "alphabetagamma");
1401 #[stable(feature = "rust1", since = "1.0.0")]
1402 fn flat_map<U, F>(self, f: F) -> FlatMap<Self, U, F>
1406 F: FnMut(Self::Item) -> U,
1408 FlatMap::new(self, f)
1411 /// Creates an iterator that flattens nested structure.
1413 /// This is useful when you have an iterator of iterators or an iterator of
1414 /// things that can be turned into iterators and you want to remove one
1415 /// level of indirection.
1422 /// let data = vec![vec![1, 2, 3, 4], vec![5, 6]];
1423 /// let flattened = data.into_iter().flatten().collect::<Vec<u8>>();
1424 /// assert_eq!(flattened, &[1, 2, 3, 4, 5, 6]);
1427 /// Mapping and then flattening:
1430 /// let words = ["alpha", "beta", "gamma"];
1432 /// // chars() returns an iterator
1433 /// let merged: String = words.iter()
1434 /// .map(|s| s.chars())
1437 /// assert_eq!(merged, "alphabetagamma");
1440 /// You can also rewrite this in terms of [`flat_map()`], which is preferable
1441 /// in this case since it conveys intent more clearly:
1444 /// let words = ["alpha", "beta", "gamma"];
1446 /// // chars() returns an iterator
1447 /// let merged: String = words.iter()
1448 /// .flat_map(|s| s.chars())
1450 /// assert_eq!(merged, "alphabetagamma");
1453 /// Flattening only removes one level of nesting at a time:
1456 /// let d3 = [[[1, 2], [3, 4]], [[5, 6], [7, 8]]];
1458 /// let d2 = d3.iter().flatten().collect::<Vec<_>>();
1459 /// assert_eq!(d2, [&[1, 2], &[3, 4], &[5, 6], &[7, 8]]);
1461 /// let d1 = d3.iter().flatten().flatten().collect::<Vec<_>>();
1462 /// assert_eq!(d1, [&1, &2, &3, &4, &5, &6, &7, &8]);
1465 /// Here we see that `flatten()` does not perform a "deep" flatten.
1466 /// Instead, only one level of nesting is removed. That is, if you
1467 /// `flatten()` a three-dimensional array, the result will be
1468 /// two-dimensional and not one-dimensional. To get a one-dimensional
1469 /// structure, you have to `flatten()` again.
1471 /// [`flat_map()`]: Iterator::flat_map
1473 #[stable(feature = "iterator_flatten", since = "1.29.0")]
1474 fn flatten(self) -> Flatten<Self>
1477 Self::Item: IntoIterator,
1482 /// Creates an iterator which ends after the first [`None`].
1484 /// After an iterator returns [`None`], future calls may or may not yield
1485 /// [`Some(T)`] again. `fuse()` adapts an iterator, ensuring that after a
1486 /// [`None`] is given, it will always return [`None`] forever.
1488 /// Note that the [`Fuse`] wrapper is a no-op on iterators that implement
1489 /// the [`FusedIterator`] trait. `fuse()` may therefore behave incorrectly
1490 /// if the [`FusedIterator`] trait is improperly implemented.
1492 /// [`Some(T)`]: Some
1493 /// [`FusedIterator`]: crate::iter::FusedIterator
1500 /// // an iterator which alternates between Some and None
1501 /// struct Alternate {
1505 /// impl Iterator for Alternate {
1506 /// type Item = i32;
1508 /// fn next(&mut self) -> Option<i32> {
1509 /// let val = self.state;
1510 /// self.state = self.state + 1;
1512 /// // if it's even, Some(i32), else None
1513 /// if val % 2 == 0 {
1521 /// let mut iter = Alternate { state: 0 };
1523 /// // we can see our iterator going back and forth
1524 /// assert_eq!(iter.next(), Some(0));
1525 /// assert_eq!(iter.next(), None);
1526 /// assert_eq!(iter.next(), Some(2));
1527 /// assert_eq!(iter.next(), None);
1529 /// // however, once we fuse it...
1530 /// let mut iter = iter.fuse();
1532 /// assert_eq!(iter.next(), Some(4));
1533 /// assert_eq!(iter.next(), None);
1535 /// // it will always return `None` after the first time.
1536 /// assert_eq!(iter.next(), None);
1537 /// assert_eq!(iter.next(), None);
1538 /// assert_eq!(iter.next(), None);
1541 #[stable(feature = "rust1", since = "1.0.0")]
1542 fn fuse(self) -> Fuse<Self>
1549 /// Does something with each element of an iterator, passing the value on.
1551 /// When using iterators, you'll often chain several of them together.
1552 /// While working on such code, you might want to check out what's
1553 /// happening at various parts in the pipeline. To do that, insert
1554 /// a call to `inspect()`.
1556 /// It's more common for `inspect()` to be used as a debugging tool than to
1557 /// exist in your final code, but applications may find it useful in certain
1558 /// situations when errors need to be logged before being discarded.
1565 /// let a = [1, 4, 2, 3];
1567 /// // this iterator sequence is complex.
1568 /// let sum = a.iter()
1570 /// .filter(|x| x % 2 == 0)
1571 /// .fold(0, |sum, i| sum + i);
1573 /// println!("{}", sum);
1575 /// // let's add some inspect() calls to investigate what's happening
1576 /// let sum = a.iter()
1578 /// .inspect(|x| println!("about to filter: {}", x))
1579 /// .filter(|x| x % 2 == 0)
1580 /// .inspect(|x| println!("made it through filter: {}", x))
1581 /// .fold(0, |sum, i| sum + i);
1583 /// println!("{}", sum);
1586 /// This will print:
1590 /// about to filter: 1
1591 /// about to filter: 4
1592 /// made it through filter: 4
1593 /// about to filter: 2
1594 /// made it through filter: 2
1595 /// about to filter: 3
1599 /// Logging errors before discarding them:
1602 /// let lines = ["1", "2", "a"];
1604 /// let sum: i32 = lines
1606 /// .map(|line| line.parse::<i32>())
1607 /// .inspect(|num| {
1608 /// if let Err(ref e) = *num {
1609 /// println!("Parsing error: {}", e);
1612 /// .filter_map(Result::ok)
1615 /// println!("Sum: {}", sum);
1618 /// This will print:
1621 /// Parsing error: invalid digit found in string
1625 #[stable(feature = "rust1", since = "1.0.0")]
1626 fn inspect<F>(self, f: F) -> Inspect<Self, F>
1629 F: FnMut(&Self::Item),
1631 Inspect::new(self, f)
1634 /// Borrows an iterator, rather than consuming it.
1636 /// This is useful to allow applying iterator adapters while still
1637 /// retaining ownership of the original iterator.
1644 /// let mut words = ["hello", "world", "of", "Rust"].into_iter();
1646 /// // Take the first two words.
1647 /// let hello_world: Vec<_> = words.by_ref().take(2).collect();
1648 /// assert_eq!(hello_world, vec!["hello", "world"]);
1650 /// // Collect the rest of the words.
1651 /// // We can only do this because we used `by_ref` earlier.
1652 /// let of_rust: Vec<_> = words.collect();
1653 /// assert_eq!(of_rust, vec!["of", "Rust"]);
1655 #[stable(feature = "rust1", since = "1.0.0")]
1656 fn by_ref(&mut self) -> &mut Self
1663 /// Transforms an iterator into a collection.
1665 /// `collect()` can take anything iterable, and turn it into a relevant
1666 /// collection. This is one of the more powerful methods in the standard
1667 /// library, used in a variety of contexts.
1669 /// The most basic pattern in which `collect()` is used is to turn one
1670 /// collection into another. You take a collection, call [`iter`] on it,
1671 /// do a bunch of transformations, and then `collect()` at the end.
1673 /// `collect()` can also create instances of types that are not typical
1674 /// collections. For example, a [`String`] can be built from [`char`]s,
1675 /// and an iterator of [`Result<T, E>`][`Result`] items can be collected
1676 /// into `Result<Collection<T>, E>`. See the examples below for more.
1678 /// Because `collect()` is so general, it can cause problems with type
1679 /// inference. As such, `collect()` is one of the few times you'll see
1680 /// the syntax affectionately known as the 'turbofish': `::<>`. This
1681 /// helps the inference algorithm understand specifically which collection
1682 /// you're trying to collect into.
1689 /// let a = [1, 2, 3];
1691 /// let doubled: Vec<i32> = a.iter()
1692 /// .map(|&x| x * 2)
1695 /// assert_eq!(vec![2, 4, 6], doubled);
1698 /// Note that we needed the `: Vec<i32>` on the left-hand side. This is because
1699 /// we could collect into, for example, a [`VecDeque<T>`] instead:
1701 /// [`VecDeque<T>`]: ../../std/collections/struct.VecDeque.html
1704 /// use std::collections::VecDeque;
1706 /// let a = [1, 2, 3];
1708 /// let doubled: VecDeque<i32> = a.iter().map(|&x| x * 2).collect();
1710 /// assert_eq!(2, doubled[0]);
1711 /// assert_eq!(4, doubled[1]);
1712 /// assert_eq!(6, doubled[2]);
1715 /// Using the 'turbofish' instead of annotating `doubled`:
1718 /// let a = [1, 2, 3];
1720 /// let doubled = a.iter().map(|x| x * 2).collect::<Vec<i32>>();
1722 /// assert_eq!(vec![2, 4, 6], doubled);
1725 /// Because `collect()` only cares about what you're collecting into, you can
1726 /// still use a partial type hint, `_`, with the turbofish:
1729 /// let a = [1, 2, 3];
1731 /// let doubled = a.iter().map(|x| x * 2).collect::<Vec<_>>();
1733 /// assert_eq!(vec![2, 4, 6], doubled);
1736 /// Using `collect()` to make a [`String`]:
1739 /// let chars = ['g', 'd', 'k', 'k', 'n'];
1741 /// let hello: String = chars.iter()
1742 /// .map(|&x| x as u8)
1743 /// .map(|x| (x + 1) as char)
1746 /// assert_eq!("hello", hello);
1749 /// If you have a list of [`Result<T, E>`][`Result`]s, you can use `collect()` to
1750 /// see if any of them failed:
1753 /// let results = [Ok(1), Err("nope"), Ok(3), Err("bad")];
1755 /// let result: Result<Vec<_>, &str> = results.iter().cloned().collect();
1757 /// // gives us the first error
1758 /// assert_eq!(Err("nope"), result);
1760 /// let results = [Ok(1), Ok(3)];
1762 /// let result: Result<Vec<_>, &str> = results.iter().cloned().collect();
1764 /// // gives us the list of answers
1765 /// assert_eq!(Ok(vec![1, 3]), result);
1768 /// [`iter`]: Iterator::next
1769 /// [`String`]: ../../std/string/struct.String.html
1770 /// [`char`]: type@char
1772 #[stable(feature = "rust1", since = "1.0.0")]
1773 #[must_use = "if you really need to exhaust the iterator, consider `.for_each(drop)` instead"]
1774 fn collect<B: FromIterator<Self::Item>>(self) -> B
1778 FromIterator::from_iter(self)
1781 /// Fallibly transforms an iterator into a collection, short circuiting if
1782 /// a failure is encountered.
1784 /// `try_collect()` is a variation of [`collect()`][`collect`] that allows fallible
1785 /// conversions during collection. Its main use case is simplifying conversions from
1786 /// iterators yielding [`Option<T>`][`Option`] into `Option<Collection<T>>`, or similarly for other [`Try`]
1787 /// types (e.g. [`Result`]).
1789 /// Importantly, `try_collect()` doesn't require that the outer [`Try`] type also implements [`FromIterator`];
1790 /// only the inner type produced on `Try::Output` must implement it. Concretely,
1791 /// this means that collecting into `ControlFlow<_, Vec<i32>>` is valid because `Vec<i32>` implements
1792 /// [`FromIterator`], even though [`ControlFlow`] doesn't.
1794 /// Also, if a failure is encountered during `try_collect()`, the iterator is still valid and
1795 /// may continue to be used, in which case it will continue iterating starting after the element that
1796 /// triggered the failure. See the last example below for an example of how this works.
1799 /// Successfully collecting an iterator of `Option<i32>` into `Option<Vec<i32>>`:
1801 /// #![feature(iterator_try_collect)]
1803 /// let u = vec![Some(1), Some(2), Some(3)];
1804 /// let v = u.into_iter().try_collect::<Vec<i32>>();
1805 /// assert_eq!(v, Some(vec![1, 2, 3]));
1808 /// Failing to collect in the same way:
1810 /// #![feature(iterator_try_collect)]
1812 /// let u = vec![Some(1), Some(2), None, Some(3)];
1813 /// let v = u.into_iter().try_collect::<Vec<i32>>();
1814 /// assert_eq!(v, None);
1817 /// A similar example, but with `Result`:
1819 /// #![feature(iterator_try_collect)]
1821 /// let u: Vec<Result<i32, ()>> = vec![Ok(1), Ok(2), Ok(3)];
1822 /// let v = u.into_iter().try_collect::<Vec<i32>>();
1823 /// assert_eq!(v, Ok(vec![1, 2, 3]));
1825 /// let u = vec![Ok(1), Ok(2), Err(()), Ok(3)];
1826 /// let v = u.into_iter().try_collect::<Vec<i32>>();
1827 /// assert_eq!(v, Err(()));
1830 /// Finally, even [`ControlFlow`] works, despite the fact that it
1831 /// doesn't implement [`FromIterator`]. Note also that the iterator can
1832 /// continue to be used, even if a failure is encountered:
1835 /// #![feature(iterator_try_collect)]
1837 /// use core::ops::ControlFlow::{Break, Continue};
1839 /// let u = [Continue(1), Continue(2), Break(3), Continue(4), Continue(5)];
1840 /// let mut it = u.into_iter();
1842 /// let v = it.try_collect::<Vec<_>>();
1843 /// assert_eq!(v, Break(3));
1845 /// let v = it.try_collect::<Vec<_>>();
1846 /// assert_eq!(v, Continue(vec![4, 5]));
1849 /// [`collect`]: Iterator::collect
1851 #[unstable(feature = "iterator_try_collect", issue = "94047")]
1852 fn try_collect<B>(&mut self) -> ChangeOutputType<Self::Item, B>
1855 <Self as Iterator>::Item: Try,
1856 <<Self as Iterator>::Item as Try>::Residual: Residual<B>,
1857 B: FromIterator<<Self::Item as Try>::Output>,
1859 try_process(self, |i| i.collect())
1862 /// Consumes an iterator, creating two collections from it.
1864 /// The predicate passed to `partition()` can return `true`, or `false`.
1865 /// `partition()` returns a pair, all of the elements for which it returned
1866 /// `true`, and all of the elements for which it returned `false`.
1868 /// See also [`is_partitioned()`] and [`partition_in_place()`].
1870 /// [`is_partitioned()`]: Iterator::is_partitioned
1871 /// [`partition_in_place()`]: Iterator::partition_in_place
1878 /// let a = [1, 2, 3];
1880 /// let (even, odd): (Vec<i32>, Vec<i32>) = a
1882 /// .partition(|&n| n % 2 == 0);
1884 /// assert_eq!(even, vec![2]);
1885 /// assert_eq!(odd, vec![1, 3]);
1887 #[stable(feature = "rust1", since = "1.0.0")]
1888 fn partition<B, F>(self, f: F) -> (B, B)
1891 B: Default + Extend<Self::Item>,
1892 F: FnMut(&Self::Item) -> bool,
1895 fn extend<'a, T, B: Extend<T>>(
1896 mut f: impl FnMut(&T) -> bool + 'a,
1899 ) -> impl FnMut((), T) + 'a {
1904 right.extend_one(x);
1909 let mut left: B = Default::default();
1910 let mut right: B = Default::default();
1912 self.fold((), extend(f, &mut left, &mut right));
1917 /// Reorders the elements of this iterator *in-place* according to the given predicate,
1918 /// such that all those that return `true` precede all those that return `false`.
1919 /// Returns the number of `true` elements found.
1921 /// The relative order of partitioned items is not maintained.
1923 /// # Current implementation
1925 /// Current algorithms tries finding the first element for which the predicate evaluates
1926 /// to false, and the last element for which it evaluates to true and repeatedly swaps them.
1928 /// Time complexity: *O*(*n*)
1930 /// See also [`is_partitioned()`] and [`partition()`].
1932 /// [`is_partitioned()`]: Iterator::is_partitioned
1933 /// [`partition()`]: Iterator::partition
1938 /// #![feature(iter_partition_in_place)]
1940 /// let mut a = [1, 2, 3, 4, 5, 6, 7];
1942 /// // Partition in-place between evens and odds
1943 /// let i = a.iter_mut().partition_in_place(|&n| n % 2 == 0);
1945 /// assert_eq!(i, 3);
1946 /// assert!(a[..i].iter().all(|&n| n % 2 == 0)); // evens
1947 /// assert!(a[i..].iter().all(|&n| n % 2 == 1)); // odds
1949 #[unstable(feature = "iter_partition_in_place", reason = "new API", issue = "62543")]
1950 fn partition_in_place<'a, T: 'a, P>(mut self, ref mut predicate: P) -> usize
1952 Self: Sized + DoubleEndedIterator<Item = &'a mut T>,
1953 P: FnMut(&T) -> bool,
1955 // FIXME: should we worry about the count overflowing? The only way to have more than
1956 // `usize::MAX` mutable references is with ZSTs, which aren't useful to partition...
1958 // These closure "factory" functions exist to avoid genericity in `Self`.
1962 predicate: &'a mut impl FnMut(&T) -> bool,
1963 true_count: &'a mut usize,
1964 ) -> impl FnMut(&&mut T) -> bool + 'a {
1966 let p = predicate(&**x);
1967 *true_count += p as usize;
1973 fn is_true<T>(predicate: &mut impl FnMut(&T) -> bool) -> impl FnMut(&&mut T) -> bool + '_ {
1974 move |x| predicate(&**x)
1977 // Repeatedly find the first `false` and swap it with the last `true`.
1978 let mut true_count = 0;
1979 while let Some(head) = self.find(is_false(predicate, &mut true_count)) {
1980 if let Some(tail) = self.rfind(is_true(predicate)) {
1981 crate::mem::swap(head, tail);
1990 /// Checks if the elements of this iterator are partitioned according to the given predicate,
1991 /// such that all those that return `true` precede all those that return `false`.
1993 /// See also [`partition()`] and [`partition_in_place()`].
1995 /// [`partition()`]: Iterator::partition
1996 /// [`partition_in_place()`]: Iterator::partition_in_place
2001 /// #![feature(iter_is_partitioned)]
2003 /// assert!("Iterator".chars().is_partitioned(char::is_uppercase));
2004 /// assert!(!"IntoIterator".chars().is_partitioned(char::is_uppercase));
2006 #[unstable(feature = "iter_is_partitioned", reason = "new API", issue = "62544")]
2007 fn is_partitioned<P>(mut self, mut predicate: P) -> bool
2010 P: FnMut(Self::Item) -> bool,
2012 // Either all items test `true`, or the first clause stops at `false`
2013 // and we check that there are no more `true` items after that.
2014 self.all(&mut predicate) || !self.any(predicate)
2017 /// An iterator method that applies a function as long as it returns
2018 /// successfully, producing a single, final value.
2020 /// `try_fold()` takes two arguments: an initial value, and a closure with
2021 /// two arguments: an 'accumulator', and an element. The closure either
2022 /// returns successfully, with the value that the accumulator should have
2023 /// for the next iteration, or it returns failure, with an error value that
2024 /// is propagated back to the caller immediately (short-circuiting).
2026 /// The initial value is the value the accumulator will have on the first
2027 /// call. If applying the closure succeeded against every element of the
2028 /// iterator, `try_fold()` returns the final accumulator as success.
2030 /// Folding is useful whenever you have a collection of something, and want
2031 /// to produce a single value from it.
2033 /// # Note to Implementors
2035 /// Several of the other (forward) methods have default implementations in
2036 /// terms of this one, so try to implement this explicitly if it can
2037 /// do something better than the default `for` loop implementation.
2039 /// In particular, try to have this call `try_fold()` on the internal parts
2040 /// from which this iterator is composed. If multiple calls are needed,
2041 /// the `?` operator may be convenient for chaining the accumulator value
2042 /// along, but beware any invariants that need to be upheld before those
2043 /// early returns. This is a `&mut self` method, so iteration needs to be
2044 /// resumable after hitting an error here.
2051 /// let a = [1, 2, 3];
2053 /// // the checked sum of all of the elements of the array
2054 /// let sum = a.iter().try_fold(0i8, |acc, &x| acc.checked_add(x));
2056 /// assert_eq!(sum, Some(6));
2059 /// Short-circuiting:
2062 /// let a = [10, 20, 30, 100, 40, 50];
2063 /// let mut it = a.iter();
2065 /// // This sum overflows when adding the 100 element
2066 /// let sum = it.try_fold(0i8, |acc, &x| acc.checked_add(x));
2067 /// assert_eq!(sum, None);
2069 /// // Because it short-circuited, the remaining elements are still
2070 /// // available through the iterator.
2071 /// assert_eq!(it.len(), 2);
2072 /// assert_eq!(it.next(), Some(&40));
2075 /// While you cannot `break` from a closure, the [`ControlFlow`] type allows
2079 /// use std::ops::ControlFlow;
2081 /// let triangular = (1..30).try_fold(0_i8, |prev, x| {
2082 /// if let Some(next) = prev.checked_add(x) {
2083 /// ControlFlow::Continue(next)
2085 /// ControlFlow::Break(prev)
2088 /// assert_eq!(triangular, ControlFlow::Break(120));
2090 /// let triangular = (1..30).try_fold(0_u64, |prev, x| {
2091 /// if let Some(next) = prev.checked_add(x) {
2092 /// ControlFlow::Continue(next)
2094 /// ControlFlow::Break(prev)
2097 /// assert_eq!(triangular, ControlFlow::Continue(435));
2100 #[stable(feature = "iterator_try_fold", since = "1.27.0")]
2101 fn try_fold<B, F, R>(&mut self, init: B, mut f: F) -> R
2104 F: FnMut(B, Self::Item) -> R,
2107 let mut accum = init;
2108 while let Some(x) = self.next() {
2109 accum = f(accum, x)?;
2114 /// An iterator method that applies a fallible function to each item in the
2115 /// iterator, stopping at the first error and returning that error.
2117 /// This can also be thought of as the fallible form of [`for_each()`]
2118 /// or as the stateless version of [`try_fold()`].
2120 /// [`for_each()`]: Iterator::for_each
2121 /// [`try_fold()`]: Iterator::try_fold
2126 /// use std::fs::rename;
2127 /// use std::io::{stdout, Write};
2128 /// use std::path::Path;
2130 /// let data = ["no_tea.txt", "stale_bread.json", "torrential_rain.png"];
2132 /// let res = data.iter().try_for_each(|x| writeln!(stdout(), "{}", x));
2133 /// assert!(res.is_ok());
2135 /// let mut it = data.iter().cloned();
2136 /// let res = it.try_for_each(|x| rename(x, Path::new(x).with_extension("old")));
2137 /// assert!(res.is_err());
2138 /// // It short-circuited, so the remaining items are still in the iterator:
2139 /// assert_eq!(it.next(), Some("stale_bread.json"));
2142 /// The [`ControlFlow`] type can be used with this method for the situations
2143 /// in which you'd use `break` and `continue` in a normal loop:
2146 /// use std::ops::ControlFlow;
2148 /// let r = (2..100).try_for_each(|x| {
2149 /// if 323 % x == 0 {
2150 /// return ControlFlow::Break(x)
2153 /// ControlFlow::Continue(())
2155 /// assert_eq!(r, ControlFlow::Break(17));
2158 #[stable(feature = "iterator_try_fold", since = "1.27.0")]
2159 fn try_for_each<F, R>(&mut self, f: F) -> R
2162 F: FnMut(Self::Item) -> R,
2163 R: Try<Output = ()>,
2166 fn call<T, R>(mut f: impl FnMut(T) -> R) -> impl FnMut((), T) -> R {
2170 self.try_fold((), call(f))
2173 /// Folds every element into an accumulator by applying an operation,
2174 /// returning the final result.
2176 /// `fold()` takes two arguments: an initial value, and a closure with two
2177 /// arguments: an 'accumulator', and an element. The closure returns the value that
2178 /// the accumulator should have for the next iteration.
2180 /// The initial value is the value the accumulator will have on the first
2183 /// After applying this closure to every element of the iterator, `fold()`
2184 /// returns the accumulator.
2186 /// This operation is sometimes called 'reduce' or 'inject'.
2188 /// Folding is useful whenever you have a collection of something, and want
2189 /// to produce a single value from it.
2191 /// Note: `fold()`, and similar methods that traverse the entire iterator,
2192 /// might not terminate for infinite iterators, even on traits for which a
2193 /// result is determinable in finite time.
2195 /// Note: [`reduce()`] can be used to use the first element as the initial
2196 /// value, if the accumulator type and item type is the same.
2198 /// Note: `fold()` combines elements in a *left-associative* fashion. For associative
2199 /// operators like `+`, the order the elements are combined in is not important, but for non-associative
2200 /// operators like `-` the order will affect the final result.
2201 /// For a *right-associative* version of `fold()`, see [`DoubleEndedIterator::rfold()`].
2203 /// # Note to Implementors
2205 /// Several of the other (forward) methods have default implementations in
2206 /// terms of this one, so try to implement this explicitly if it can
2207 /// do something better than the default `for` loop implementation.
2209 /// In particular, try to have this call `fold()` on the internal parts
2210 /// from which this iterator is composed.
2217 /// let a = [1, 2, 3];
2219 /// // the sum of all of the elements of the array
2220 /// let sum = a.iter().fold(0, |acc, x| acc + x);
2222 /// assert_eq!(sum, 6);
2225 /// Let's walk through each step of the iteration here:
2227 /// | element | acc | x | result |
2228 /// |---------|-----|---|--------|
2230 /// | 1 | 0 | 1 | 1 |
2231 /// | 2 | 1 | 2 | 3 |
2232 /// | 3 | 3 | 3 | 6 |
2234 /// And so, our final result, `6`.
2236 /// This example demonstrates the left-associative nature of `fold()`:
2237 /// it builds a string, starting with an initial value
2238 /// and continuing with each element from the front until the back:
2241 /// let numbers = [1, 2, 3, 4, 5];
2243 /// let zero = "0".to_string();
2245 /// let result = numbers.iter().fold(zero, |acc, &x| {
2246 /// format!("({} + {})", acc, x)
2249 /// assert_eq!(result, "(((((0 + 1) + 2) + 3) + 4) + 5)");
2251 /// It's common for people who haven't used iterators a lot to
2252 /// use a `for` loop with a list of things to build up a result. Those
2253 /// can be turned into `fold()`s:
2255 /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
2258 /// let numbers = [1, 2, 3, 4, 5];
2260 /// let mut result = 0;
2263 /// for i in &numbers {
2264 /// result = result + i;
2268 /// let result2 = numbers.iter().fold(0, |acc, &x| acc + x);
2270 /// // they're the same
2271 /// assert_eq!(result, result2);
2274 /// [`reduce()`]: Iterator::reduce
2275 #[doc(alias = "inject", alias = "foldl")]
2277 #[stable(feature = "rust1", since = "1.0.0")]
2278 fn fold<B, F>(mut self, init: B, mut f: F) -> B
2281 F: FnMut(B, Self::Item) -> B,
2283 let mut accum = init;
2284 while let Some(x) = self.next() {
2285 accum = f(accum, x);
2290 /// Reduces the elements to a single one, by repeatedly applying a reducing
2293 /// If the iterator is empty, returns [`None`]; otherwise, returns the
2294 /// result of the reduction.
2296 /// The reducing function is a closure with two arguments: an 'accumulator', and an element.
2297 /// For iterators with at least one element, this is the same as [`fold()`]
2298 /// with the first element of the iterator as the initial accumulator value, folding
2299 /// every subsequent element into it.
2301 /// [`fold()`]: Iterator::fold
2305 /// Find the maximum value:
2308 /// fn find_max<I>(iter: I) -> Option<I::Item>
2309 /// where I: Iterator,
2312 /// iter.reduce(|accum, item| {
2313 /// if accum >= item { accum } else { item }
2316 /// let a = [10, 20, 5, -23, 0];
2317 /// let b: [u32; 0] = [];
2319 /// assert_eq!(find_max(a.iter()), Some(&20));
2320 /// assert_eq!(find_max(b.iter()), None);
2323 #[stable(feature = "iterator_fold_self", since = "1.51.0")]
2324 fn reduce<F>(mut self, f: F) -> Option<Self::Item>
2327 F: FnMut(Self::Item, Self::Item) -> Self::Item,
2329 let first = self.next()?;
2330 Some(self.fold(first, f))
2333 /// Reduces the elements to a single one by repeatedly applying a reducing operation. If the
2334 /// closure returns a failure, the failure is propagated back to the caller immediately.
2336 /// The return type of this method depends on the return type of the closure. If the closure
2337 /// returns `Result<Self::Item, E>`, then this function will return `Result<Option<Self::Item>,
2338 /// E>`. If the closure returns `Option<Self::Item>`, then this function will return
2339 /// `Option<Option<Self::Item>>`.
2341 /// When called on an empty iterator, this function will return either `Some(None)` or
2342 /// `Ok(None)` depending on the type of the provided closure.
2344 /// For iterators with at least one element, this is essentially the same as calling
2345 /// [`try_fold()`] with the first element of the iterator as the initial accumulator value.
2347 /// [`try_fold()`]: Iterator::try_fold
2351 /// Safely calculate the sum of a series of numbers:
2354 /// #![feature(iterator_try_reduce)]
2356 /// let numbers: Vec<usize> = vec![10, 20, 5, 23, 0];
2357 /// let sum = numbers.into_iter().try_reduce(|x, y| x.checked_add(y));
2358 /// assert_eq!(sum, Some(Some(58)));
2361 /// Determine when a reduction short circuited:
2364 /// #![feature(iterator_try_reduce)]
2366 /// let numbers = vec![1, 2, 3, usize::MAX, 4, 5];
2367 /// let sum = numbers.into_iter().try_reduce(|x, y| x.checked_add(y));
2368 /// assert_eq!(sum, None);
2371 /// Determine when a reduction was not performed because there are no elements:
2374 /// #![feature(iterator_try_reduce)]
2376 /// let numbers: Vec<usize> = Vec::new();
2377 /// let sum = numbers.into_iter().try_reduce(|x, y| x.checked_add(y));
2378 /// assert_eq!(sum, Some(None));
2381 /// Use a [`Result`] instead of an [`Option`]:
2384 /// #![feature(iterator_try_reduce)]
2386 /// let numbers = vec!["1", "2", "3", "4", "5"];
2387 /// let max: Result<Option<_>, <usize as std::str::FromStr>::Err> =
2388 /// numbers.into_iter().try_reduce(|x, y| {
2389 /// if x.parse::<usize>()? > y.parse::<usize>()? { Ok(x) } else { Ok(y) }
2391 /// assert_eq!(max, Ok(Some("5")));
2394 #[unstable(feature = "iterator_try_reduce", reason = "new API", issue = "87053")]
2395 fn try_reduce<F, R>(&mut self, f: F) -> ChangeOutputType<R, Option<R::Output>>
2398 F: FnMut(Self::Item, Self::Item) -> R,
2399 R: Try<Output = Self::Item>,
2400 R::Residual: Residual<Option<Self::Item>>,
2402 let first = match self.next() {
2404 None => return Try::from_output(None),
2407 match self.try_fold(first, f).branch() {
2408 ControlFlow::Break(r) => FromResidual::from_residual(r),
2409 ControlFlow::Continue(i) => Try::from_output(Some(i)),
2413 /// Tests if every element of the iterator matches a predicate.
2415 /// `all()` takes a closure that returns `true` or `false`. It applies
2416 /// this closure to each element of the iterator, and if they all return
2417 /// `true`, then so does `all()`. If any of them return `false`, it
2418 /// returns `false`.
2420 /// `all()` is short-circuiting; in other words, it will stop processing
2421 /// as soon as it finds a `false`, given that no matter what else happens,
2422 /// the result will also be `false`.
2424 /// An empty iterator returns `true`.
2431 /// let a = [1, 2, 3];
2433 /// assert!(a.iter().all(|&x| x > 0));
2435 /// assert!(!a.iter().all(|&x| x > 2));
2438 /// Stopping at the first `false`:
2441 /// let a = [1, 2, 3];
2443 /// let mut iter = a.iter();
2445 /// assert!(!iter.all(|&x| x != 2));
2447 /// // we can still use `iter`, as there are more elements.
2448 /// assert_eq!(iter.next(), Some(&3));
2451 #[stable(feature = "rust1", since = "1.0.0")]
2452 fn all<F>(&mut self, f: F) -> bool
2455 F: FnMut(Self::Item) -> bool,
2458 fn check<T>(mut f: impl FnMut(T) -> bool) -> impl FnMut((), T) -> ControlFlow<()> {
2460 if f(x) { ControlFlow::CONTINUE } else { ControlFlow::BREAK }
2463 self.try_fold((), check(f)) == ControlFlow::CONTINUE
2466 /// Tests if any element of the iterator matches a predicate.
2468 /// `any()` takes a closure that returns `true` or `false`. It applies
2469 /// this closure to each element of the iterator, and if any of them return
2470 /// `true`, then so does `any()`. If they all return `false`, it
2471 /// returns `false`.
2473 /// `any()` is short-circuiting; in other words, it will stop processing
2474 /// as soon as it finds a `true`, given that no matter what else happens,
2475 /// the result will also be `true`.
2477 /// An empty iterator returns `false`.
2484 /// let a = [1, 2, 3];
2486 /// assert!(a.iter().any(|&x| x > 0));
2488 /// assert!(!a.iter().any(|&x| x > 5));
2491 /// Stopping at the first `true`:
2494 /// let a = [1, 2, 3];
2496 /// let mut iter = a.iter();
2498 /// assert!(iter.any(|&x| x != 2));
2500 /// // we can still use `iter`, as there are more elements.
2501 /// assert_eq!(iter.next(), Some(&2));
2504 #[stable(feature = "rust1", since = "1.0.0")]
2505 fn any<F>(&mut self, f: F) -> bool
2508 F: FnMut(Self::Item) -> bool,
2511 fn check<T>(mut f: impl FnMut(T) -> bool) -> impl FnMut((), T) -> ControlFlow<()> {
2513 if f(x) { ControlFlow::BREAK } else { ControlFlow::CONTINUE }
2517 self.try_fold((), check(f)) == ControlFlow::BREAK
2520 /// Searches for an element of an iterator that satisfies a predicate.
2522 /// `find()` takes a closure that returns `true` or `false`. It applies
2523 /// this closure to each element of the iterator, and if any of them return
2524 /// `true`, then `find()` returns [`Some(element)`]. If they all return
2525 /// `false`, it returns [`None`].
2527 /// `find()` is short-circuiting; in other words, it will stop processing
2528 /// as soon as the closure returns `true`.
2530 /// Because `find()` takes a reference, and many iterators iterate over
2531 /// references, this leads to a possibly confusing situation where the
2532 /// argument is a double reference. You can see this effect in the
2533 /// examples below, with `&&x`.
2535 /// [`Some(element)`]: Some
2542 /// let a = [1, 2, 3];
2544 /// assert_eq!(a.iter().find(|&&x| x == 2), Some(&2));
2546 /// assert_eq!(a.iter().find(|&&x| x == 5), None);
2549 /// Stopping at the first `true`:
2552 /// let a = [1, 2, 3];
2554 /// let mut iter = a.iter();
2556 /// assert_eq!(iter.find(|&&x| x == 2), Some(&2));
2558 /// // we can still use `iter`, as there are more elements.
2559 /// assert_eq!(iter.next(), Some(&3));
2562 /// Note that `iter.find(f)` is equivalent to `iter.filter(f).next()`.
2564 #[stable(feature = "rust1", since = "1.0.0")]
2565 fn find<P>(&mut self, predicate: P) -> Option<Self::Item>
2568 P: FnMut(&Self::Item) -> bool,
2571 fn check<T>(mut predicate: impl FnMut(&T) -> bool) -> impl FnMut((), T) -> ControlFlow<T> {
2573 if predicate(&x) { ControlFlow::Break(x) } else { ControlFlow::CONTINUE }
2577 self.try_fold((), check(predicate)).break_value()
2580 /// Applies function to the elements of iterator and returns
2581 /// the first non-none result.
2583 /// `iter.find_map(f)` is equivalent to `iter.filter_map(f).next()`.
2588 /// let a = ["lol", "NaN", "2", "5"];
2590 /// let first_number = a.iter().find_map(|s| s.parse().ok());
2592 /// assert_eq!(first_number, Some(2));
2595 #[stable(feature = "iterator_find_map", since = "1.30.0")]
2596 fn find_map<B, F>(&mut self, f: F) -> Option<B>
2599 F: FnMut(Self::Item) -> Option<B>,
2602 fn check<T, B>(mut f: impl FnMut(T) -> Option<B>) -> impl FnMut((), T) -> ControlFlow<B> {
2603 move |(), x| match f(x) {
2604 Some(x) => ControlFlow::Break(x),
2605 None => ControlFlow::CONTINUE,
2609 self.try_fold((), check(f)).break_value()
2612 /// Applies function to the elements of iterator and returns
2613 /// the first true result or the first error.
2615 /// The return type of this method depends on the return type of the closure.
2616 /// If you return `Result<bool, E>` from the closure, you'll get a `Result<Option<Self::Item>; E>`.
2617 /// If you return `Option<bool>` from the closure, you'll get an `Option<Option<Self::Item>>`.
2622 /// #![feature(try_find)]
2624 /// let a = ["1", "2", "lol", "NaN", "5"];
2626 /// let is_my_num = |s: &str, search: i32| -> Result<bool, std::num::ParseIntError> {
2627 /// Ok(s.parse::<i32>()? == search)
2630 /// let result = a.iter().try_find(|&&s| is_my_num(s, 2));
2631 /// assert_eq!(result, Ok(Some(&"2")));
2633 /// let result = a.iter().try_find(|&&s| is_my_num(s, 5));
2634 /// assert!(result.is_err());
2637 /// This also supports other types which implement `Try`, not just `Result`.
2639 /// #![feature(try_find)]
2641 /// use std::num::NonZeroU32;
2642 /// let a = [3, 5, 7, 4, 9, 0, 11];
2643 /// let result = a.iter().try_find(|&&x| NonZeroU32::new(x).map(|y| y.is_power_of_two()));
2644 /// assert_eq!(result, Some(Some(&4)));
2645 /// let result = a.iter().take(3).try_find(|&&x| NonZeroU32::new(x).map(|y| y.is_power_of_two()));
2646 /// assert_eq!(result, Some(None));
2647 /// let result = a.iter().rev().try_find(|&&x| NonZeroU32::new(x).map(|y| y.is_power_of_two()));
2648 /// assert_eq!(result, None);
2651 #[unstable(feature = "try_find", reason = "new API", issue = "63178")]
2652 fn try_find<F, R>(&mut self, f: F) -> ChangeOutputType<R, Option<Self::Item>>
2655 F: FnMut(&Self::Item) -> R,
2656 R: Try<Output = bool>,
2657 R::Residual: Residual<Option<Self::Item>>,
2661 mut f: impl FnMut(&I) -> V,
2662 ) -> impl FnMut((), I) -> ControlFlow<R::TryType>
2664 V: Try<Output = bool, Residual = R>,
2665 R: Residual<Option<I>>,
2667 move |(), x| match f(&x).branch() {
2668 ControlFlow::Continue(false) => ControlFlow::CONTINUE,
2669 ControlFlow::Continue(true) => ControlFlow::Break(Try::from_output(Some(x))),
2670 ControlFlow::Break(r) => ControlFlow::Break(FromResidual::from_residual(r)),
2674 match self.try_fold((), check(f)) {
2675 ControlFlow::Break(x) => x,
2676 ControlFlow::Continue(()) => Try::from_output(None),
2680 /// Searches for an element in an iterator, returning its index.
2682 /// `position()` takes a closure that returns `true` or `false`. It applies
2683 /// this closure to each element of the iterator, and if one of them
2684 /// returns `true`, then `position()` returns [`Some(index)`]. If all of
2685 /// them return `false`, it returns [`None`].
2687 /// `position()` is short-circuiting; in other words, it will stop
2688 /// processing as soon as it finds a `true`.
2690 /// # Overflow Behavior
2692 /// The method does no guarding against overflows, so if there are more
2693 /// than [`usize::MAX`] non-matching elements, it either produces the wrong
2694 /// result or panics. If debug assertions are enabled, a panic is
2699 /// This function might panic if the iterator has more than `usize::MAX`
2700 /// non-matching elements.
2702 /// [`Some(index)`]: Some
2709 /// let a = [1, 2, 3];
2711 /// assert_eq!(a.iter().position(|&x| x == 2), Some(1));
2713 /// assert_eq!(a.iter().position(|&x| x == 5), None);
2716 /// Stopping at the first `true`:
2719 /// let a = [1, 2, 3, 4];
2721 /// let mut iter = a.iter();
2723 /// assert_eq!(iter.position(|&x| x >= 2), Some(1));
2725 /// // we can still use `iter`, as there are more elements.
2726 /// assert_eq!(iter.next(), Some(&3));
2728 /// // The returned index depends on iterator state
2729 /// assert_eq!(iter.position(|&x| x == 4), Some(0));
2733 #[stable(feature = "rust1", since = "1.0.0")]
2734 fn position<P>(&mut self, predicate: P) -> Option<usize>
2737 P: FnMut(Self::Item) -> bool,
2741 mut predicate: impl FnMut(T) -> bool,
2742 ) -> impl FnMut(usize, T) -> ControlFlow<usize, usize> {
2743 #[rustc_inherit_overflow_checks]
2745 if predicate(x) { ControlFlow::Break(i) } else { ControlFlow::Continue(i + 1) }
2749 self.try_fold(0, check(predicate)).break_value()
2752 /// Searches for an element in an iterator from the right, returning its
2755 /// `rposition()` takes a closure that returns `true` or `false`. It applies
2756 /// this closure to each element of the iterator, starting from the end,
2757 /// and if one of them returns `true`, then `rposition()` returns
2758 /// [`Some(index)`]. If all of them return `false`, it returns [`None`].
2760 /// `rposition()` is short-circuiting; in other words, it will stop
2761 /// processing as soon as it finds a `true`.
2763 /// [`Some(index)`]: Some
2770 /// let a = [1, 2, 3];
2772 /// assert_eq!(a.iter().rposition(|&x| x == 3), Some(2));
2774 /// assert_eq!(a.iter().rposition(|&x| x == 5), None);
2777 /// Stopping at the first `true`:
2780 /// let a = [1, 2, 3];
2782 /// let mut iter = a.iter();
2784 /// assert_eq!(iter.rposition(|&x| x == 2), Some(1));
2786 /// // we can still use `iter`, as there are more elements.
2787 /// assert_eq!(iter.next(), Some(&1));
2790 #[stable(feature = "rust1", since = "1.0.0")]
2791 fn rposition<P>(&mut self, predicate: P) -> Option<usize>
2793 P: FnMut(Self::Item) -> bool,
2794 Self: Sized + ExactSizeIterator + DoubleEndedIterator,
2796 // No need for an overflow check here, because `ExactSizeIterator`
2797 // implies that the number of elements fits into a `usize`.
2800 mut predicate: impl FnMut(T) -> bool,
2801 ) -> impl FnMut(usize, T) -> ControlFlow<usize, usize> {
2804 if predicate(x) { ControlFlow::Break(i) } else { ControlFlow::Continue(i) }
2809 self.try_rfold(n, check(predicate)).break_value()
2812 /// Returns the maximum element of an iterator.
2814 /// If several elements are equally maximum, the last element is
2815 /// returned. If the iterator is empty, [`None`] is returned.
2817 /// Note that [`f32`]/[`f64`] doesn't implement [`Ord`] due to NaN being
2818 /// incomparable. You can work around this by using [`Iterator::reduce`]:
2821 /// [2.4, f32::NAN, 1.3]
2823 /// .reduce(f32::max)
2834 /// let a = [1, 2, 3];
2835 /// let b: Vec<u32> = Vec::new();
2837 /// assert_eq!(a.iter().max(), Some(&3));
2838 /// assert_eq!(b.iter().max(), None);
2841 #[stable(feature = "rust1", since = "1.0.0")]
2842 fn max(self) -> Option<Self::Item>
2847 self.max_by(Ord::cmp)
2850 /// Returns the minimum element of an iterator.
2852 /// If several elements are equally minimum, the first element is returned.
2853 /// If the iterator is empty, [`None`] is returned.
2855 /// Note that [`f32`]/[`f64`] doesn't implement [`Ord`] due to NaN being
2856 /// incomparable. You can work around this by using [`Iterator::reduce`]:
2859 /// [2.4, f32::NAN, 1.3]
2861 /// .reduce(f32::min)
2872 /// let a = [1, 2, 3];
2873 /// let b: Vec<u32> = Vec::new();
2875 /// assert_eq!(a.iter().min(), Some(&1));
2876 /// assert_eq!(b.iter().min(), None);
2879 #[stable(feature = "rust1", since = "1.0.0")]
2880 fn min(self) -> Option<Self::Item>
2885 self.min_by(Ord::cmp)
2888 /// Returns the element that gives the maximum value from the
2889 /// specified function.
2891 /// If several elements are equally maximum, the last element is
2892 /// returned. If the iterator is empty, [`None`] is returned.
2897 /// let a = [-3_i32, 0, 1, 5, -10];
2898 /// assert_eq!(*a.iter().max_by_key(|x| x.abs()).unwrap(), -10);
2901 #[stable(feature = "iter_cmp_by_key", since = "1.6.0")]
2902 fn max_by_key<B: Ord, F>(self, f: F) -> Option<Self::Item>
2905 F: FnMut(&Self::Item) -> B,
2908 fn key<T, B>(mut f: impl FnMut(&T) -> B) -> impl FnMut(T) -> (B, T) {
2913 fn compare<T, B: Ord>((x_p, _): &(B, T), (y_p, _): &(B, T)) -> Ordering {
2917 let (_, x) = self.map(key(f)).max_by(compare)?;
2921 /// Returns the element that gives the maximum value with respect to the
2922 /// specified comparison function.
2924 /// If several elements are equally maximum, the last element is
2925 /// returned. If the iterator is empty, [`None`] is returned.
2930 /// let a = [-3_i32, 0, 1, 5, -10];
2931 /// assert_eq!(*a.iter().max_by(|x, y| x.cmp(y)).unwrap(), 5);
2934 #[stable(feature = "iter_max_by", since = "1.15.0")]
2935 fn max_by<F>(self, compare: F) -> Option<Self::Item>
2938 F: FnMut(&Self::Item, &Self::Item) -> Ordering,
2941 fn fold<T>(mut compare: impl FnMut(&T, &T) -> Ordering) -> impl FnMut(T, T) -> T {
2942 move |x, y| cmp::max_by(x, y, &mut compare)
2945 self.reduce(fold(compare))
2948 /// Returns the element that gives the minimum value from the
2949 /// specified function.
2951 /// If several elements are equally minimum, the first element is
2952 /// returned. If the iterator is empty, [`None`] is returned.
2957 /// let a = [-3_i32, 0, 1, 5, -10];
2958 /// assert_eq!(*a.iter().min_by_key(|x| x.abs()).unwrap(), 0);
2961 #[stable(feature = "iter_cmp_by_key", since = "1.6.0")]
2962 fn min_by_key<B: Ord, F>(self, f: F) -> Option<Self::Item>
2965 F: FnMut(&Self::Item) -> B,
2968 fn key<T, B>(mut f: impl FnMut(&T) -> B) -> impl FnMut(T) -> (B, T) {
2973 fn compare<T, B: Ord>((x_p, _): &(B, T), (y_p, _): &(B, T)) -> Ordering {
2977 let (_, x) = self.map(key(f)).min_by(compare)?;
2981 /// Returns the element that gives the minimum value with respect to the
2982 /// specified comparison function.
2984 /// If several elements are equally minimum, the first element is
2985 /// returned. If the iterator is empty, [`None`] is returned.
2990 /// let a = [-3_i32, 0, 1, 5, -10];
2991 /// assert_eq!(*a.iter().min_by(|x, y| x.cmp(y)).unwrap(), -10);
2994 #[stable(feature = "iter_min_by", since = "1.15.0")]
2995 fn min_by<F>(self, compare: F) -> Option<Self::Item>
2998 F: FnMut(&Self::Item, &Self::Item) -> Ordering,
3001 fn fold<T>(mut compare: impl FnMut(&T, &T) -> Ordering) -> impl FnMut(T, T) -> T {
3002 move |x, y| cmp::min_by(x, y, &mut compare)
3005 self.reduce(fold(compare))
3008 /// Reverses an iterator's direction.
3010 /// Usually, iterators iterate from left to right. After using `rev()`,
3011 /// an iterator will instead iterate from right to left.
3013 /// This is only possible if the iterator has an end, so `rev()` only
3014 /// works on [`DoubleEndedIterator`]s.
3019 /// let a = [1, 2, 3];
3021 /// let mut iter = a.iter().rev();
3023 /// assert_eq!(iter.next(), Some(&3));
3024 /// assert_eq!(iter.next(), Some(&2));
3025 /// assert_eq!(iter.next(), Some(&1));
3027 /// assert_eq!(iter.next(), None);
3030 #[doc(alias = "reverse")]
3031 #[stable(feature = "rust1", since = "1.0.0")]
3032 fn rev(self) -> Rev<Self>
3034 Self: Sized + DoubleEndedIterator,
3039 /// Converts an iterator of pairs into a pair of containers.
3041 /// `unzip()` consumes an entire iterator of pairs, producing two
3042 /// collections: one from the left elements of the pairs, and one
3043 /// from the right elements.
3045 /// This function is, in some sense, the opposite of [`zip`].
3047 /// [`zip`]: Iterator::zip
3054 /// let a = [(1, 2), (3, 4), (5, 6)];
3056 /// let (left, right): (Vec<_>, Vec<_>) = a.iter().cloned().unzip();
3058 /// assert_eq!(left, [1, 3, 5]);
3059 /// assert_eq!(right, [2, 4, 6]);
3061 /// // you can also unzip multiple nested tuples at once
3062 /// let a = [(1, (2, 3)), (4, (5, 6))];
3064 /// let (x, (y, z)): (Vec<_>, (Vec<_>, Vec<_>)) = a.iter().cloned().unzip();
3065 /// assert_eq!(x, [1, 4]);
3066 /// assert_eq!(y, [2, 5]);
3067 /// assert_eq!(z, [3, 6]);
3069 #[stable(feature = "rust1", since = "1.0.0")]
3070 fn unzip<A, B, FromA, FromB>(self) -> (FromA, FromB)
3072 FromA: Default + Extend<A>,
3073 FromB: Default + Extend<B>,
3074 Self: Sized + Iterator<Item = (A, B)>,
3076 let mut unzipped: (FromA, FromB) = Default::default();
3077 unzipped.extend(self);
3081 /// Creates an iterator which copies all of its elements.
3083 /// This is useful when you have an iterator over `&T`, but you need an
3084 /// iterator over `T`.
3091 /// let a = [1, 2, 3];
3093 /// let v_copied: Vec<_> = a.iter().copied().collect();
3095 /// // copied is the same as .map(|&x| x)
3096 /// let v_map: Vec<_> = a.iter().map(|&x| x).collect();
3098 /// assert_eq!(v_copied, vec![1, 2, 3]);
3099 /// assert_eq!(v_map, vec![1, 2, 3]);
3101 #[stable(feature = "iter_copied", since = "1.36.0")]
3102 fn copied<'a, T: 'a>(self) -> Copied<Self>
3104 Self: Sized + Iterator<Item = &'a T>,
3110 /// Creates an iterator which [`clone`]s all of its elements.
3112 /// This is useful when you have an iterator over `&T`, but you need an
3113 /// iterator over `T`.
3115 /// [`clone`]: Clone::clone
3122 /// let a = [1, 2, 3];
3124 /// let v_cloned: Vec<_> = a.iter().cloned().collect();
3126 /// // cloned is the same as .map(|&x| x), for integers
3127 /// let v_map: Vec<_> = a.iter().map(|&x| x).collect();
3129 /// assert_eq!(v_cloned, vec![1, 2, 3]);
3130 /// assert_eq!(v_map, vec![1, 2, 3]);
3132 #[stable(feature = "rust1", since = "1.0.0")]
3133 fn cloned<'a, T: 'a>(self) -> Cloned<Self>
3135 Self: Sized + Iterator<Item = &'a T>,
3141 /// Repeats an iterator endlessly.
3143 /// Instead of stopping at [`None`], the iterator will instead start again,
3144 /// from the beginning. After iterating again, it will start at the
3145 /// beginning again. And again. And again. Forever. Note that in case the
3146 /// original iterator is empty, the resulting iterator will also be empty.
3153 /// let a = [1, 2, 3];
3155 /// let mut it = a.iter().cycle();
3157 /// assert_eq!(it.next(), Some(&1));
3158 /// assert_eq!(it.next(), Some(&2));
3159 /// assert_eq!(it.next(), Some(&3));
3160 /// assert_eq!(it.next(), Some(&1));
3161 /// assert_eq!(it.next(), Some(&2));
3162 /// assert_eq!(it.next(), Some(&3));
3163 /// assert_eq!(it.next(), Some(&1));
3165 #[stable(feature = "rust1", since = "1.0.0")]
3167 fn cycle(self) -> Cycle<Self>
3169 Self: Sized + Clone,
3174 /// Sums the elements of an iterator.
3176 /// Takes each element, adds them together, and returns the result.
3178 /// An empty iterator returns the zero value of the type.
3182 /// When calling `sum()` and a primitive integer type is being returned, this
3183 /// method will panic if the computation overflows and debug assertions are
3191 /// let a = [1, 2, 3];
3192 /// let sum: i32 = a.iter().sum();
3194 /// assert_eq!(sum, 6);
3196 #[stable(feature = "iter_arith", since = "1.11.0")]
3197 fn sum<S>(self) -> S
3205 /// Iterates over the entire iterator, multiplying all the elements
3207 /// An empty iterator returns the one value of the type.
3211 /// When calling `product()` and a primitive integer type is being returned,
3212 /// method will panic if the computation overflows and debug assertions are
3218 /// fn factorial(n: u32) -> u32 {
3219 /// (1..=n).product()
3221 /// assert_eq!(factorial(0), 1);
3222 /// assert_eq!(factorial(1), 1);
3223 /// assert_eq!(factorial(5), 120);
3225 #[stable(feature = "iter_arith", since = "1.11.0")]
3226 fn product<P>(self) -> P
3229 P: Product<Self::Item>,
3231 Product::product(self)
3234 /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
3240 /// use std::cmp::Ordering;
3242 /// assert_eq!([1].iter().cmp([1].iter()), Ordering::Equal);
3243 /// assert_eq!([1].iter().cmp([1, 2].iter()), Ordering::Less);
3244 /// assert_eq!([1, 2].iter().cmp([1].iter()), Ordering::Greater);
3246 #[stable(feature = "iter_order", since = "1.5.0")]
3247 fn cmp<I>(self, other: I) -> Ordering
3249 I: IntoIterator<Item = Self::Item>,
3253 self.cmp_by(other, |x, y| x.cmp(&y))
3256 /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
3257 /// of another with respect to the specified comparison function.
3264 /// #![feature(iter_order_by)]
3266 /// use std::cmp::Ordering;
3268 /// let xs = [1, 2, 3, 4];
3269 /// let ys = [1, 4, 9, 16];
3271 /// assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| x.cmp(&y)), Ordering::Less);
3272 /// assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| (x * x).cmp(&y)), Ordering::Equal);
3273 /// assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| (2 * x).cmp(&y)), Ordering::Greater);
3275 #[unstable(feature = "iter_order_by", issue = "64295")]
3276 fn cmp_by<I, F>(mut self, other: I, mut cmp: F) -> Ordering
3280 F: FnMut(Self::Item, I::Item) -> Ordering,
3282 let mut other = other.into_iter();
3285 let x = match self.next() {
3287 if other.next().is_none() {
3288 return Ordering::Equal;
3290 return Ordering::Less;
3296 let y = match other.next() {
3297 None => return Ordering::Greater,
3302 Ordering::Equal => (),
3303 non_eq => return non_eq,
3308 /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
3314 /// use std::cmp::Ordering;
3316 /// assert_eq!([1.].iter().partial_cmp([1.].iter()), Some(Ordering::Equal));
3317 /// assert_eq!([1.].iter().partial_cmp([1., 2.].iter()), Some(Ordering::Less));
3318 /// assert_eq!([1., 2.].iter().partial_cmp([1.].iter()), Some(Ordering::Greater));
3320 /// assert_eq!([f64::NAN].iter().partial_cmp([1.].iter()), None);
3322 #[stable(feature = "iter_order", since = "1.5.0")]
3323 fn partial_cmp<I>(self, other: I) -> Option<Ordering>
3326 Self::Item: PartialOrd<I::Item>,
3329 self.partial_cmp_by(other, |x, y| x.partial_cmp(&y))
3332 /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
3333 /// of another with respect to the specified comparison function.
3340 /// #![feature(iter_order_by)]
3342 /// use std::cmp::Ordering;
3344 /// let xs = [1.0, 2.0, 3.0, 4.0];
3345 /// let ys = [1.0, 4.0, 9.0, 16.0];
3348 /// xs.iter().partial_cmp_by(&ys, |&x, &y| x.partial_cmp(&y)),
3349 /// Some(Ordering::Less)
3352 /// xs.iter().partial_cmp_by(&ys, |&x, &y| (x * x).partial_cmp(&y)),
3353 /// Some(Ordering::Equal)
3356 /// xs.iter().partial_cmp_by(&ys, |&x, &y| (2.0 * x).partial_cmp(&y)),
3357 /// Some(Ordering::Greater)
3360 #[unstable(feature = "iter_order_by", issue = "64295")]
3361 fn partial_cmp_by<I, F>(mut self, other: I, mut partial_cmp: F) -> Option<Ordering>
3365 F: FnMut(Self::Item, I::Item) -> Option<Ordering>,
3367 let mut other = other.into_iter();
3370 let x = match self.next() {
3372 if other.next().is_none() {
3373 return Some(Ordering::Equal);
3375 return Some(Ordering::Less);
3381 let y = match other.next() {
3382 None => return Some(Ordering::Greater),
3386 match partial_cmp(x, y) {
3387 Some(Ordering::Equal) => (),
3388 non_eq => return non_eq,
3393 /// Determines if the elements of this [`Iterator`] are equal to those of
3399 /// assert_eq!([1].iter().eq([1].iter()), true);
3400 /// assert_eq!([1].iter().eq([1, 2].iter()), false);
3402 #[stable(feature = "iter_order", since = "1.5.0")]
3403 fn eq<I>(self, other: I) -> bool
3406 Self::Item: PartialEq<I::Item>,
3409 self.eq_by(other, |x, y| x == y)
3412 /// Determines if the elements of this [`Iterator`] are equal to those of
3413 /// another with respect to the specified equality function.
3420 /// #![feature(iter_order_by)]
3422 /// let xs = [1, 2, 3, 4];
3423 /// let ys = [1, 4, 9, 16];
3425 /// assert!(xs.iter().eq_by(&ys, |&x, &y| x * x == y));
3427 #[unstable(feature = "iter_order_by", issue = "64295")]
3428 fn eq_by<I, F>(mut self, other: I, mut eq: F) -> bool
3432 F: FnMut(Self::Item, I::Item) -> bool,
3434 let mut other = other.into_iter();
3437 let x = match self.next() {
3438 None => return other.next().is_none(),
3442 let y = match other.next() {
3443 None => return false,
3453 /// Determines if the elements of this [`Iterator`] are unequal to those of
3459 /// assert_eq!([1].iter().ne([1].iter()), false);
3460 /// assert_eq!([1].iter().ne([1, 2].iter()), true);
3462 #[stable(feature = "iter_order", since = "1.5.0")]
3463 fn ne<I>(self, other: I) -> bool
3466 Self::Item: PartialEq<I::Item>,
3472 /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
3473 /// less than those of another.
3478 /// assert_eq!([1].iter().lt([1].iter()), false);
3479 /// assert_eq!([1].iter().lt([1, 2].iter()), true);
3480 /// assert_eq!([1, 2].iter().lt([1].iter()), false);
3481 /// assert_eq!([1, 2].iter().lt([1, 2].iter()), false);
3483 #[stable(feature = "iter_order", since = "1.5.0")]
3484 fn lt<I>(self, other: I) -> bool
3487 Self::Item: PartialOrd<I::Item>,
3490 self.partial_cmp(other) == Some(Ordering::Less)
3493 /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
3494 /// less or equal to those of another.
3499 /// assert_eq!([1].iter().le([1].iter()), true);
3500 /// assert_eq!([1].iter().le([1, 2].iter()), true);
3501 /// assert_eq!([1, 2].iter().le([1].iter()), false);
3502 /// assert_eq!([1, 2].iter().le([1, 2].iter()), true);
3504 #[stable(feature = "iter_order", since = "1.5.0")]
3505 fn le<I>(self, other: I) -> bool
3508 Self::Item: PartialOrd<I::Item>,
3511 matches!(self.partial_cmp(other), Some(Ordering::Less | Ordering::Equal))
3514 /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
3515 /// greater than those of another.
3520 /// assert_eq!([1].iter().gt([1].iter()), false);
3521 /// assert_eq!([1].iter().gt([1, 2].iter()), false);
3522 /// assert_eq!([1, 2].iter().gt([1].iter()), true);
3523 /// assert_eq!([1, 2].iter().gt([1, 2].iter()), false);
3525 #[stable(feature = "iter_order", since = "1.5.0")]
3526 fn gt<I>(self, other: I) -> bool
3529 Self::Item: PartialOrd<I::Item>,
3532 self.partial_cmp(other) == Some(Ordering::Greater)
3535 /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
3536 /// greater than or equal to those of another.
3541 /// assert_eq!([1].iter().ge([1].iter()), true);
3542 /// assert_eq!([1].iter().ge([1, 2].iter()), false);
3543 /// assert_eq!([1, 2].iter().ge([1].iter()), true);
3544 /// assert_eq!([1, 2].iter().ge([1, 2].iter()), true);
3546 #[stable(feature = "iter_order", since = "1.5.0")]
3547 fn ge<I>(self, other: I) -> bool
3550 Self::Item: PartialOrd<I::Item>,
3553 matches!(self.partial_cmp(other), Some(Ordering::Greater | Ordering::Equal))
3556 /// Checks if the elements of this iterator are sorted.
3558 /// That is, for each element `a` and its following element `b`, `a <= b` must hold. If the
3559 /// iterator yields exactly zero or one element, `true` is returned.
3561 /// Note that if `Self::Item` is only `PartialOrd`, but not `Ord`, the above definition
3562 /// implies that this function returns `false` if any two consecutive items are not
3568 /// #![feature(is_sorted)]
3570 /// assert!([1, 2, 2, 9].iter().is_sorted());
3571 /// assert!(![1, 3, 2, 4].iter().is_sorted());
3572 /// assert!([0].iter().is_sorted());
3573 /// assert!(std::iter::empty::<i32>().is_sorted());
3574 /// assert!(![0.0, 1.0, f32::NAN].iter().is_sorted());
3577 #[unstable(feature = "is_sorted", reason = "new API", issue = "53485")]
3578 fn is_sorted(self) -> bool
3581 Self::Item: PartialOrd,
3583 self.is_sorted_by(PartialOrd::partial_cmp)
3586 /// Checks if the elements of this iterator are sorted using the given comparator function.
3588 /// Instead of using `PartialOrd::partial_cmp`, this function uses the given `compare`
3589 /// function to determine the ordering of two elements. Apart from that, it's equivalent to
3590 /// [`is_sorted`]; see its documentation for more information.
3595 /// #![feature(is_sorted)]
3597 /// assert!([1, 2, 2, 9].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
3598 /// assert!(![1, 3, 2, 4].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
3599 /// assert!([0].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
3600 /// assert!(std::iter::empty::<i32>().is_sorted_by(|a, b| a.partial_cmp(b)));
3601 /// assert!(![0.0, 1.0, f32::NAN].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
3604 /// [`is_sorted`]: Iterator::is_sorted
3605 #[unstable(feature = "is_sorted", reason = "new API", issue = "53485")]
3606 fn is_sorted_by<F>(mut self, compare: F) -> bool
3609 F: FnMut(&Self::Item, &Self::Item) -> Option<Ordering>,
3614 mut compare: impl FnMut(&T, &T) -> Option<Ordering> + 'a,
3615 ) -> impl FnMut(T) -> bool + 'a {
3617 if let Some(Ordering::Greater) | None = compare(&last, &curr) {
3625 let mut last = match self.next() {
3627 None => return true,
3630 self.all(check(&mut last, compare))
3633 /// Checks if the elements of this iterator are sorted using the given key extraction
3636 /// Instead of comparing the iterator's elements directly, this function compares the keys of
3637 /// the elements, as determined by `f`. Apart from that, it's equivalent to [`is_sorted`]; see
3638 /// its documentation for more information.
3640 /// [`is_sorted`]: Iterator::is_sorted
3645 /// #![feature(is_sorted)]
3647 /// assert!(["c", "bb", "aaa"].iter().is_sorted_by_key(|s| s.len()));
3648 /// assert!(![-2i32, -1, 0, 3].iter().is_sorted_by_key(|n| n.abs()));
3651 #[unstable(feature = "is_sorted", reason = "new API", issue = "53485")]
3652 fn is_sorted_by_key<F, K>(self, f: F) -> bool
3655 F: FnMut(Self::Item) -> K,
3658 self.map(f).is_sorted()
3661 /// See [TrustedRandomAccess][super::super::TrustedRandomAccess]
3662 // The unusual name is to avoid name collisions in method resolution
3666 #[unstable(feature = "trusted_random_access", issue = "none")]
3667 unsafe fn __iterator_get_unchecked(&mut self, _idx: usize) -> Self::Item
3669 Self: TrustedRandomAccessNoCoerce,
3671 unreachable!("Always specialized");
3675 #[stable(feature = "rust1", since = "1.0.0")]
3676 impl<I: Iterator + ?Sized> Iterator for &mut I {
3677 type Item = I::Item;
3679 fn next(&mut self) -> Option<I::Item> {
3682 fn size_hint(&self) -> (usize, Option<usize>) {
3683 (**self).size_hint()
3685 fn advance_by(&mut self, n: usize) -> Result<(), usize> {
3686 (**self).advance_by(n)
3688 fn nth(&mut self, n: usize) -> Option<Self::Item> {