1 use crate::cmp::{self, Ordering};
2 use crate::ops::{ControlFlow, Try};
4 use super::super::TrustedRandomAccessNoCoerce;
5 use super::super::{Chain, Cloned, Copied, Cycle, Enumerate, Filter, FilterMap, Fuse};
6 use super::super::{FlatMap, Flatten};
7 use super::super::{FromIterator, Intersperse, IntersperseWith, Product, Sum, Zip};
9 Inspect, Map, MapWhile, Peekable, Rev, Scan, Skip, SkipWhile, StepBy, Take, TakeWhile,
12 fn _assert_is_object_safe(_: &dyn Iterator<Item = ()>) {}
14 /// An interface for dealing with iterators.
16 /// This is the main iterator trait. For more about the concept of iterators
17 /// generally, please see the [module-level documentation]. In particular, you
18 /// may want to know how to [implement `Iterator`][impl].
20 /// [module-level documentation]: crate::iter
21 /// [impl]: crate::iter#implementing-iterator
22 #[stable(feature = "rust1", since = "1.0.0")]
23 #[rustc_on_unimplemented(
25 _Self = "std::ops::RangeTo<Idx>",
26 label = "if you meant to iterate until a value, add a starting value",
27 note = "`..end` is a `RangeTo`, which cannot be iterated on; you might have meant to have a \
28 bounded `Range`: `0..end`"
31 _Self = "std::ops::RangeToInclusive<Idx>",
32 label = "if you meant to iterate until a value (including it), add a starting value",
33 note = "`..=end` is a `RangeToInclusive`, which cannot be iterated on; you might have meant \
34 to have a bounded `RangeInclusive`: `0..=end`"
38 label = "`{Self}` is not an iterator; try calling `.chars()` or `.bytes()`"
41 _Self = "std::string::String",
42 label = "`{Self}` is not an iterator; try calling `.chars()` or `.bytes()`"
46 note = "if you want to iterate between `start` until a value `end`, use the exclusive range \
47 syntax `start..end` or the inclusive range syntax `start..=end`"
49 label = "`{Self}` is not an iterator",
50 message = "`{Self}` is not an iterator"
53 #[rustc_diagnostic_item = "Iterator"]
54 #[must_use = "iterators are lazy and do nothing unless consumed"]
56 /// The type of the elements being iterated over.
57 #[stable(feature = "rust1", since = "1.0.0")]
60 /// Advances the iterator and returns the next value.
62 /// Returns [`None`] when iteration is finished. Individual iterator
63 /// implementations may choose to resume iteration, and so calling `next()`
64 /// again may or may not eventually start returning [`Some(Item)`] again at some
67 /// [`Some(Item)`]: Some
74 /// let a = [1, 2, 3];
76 /// let mut iter = a.iter();
78 /// // A call to next() returns the next value...
79 /// assert_eq!(Some(&1), iter.next());
80 /// assert_eq!(Some(&2), iter.next());
81 /// assert_eq!(Some(&3), iter.next());
83 /// // ... and then None once it's over.
84 /// assert_eq!(None, iter.next());
86 /// // More calls may or may not return `None`. Here, they always will.
87 /// assert_eq!(None, iter.next());
88 /// assert_eq!(None, iter.next());
91 #[stable(feature = "rust1", since = "1.0.0")]
92 fn next(&mut self) -> Option<Self::Item>;
94 /// Returns the bounds on the remaining length of the iterator.
96 /// Specifically, `size_hint()` returns a tuple where the first element
97 /// is the lower bound, and the second element is the upper bound.
99 /// The second half of the tuple that is returned is an [`Option`]`<`[`usize`]`>`.
100 /// A [`None`] here means that either there is no known upper bound, or the
101 /// upper bound is larger than [`usize`].
103 /// # Implementation notes
105 /// It is not enforced that an iterator implementation yields the declared
106 /// number of elements. A buggy iterator may yield less than the lower bound
107 /// or more than the upper bound of elements.
109 /// `size_hint()` is primarily intended to be used for optimizations such as
110 /// reserving space for the elements of the iterator, but must not be
111 /// trusted to e.g., omit bounds checks in unsafe code. An incorrect
112 /// implementation of `size_hint()` should not lead to memory safety
115 /// That said, the implementation should provide a correct estimation,
116 /// because otherwise it would be a violation of the trait's protocol.
118 /// The default implementation returns `(0, `[`None`]`)` which is correct for any
121 /// [`usize`]: type@usize
128 /// let a = [1, 2, 3];
129 /// let iter = a.iter();
131 /// assert_eq!((3, Some(3)), iter.size_hint());
134 /// A more complex example:
137 /// // The even numbers in the range of zero to nine.
138 /// let iter = (0..10).filter(|x| x % 2 == 0);
140 /// // We might iterate from zero to ten times. Knowing that it's five
141 /// // exactly wouldn't be possible without executing filter().
142 /// assert_eq!((0, Some(10)), iter.size_hint());
144 /// // Let's add five more numbers with chain()
145 /// let iter = (0..10).filter(|x| x % 2 == 0).chain(15..20);
147 /// // now both bounds are increased by five
148 /// assert_eq!((5, Some(15)), iter.size_hint());
151 /// Returning `None` for an upper bound:
154 /// // an infinite iterator has no upper bound
155 /// // and the maximum possible lower bound
158 /// assert_eq!((usize::MAX, None), iter.size_hint());
161 #[stable(feature = "rust1", since = "1.0.0")]
162 fn size_hint(&self) -> (usize, Option<usize>) {
166 /// Consumes the iterator, counting the number of iterations and returning it.
168 /// This method will call [`next`] repeatedly until [`None`] is encountered,
169 /// returning the number of times it saw [`Some`]. Note that [`next`] has to be
170 /// called at least once even if the iterator does not have any elements.
172 /// [`next`]: Iterator::next
174 /// # Overflow Behavior
176 /// The method does no guarding against overflows, so counting elements of
177 /// an iterator with more than [`usize::MAX`] elements either produces the
178 /// wrong result or panics. If debug assertions are enabled, a panic is
183 /// This function might panic if the iterator has more than [`usize::MAX`]
191 /// let a = [1, 2, 3];
192 /// assert_eq!(a.iter().count(), 3);
194 /// let a = [1, 2, 3, 4, 5];
195 /// assert_eq!(a.iter().count(), 5);
198 #[stable(feature = "rust1", since = "1.0.0")]
199 fn count(self) -> usize
205 #[rustc_inherit_overflow_checks]
206 |count, _| count + 1,
210 /// Consumes the iterator, returning the last element.
212 /// This method will evaluate the iterator until it returns [`None`]. While
213 /// doing so, it keeps track of the current element. After [`None`] is
214 /// returned, `last()` will then return the last element it saw.
221 /// let a = [1, 2, 3];
222 /// assert_eq!(a.iter().last(), Some(&3));
224 /// let a = [1, 2, 3, 4, 5];
225 /// assert_eq!(a.iter().last(), Some(&5));
228 #[stable(feature = "rust1", since = "1.0.0")]
229 fn last(self) -> Option<Self::Item>
234 fn some<T>(_: Option<T>, x: T) -> Option<T> {
238 self.fold(None, some)
241 /// Advances the iterator by `n` elements.
243 /// This method will eagerly skip `n` elements by calling [`next`] up to `n`
244 /// times until [`None`] is encountered.
246 /// `advance_by(n)` will return [`Ok(())`][Ok] if the iterator successfully advances by
247 /// `n` elements, or [`Err(k)`][Err] if [`None`] is encountered, where `k` is the number
248 /// of elements the iterator is advanced by before running out of elements (i.e. the
249 /// length of the iterator). Note that `k` is always less than `n`.
251 /// Calling `advance_by(0)` does not consume any elements and always returns [`Ok(())`][Ok].
253 /// [`next`]: Iterator::next
260 /// #![feature(iter_advance_by)]
262 /// let a = [1, 2, 3, 4];
263 /// let mut iter = a.iter();
265 /// assert_eq!(iter.advance_by(2), Ok(()));
266 /// assert_eq!(iter.next(), Some(&3));
267 /// assert_eq!(iter.advance_by(0), Ok(()));
268 /// assert_eq!(iter.advance_by(100), Err(1)); // only `&4` was skipped
271 #[unstable(feature = "iter_advance_by", reason = "recently added", issue = "77404")]
272 fn advance_by(&mut self, n: usize) -> Result<(), usize> {
274 self.next().ok_or(i)?;
279 /// Returns the `n`th element of the iterator.
281 /// Like most indexing operations, the count starts from zero, so `nth(0)`
282 /// returns the first value, `nth(1)` the second, and so on.
284 /// Note that all preceding elements, as well as the returned element, will be
285 /// consumed from the iterator. That means that the preceding elements will be
286 /// discarded, and also that calling `nth(0)` multiple times on the same iterator
287 /// will return different elements.
289 /// `nth()` will return [`None`] if `n` is greater than or equal to the length of the
297 /// let a = [1, 2, 3];
298 /// assert_eq!(a.iter().nth(1), Some(&2));
301 /// Calling `nth()` multiple times doesn't rewind the iterator:
304 /// let a = [1, 2, 3];
306 /// let mut iter = a.iter();
308 /// assert_eq!(iter.nth(1), Some(&2));
309 /// assert_eq!(iter.nth(1), None);
312 /// Returning `None` if there are less than `n + 1` elements:
315 /// let a = [1, 2, 3];
316 /// assert_eq!(a.iter().nth(10), None);
319 #[stable(feature = "rust1", since = "1.0.0")]
320 fn nth(&mut self, n: usize) -> Option<Self::Item> {
321 self.advance_by(n).ok()?;
325 /// Creates an iterator starting at the same point, but stepping by
326 /// the given amount at each iteration.
328 /// Note 1: The first element of the iterator will always be returned,
329 /// regardless of the step given.
331 /// Note 2: The time at which ignored elements are pulled is not fixed.
332 /// `StepBy` behaves like the sequence `self.next()`, `self.nth(step-1)`,
333 /// `self.nth(step-1)`, …, but is also free to behave like the sequence
334 /// `advance_n_and_return_first(&mut self, step)`,
335 /// `advance_n_and_return_first(&mut self, step)`, …
336 /// Which way is used may change for some iterators for performance reasons.
337 /// The second way will advance the iterator earlier and may consume more items.
339 /// `advance_n_and_return_first` is the equivalent of:
341 /// fn advance_n_and_return_first<I>(iter: &mut I, n: usize) -> Option<I::Item>
345 /// let next = iter.next();
355 /// The method will panic if the given step is `0`.
362 /// let a = [0, 1, 2, 3, 4, 5];
363 /// let mut iter = a.iter().step_by(2);
365 /// assert_eq!(iter.next(), Some(&0));
366 /// assert_eq!(iter.next(), Some(&2));
367 /// assert_eq!(iter.next(), Some(&4));
368 /// assert_eq!(iter.next(), None);
371 #[stable(feature = "iterator_step_by", since = "1.28.0")]
372 fn step_by(self, step: usize) -> StepBy<Self>
376 StepBy::new(self, step)
379 /// Takes two iterators and creates a new iterator over both in sequence.
381 /// `chain()` will return a new iterator which will first iterate over
382 /// values from the first iterator and then over values from the second
385 /// In other words, it links two iterators together, in a chain. 🔗
387 /// [`once`] is commonly used to adapt a single value into a chain of
388 /// other kinds of iteration.
395 /// let a1 = [1, 2, 3];
396 /// let a2 = [4, 5, 6];
398 /// let mut iter = a1.iter().chain(a2.iter());
400 /// assert_eq!(iter.next(), Some(&1));
401 /// assert_eq!(iter.next(), Some(&2));
402 /// assert_eq!(iter.next(), Some(&3));
403 /// assert_eq!(iter.next(), Some(&4));
404 /// assert_eq!(iter.next(), Some(&5));
405 /// assert_eq!(iter.next(), Some(&6));
406 /// assert_eq!(iter.next(), None);
409 /// Since the argument to `chain()` uses [`IntoIterator`], we can pass
410 /// anything that can be converted into an [`Iterator`], not just an
411 /// [`Iterator`] itself. For example, slices (`&[T]`) implement
412 /// [`IntoIterator`], and so can be passed to `chain()` directly:
415 /// let s1 = &[1, 2, 3];
416 /// let s2 = &[4, 5, 6];
418 /// let mut iter = s1.iter().chain(s2);
420 /// assert_eq!(iter.next(), Some(&1));
421 /// assert_eq!(iter.next(), Some(&2));
422 /// assert_eq!(iter.next(), Some(&3));
423 /// assert_eq!(iter.next(), Some(&4));
424 /// assert_eq!(iter.next(), Some(&5));
425 /// assert_eq!(iter.next(), Some(&6));
426 /// assert_eq!(iter.next(), None);
429 /// If you work with Windows API, you may wish to convert [`OsStr`] to `Vec<u16>`:
433 /// fn os_str_to_utf16(s: &std::ffi::OsStr) -> Vec<u16> {
434 /// use std::os::windows::ffi::OsStrExt;
435 /// s.encode_wide().chain(std::iter::once(0)).collect()
439 /// [`once`]: crate::iter::once
440 /// [`OsStr`]: ../../std/ffi/struct.OsStr.html
442 #[stable(feature = "rust1", since = "1.0.0")]
443 fn chain<U>(self, other: U) -> Chain<Self, U::IntoIter>
446 U: IntoIterator<Item = Self::Item>,
448 Chain::new(self, other.into_iter())
451 /// 'Zips up' two iterators into a single iterator of pairs.
453 /// `zip()` returns a new iterator that will iterate over two other
454 /// iterators, returning a tuple where the first element comes from the
455 /// first iterator, and the second element comes from the second iterator.
457 /// In other words, it zips two iterators together, into a single one.
459 /// If either iterator returns [`None`], [`next`] from the zipped iterator
460 /// will return [`None`]. If the first iterator returns [`None`], `zip` will
461 /// short-circuit and `next` will not be called on the second iterator.
468 /// let a1 = [1, 2, 3];
469 /// let a2 = [4, 5, 6];
471 /// let mut iter = a1.iter().zip(a2.iter());
473 /// assert_eq!(iter.next(), Some((&1, &4)));
474 /// assert_eq!(iter.next(), Some((&2, &5)));
475 /// assert_eq!(iter.next(), Some((&3, &6)));
476 /// assert_eq!(iter.next(), None);
479 /// Since the argument to `zip()` uses [`IntoIterator`], we can pass
480 /// anything that can be converted into an [`Iterator`], not just an
481 /// [`Iterator`] itself. For example, slices (`&[T]`) implement
482 /// [`IntoIterator`], and so can be passed to `zip()` directly:
485 /// let s1 = &[1, 2, 3];
486 /// let s2 = &[4, 5, 6];
488 /// let mut iter = s1.iter().zip(s2);
490 /// assert_eq!(iter.next(), Some((&1, &4)));
491 /// assert_eq!(iter.next(), Some((&2, &5)));
492 /// assert_eq!(iter.next(), Some((&3, &6)));
493 /// assert_eq!(iter.next(), None);
496 /// `zip()` is often used to zip an infinite iterator to a finite one.
497 /// This works because the finite iterator will eventually return [`None`],
498 /// ending the zipper. Zipping with `(0..)` can look a lot like [`enumerate`]:
501 /// let enumerate: Vec<_> = "foo".chars().enumerate().collect();
503 /// let zipper: Vec<_> = (0..).zip("foo".chars()).collect();
505 /// assert_eq!((0, 'f'), enumerate[0]);
506 /// assert_eq!((0, 'f'), zipper[0]);
508 /// assert_eq!((1, 'o'), enumerate[1]);
509 /// assert_eq!((1, 'o'), zipper[1]);
511 /// assert_eq!((2, 'o'), enumerate[2]);
512 /// assert_eq!((2, 'o'), zipper[2]);
515 /// [`enumerate`]: Iterator::enumerate
516 /// [`next`]: Iterator::next
518 #[stable(feature = "rust1", since = "1.0.0")]
519 fn zip<U>(self, other: U) -> Zip<Self, U::IntoIter>
524 Zip::new(self, other.into_iter())
527 /// Creates a new iterator which places a copy of `separator` between adjacent
528 /// items of the original iterator.
530 /// In case `separator` does not implement [`Clone`] or needs to be
531 /// computed every time, use [`intersperse_with`].
538 /// let mut a = [0, 1, 2].iter().intersperse(&100);
539 /// assert_eq!(a.next(), Some(&0)); // The first element from `a`.
540 /// assert_eq!(a.next(), Some(&100)); // The separator.
541 /// assert_eq!(a.next(), Some(&1)); // The next element from `a`.
542 /// assert_eq!(a.next(), Some(&100)); // The separator.
543 /// assert_eq!(a.next(), Some(&2)); // The last element from `a`.
544 /// assert_eq!(a.next(), None); // The iterator is finished.
547 /// `intersperse` can be very useful to join an iterator's items using a common element:
550 /// let hello = ["Hello", "World", "!"].iter().copied().intersperse(" ").collect::<String>();
551 /// assert_eq!(hello, "Hello World !");
554 /// [`Clone`]: crate::clone::Clone
555 /// [`intersperse_with`]: Iterator::intersperse_with
557 #[stable(feature = "iter_intersperse", since = "1.56.0")]
558 fn intersperse(self, separator: Self::Item) -> Intersperse<Self>
563 Intersperse::new(self, separator)
566 /// Creates a new iterator which places an item generated by `separator`
567 /// between adjacent items of the original iterator.
569 /// The closure will be called exactly once each time an item is placed
570 /// between two adjacent items from the underlying iterator; specifically,
571 /// the closure is not called if the underlying iterator yields less than
572 /// two items and after the last item is yielded.
574 /// If the iterator's item implements [`Clone`], it may be easier to use
582 /// #[derive(PartialEq, Debug)]
583 /// struct NotClone(usize);
585 /// let v = vec![NotClone(0), NotClone(1), NotClone(2)];
586 /// let mut it = v.into_iter().intersperse_with(|| NotClone(99));
588 /// assert_eq!(it.next(), Some(NotClone(0))); // The first element from `v`.
589 /// assert_eq!(it.next(), Some(NotClone(99))); // The separator.
590 /// assert_eq!(it.next(), Some(NotClone(1))); // The next element from `v`.
591 /// assert_eq!(it.next(), Some(NotClone(99))); // The separator.
592 /// assert_eq!(it.next(), Some(NotClone(2))); // The last element from from `v`.
593 /// assert_eq!(it.next(), None); // The iterator is finished.
596 /// `intersperse_with` can be used in situations where the separator needs
600 /// let src = ["Hello", "to", "all", "people", "!!"].iter().copied();
602 /// // The closure mutably borrows its context to generate an item.
603 /// let mut happy_emojis = [" ❤️ ", " 😀 "].iter().copied();
604 /// let separator = || happy_emojis.next().unwrap_or(" 🦀 ");
606 /// let result = src.intersperse_with(separator).collect::<String>();
607 /// assert_eq!(result, "Hello ❤️ to 😀 all 🦀 people 🦀 !!");
609 /// [`Clone`]: crate::clone::Clone
610 /// [`intersperse`]: Iterator::intersperse
612 #[stable(feature = "iter_intersperse", since = "1.56.0")]
613 fn intersperse_with<G>(self, separator: G) -> IntersperseWith<Self, G>
616 G: FnMut() -> Self::Item,
618 IntersperseWith::new(self, separator)
621 /// Takes a closure and creates an iterator which calls that closure on each
624 /// `map()` transforms one iterator into another, by means of its argument:
625 /// something that implements [`FnMut`]. It produces a new iterator which
626 /// calls this closure on each element of the original iterator.
628 /// If you are good at thinking in types, you can think of `map()` like this:
629 /// If you have an iterator that gives you elements of some type `A`, and
630 /// you want an iterator of some other type `B`, you can use `map()`,
631 /// passing a closure that takes an `A` and returns a `B`.
633 /// `map()` is conceptually similar to a [`for`] loop. However, as `map()` is
634 /// lazy, it is best used when you're already working with other iterators.
635 /// If you're doing some sort of looping for a side effect, it's considered
636 /// more idiomatic to use [`for`] than `map()`.
638 /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
639 /// [`FnMut`]: crate::ops::FnMut
646 /// let a = [1, 2, 3];
648 /// let mut iter = a.iter().map(|x| 2 * x);
650 /// assert_eq!(iter.next(), Some(2));
651 /// assert_eq!(iter.next(), Some(4));
652 /// assert_eq!(iter.next(), Some(6));
653 /// assert_eq!(iter.next(), None);
656 /// If you're doing some sort of side effect, prefer [`for`] to `map()`:
659 /// # #![allow(unused_must_use)]
660 /// // don't do this:
661 /// (0..5).map(|x| println!("{}", x));
663 /// // it won't even execute, as it is lazy. Rust will warn you about this.
665 /// // Instead, use for:
667 /// println!("{}", x);
671 #[stable(feature = "rust1", since = "1.0.0")]
672 fn map<B, F>(self, f: F) -> Map<Self, F>
675 F: FnMut(Self::Item) -> B,
680 /// Calls a closure on each element of an iterator.
682 /// This is equivalent to using a [`for`] loop on the iterator, although
683 /// `break` and `continue` are not possible from a closure. It's generally
684 /// more idiomatic to use a `for` loop, but `for_each` may be more legible
685 /// when processing items at the end of longer iterator chains. In some
686 /// cases `for_each` may also be faster than a loop, because it will use
687 /// internal iteration on adapters like `Chain`.
689 /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
696 /// use std::sync::mpsc::channel;
698 /// let (tx, rx) = channel();
699 /// (0..5).map(|x| x * 2 + 1)
700 /// .for_each(move |x| tx.send(x).unwrap());
702 /// let v: Vec<_> = rx.iter().collect();
703 /// assert_eq!(v, vec![1, 3, 5, 7, 9]);
706 /// For such a small example, a `for` loop may be cleaner, but `for_each`
707 /// might be preferable to keep a functional style with longer iterators:
710 /// (0..5).flat_map(|x| x * 100 .. x * 110)
712 /// .filter(|&(i, x)| (i + x) % 3 == 0)
713 /// .for_each(|(i, x)| println!("{}:{}", i, x));
716 #[stable(feature = "iterator_for_each", since = "1.21.0")]
717 fn for_each<F>(self, f: F)
720 F: FnMut(Self::Item),
723 fn call<T>(mut f: impl FnMut(T)) -> impl FnMut((), T) {
724 move |(), item| f(item)
727 self.fold((), call(f));
730 /// Creates an iterator which uses a closure to determine if an element
731 /// should be yielded.
733 /// Given an element the closure must return `true` or `false`. The returned
734 /// iterator will yield only the elements for which the closure returns
742 /// let a = [0i32, 1, 2];
744 /// let mut iter = a.iter().filter(|x| x.is_positive());
746 /// assert_eq!(iter.next(), Some(&1));
747 /// assert_eq!(iter.next(), Some(&2));
748 /// assert_eq!(iter.next(), None);
751 /// Because the closure passed to `filter()` takes a reference, and many
752 /// iterators iterate over references, this leads to a possibly confusing
753 /// situation, where the type of the closure is a double reference:
756 /// let a = [0, 1, 2];
758 /// let mut iter = a.iter().filter(|x| **x > 1); // need two *s!
760 /// assert_eq!(iter.next(), Some(&2));
761 /// assert_eq!(iter.next(), None);
764 /// It's common to instead use destructuring on the argument to strip away
768 /// let a = [0, 1, 2];
770 /// let mut iter = a.iter().filter(|&x| *x > 1); // both & and *
772 /// assert_eq!(iter.next(), Some(&2));
773 /// assert_eq!(iter.next(), None);
779 /// let a = [0, 1, 2];
781 /// let mut iter = a.iter().filter(|&&x| x > 1); // two &s
783 /// assert_eq!(iter.next(), Some(&2));
784 /// assert_eq!(iter.next(), None);
789 /// Note that `iter.filter(f).next()` is equivalent to `iter.find(f)`.
791 #[stable(feature = "rust1", since = "1.0.0")]
792 fn filter<P>(self, predicate: P) -> Filter<Self, P>
795 P: FnMut(&Self::Item) -> bool,
797 Filter::new(self, predicate)
800 /// Creates an iterator that both filters and maps.
802 /// The returned iterator yields only the `value`s for which the supplied
803 /// closure returns `Some(value)`.
805 /// `filter_map` can be used to make chains of [`filter`] and [`map`] more
806 /// concise. The example below shows how a `map().filter().map()` can be
807 /// shortened to a single call to `filter_map`.
809 /// [`filter`]: Iterator::filter
810 /// [`map`]: Iterator::map
817 /// let a = ["1", "two", "NaN", "four", "5"];
819 /// let mut iter = a.iter().filter_map(|s| s.parse().ok());
821 /// assert_eq!(iter.next(), Some(1));
822 /// assert_eq!(iter.next(), Some(5));
823 /// assert_eq!(iter.next(), None);
826 /// Here's the same example, but with [`filter`] and [`map`]:
829 /// let a = ["1", "two", "NaN", "four", "5"];
830 /// let mut iter = a.iter().map(|s| s.parse()).filter(|s| s.is_ok()).map(|s| s.unwrap());
831 /// assert_eq!(iter.next(), Some(1));
832 /// assert_eq!(iter.next(), Some(5));
833 /// assert_eq!(iter.next(), None);
836 #[stable(feature = "rust1", since = "1.0.0")]
837 fn filter_map<B, F>(self, f: F) -> FilterMap<Self, F>
840 F: FnMut(Self::Item) -> Option<B>,
842 FilterMap::new(self, f)
845 /// Creates an iterator which gives the current iteration count as well as
848 /// The iterator returned yields pairs `(i, val)`, where `i` is the
849 /// current index of iteration and `val` is the value returned by the
852 /// `enumerate()` keeps its count as a [`usize`]. If you want to count by a
853 /// different sized integer, the [`zip`] function provides similar
856 /// # Overflow Behavior
858 /// The method does no guarding against overflows, so enumerating more than
859 /// [`usize::MAX`] elements either produces the wrong result or panics. If
860 /// debug assertions are enabled, a panic is guaranteed.
864 /// The returned iterator might panic if the to-be-returned index would
865 /// overflow a [`usize`].
867 /// [`usize`]: type@usize
868 /// [`zip`]: Iterator::zip
873 /// let a = ['a', 'b', 'c'];
875 /// let mut iter = a.iter().enumerate();
877 /// assert_eq!(iter.next(), Some((0, &'a')));
878 /// assert_eq!(iter.next(), Some((1, &'b')));
879 /// assert_eq!(iter.next(), Some((2, &'c')));
880 /// assert_eq!(iter.next(), None);
883 #[stable(feature = "rust1", since = "1.0.0")]
884 fn enumerate(self) -> Enumerate<Self>
891 /// Creates an iterator which can use the [`peek`] and [`peek_mut`] methods
892 /// to look at the next element of the iterator without consuming it. See
893 /// their documentation for more information.
895 /// Note that the underlying iterator is still advanced when [`peek`] or
896 /// [`peek_mut`] are called for the first time: In order to retrieve the
897 /// next element, [`next`] is called on the underlying iterator, hence any
898 /// side effects (i.e. anything other than fetching the next value) of
899 /// the [`next`] method will occur.
907 /// let xs = [1, 2, 3];
909 /// let mut iter = xs.iter().peekable();
911 /// // peek() lets us see into the future
912 /// assert_eq!(iter.peek(), Some(&&1));
913 /// assert_eq!(iter.next(), Some(&1));
915 /// assert_eq!(iter.next(), Some(&2));
917 /// // we can peek() multiple times, the iterator won't advance
918 /// assert_eq!(iter.peek(), Some(&&3));
919 /// assert_eq!(iter.peek(), Some(&&3));
921 /// assert_eq!(iter.next(), Some(&3));
923 /// // after the iterator is finished, so is peek()
924 /// assert_eq!(iter.peek(), None);
925 /// assert_eq!(iter.next(), None);
928 /// Using [`peek_mut`] to mutate the next item without advancing the
932 /// let xs = [1, 2, 3];
934 /// let mut iter = xs.iter().peekable();
936 /// // `peek_mut()` lets us see into the future
937 /// assert_eq!(iter.peek_mut(), Some(&mut &1));
938 /// assert_eq!(iter.peek_mut(), Some(&mut &1));
939 /// assert_eq!(iter.next(), Some(&1));
941 /// if let Some(mut p) = iter.peek_mut() {
942 /// assert_eq!(*p, &2);
943 /// // put a value into the iterator
947 /// // The value reappears as the iterator continues
948 /// assert_eq!(iter.collect::<Vec<_>>(), vec![&1000, &3]);
950 /// [`peek`]: Peekable::peek
951 /// [`peek_mut`]: Peekable::peek_mut
952 /// [`next`]: Iterator::next
954 #[stable(feature = "rust1", since = "1.0.0")]
955 fn peekable(self) -> Peekable<Self>
962 /// Creates an iterator that [`skip`]s elements based on a predicate.
964 /// [`skip`]: Iterator::skip
966 /// `skip_while()` takes a closure as an argument. It will call this
967 /// closure on each element of the iterator, and ignore elements
968 /// until it returns `false`.
970 /// After `false` is returned, `skip_while()`'s job is over, and the
971 /// rest of the elements are yielded.
978 /// let a = [-1i32, 0, 1];
980 /// let mut iter = a.iter().skip_while(|x| x.is_negative());
982 /// assert_eq!(iter.next(), Some(&0));
983 /// assert_eq!(iter.next(), Some(&1));
984 /// assert_eq!(iter.next(), None);
987 /// Because the closure passed to `skip_while()` takes a reference, and many
988 /// iterators iterate over references, this leads to a possibly confusing
989 /// situation, where the type of the closure argument is a double reference:
992 /// let a = [-1, 0, 1];
994 /// let mut iter = a.iter().skip_while(|x| **x < 0); // need two *s!
996 /// assert_eq!(iter.next(), Some(&0));
997 /// assert_eq!(iter.next(), Some(&1));
998 /// assert_eq!(iter.next(), None);
1001 /// Stopping after an initial `false`:
1004 /// let a = [-1, 0, 1, -2];
1006 /// let mut iter = a.iter().skip_while(|x| **x < 0);
1008 /// assert_eq!(iter.next(), Some(&0));
1009 /// assert_eq!(iter.next(), Some(&1));
1011 /// // while this would have been false, since we already got a false,
1012 /// // skip_while() isn't used any more
1013 /// assert_eq!(iter.next(), Some(&-2));
1015 /// assert_eq!(iter.next(), None);
1018 #[stable(feature = "rust1", since = "1.0.0")]
1019 fn skip_while<P>(self, predicate: P) -> SkipWhile<Self, P>
1022 P: FnMut(&Self::Item) -> bool,
1024 SkipWhile::new(self, predicate)
1027 /// Creates an iterator that yields elements based on a predicate.
1029 /// `take_while()` takes a closure as an argument. It will call this
1030 /// closure on each element of the iterator, and yield elements
1031 /// while it returns `true`.
1033 /// After `false` is returned, `take_while()`'s job is over, and the
1034 /// rest of the elements are ignored.
1041 /// let a = [-1i32, 0, 1];
1043 /// let mut iter = a.iter().take_while(|x| x.is_negative());
1045 /// assert_eq!(iter.next(), Some(&-1));
1046 /// assert_eq!(iter.next(), None);
1049 /// Because the closure passed to `take_while()` takes a reference, and many
1050 /// iterators iterate over references, this leads to a possibly confusing
1051 /// situation, where the type of the closure is a double reference:
1054 /// let a = [-1, 0, 1];
1056 /// let mut iter = a.iter().take_while(|x| **x < 0); // need two *s!
1058 /// assert_eq!(iter.next(), Some(&-1));
1059 /// assert_eq!(iter.next(), None);
1062 /// Stopping after an initial `false`:
1065 /// let a = [-1, 0, 1, -2];
1067 /// let mut iter = a.iter().take_while(|x| **x < 0);
1069 /// assert_eq!(iter.next(), Some(&-1));
1071 /// // We have more elements that are less than zero, but since we already
1072 /// // got a false, take_while() isn't used any more
1073 /// assert_eq!(iter.next(), None);
1076 /// Because `take_while()` needs to look at the value in order to see if it
1077 /// should be included or not, consuming iterators will see that it is
1081 /// let a = [1, 2, 3, 4];
1082 /// let mut iter = a.iter();
1084 /// let result: Vec<i32> = iter.by_ref()
1085 /// .take_while(|n| **n != 3)
1089 /// assert_eq!(result, &[1, 2]);
1091 /// let result: Vec<i32> = iter.cloned().collect();
1093 /// assert_eq!(result, &[4]);
1096 /// The `3` is no longer there, because it was consumed in order to see if
1097 /// the iteration should stop, but wasn't placed back into the iterator.
1099 #[stable(feature = "rust1", since = "1.0.0")]
1100 fn take_while<P>(self, predicate: P) -> TakeWhile<Self, P>
1103 P: FnMut(&Self::Item) -> bool,
1105 TakeWhile::new(self, predicate)
1108 /// Creates an iterator that both yields elements based on a predicate and maps.
1110 /// `map_while()` takes a closure as an argument. It will call this
1111 /// closure on each element of the iterator, and yield elements
1112 /// while it returns [`Some(_)`][`Some`].
1119 /// #![feature(iter_map_while)]
1120 /// let a = [-1i32, 4, 0, 1];
1122 /// let mut iter = a.iter().map_while(|x| 16i32.checked_div(*x));
1124 /// assert_eq!(iter.next(), Some(-16));
1125 /// assert_eq!(iter.next(), Some(4));
1126 /// assert_eq!(iter.next(), None);
1129 /// Here's the same example, but with [`take_while`] and [`map`]:
1131 /// [`take_while`]: Iterator::take_while
1132 /// [`map`]: Iterator::map
1135 /// let a = [-1i32, 4, 0, 1];
1137 /// let mut iter = a.iter()
1138 /// .map(|x| 16i32.checked_div(*x))
1139 /// .take_while(|x| x.is_some())
1140 /// .map(|x| x.unwrap());
1142 /// assert_eq!(iter.next(), Some(-16));
1143 /// assert_eq!(iter.next(), Some(4));
1144 /// assert_eq!(iter.next(), None);
1147 /// Stopping after an initial [`None`]:
1150 /// #![feature(iter_map_while)]
1151 /// use std::convert::TryFrom;
1153 /// let a = [0, 1, 2, -3, 4, 5, -6];
1155 /// let iter = a.iter().map_while(|x| u32::try_from(*x).ok());
1156 /// let vec = iter.collect::<Vec<_>>();
1158 /// // We have more elements which could fit in u32 (4, 5), but `map_while` returned `None` for `-3`
1159 /// // (as the `predicate` returned `None`) and `collect` stops at the first `None` encountered.
1160 /// assert_eq!(vec, vec![0, 1, 2]);
1163 /// Because `map_while()` needs to look at the value in order to see if it
1164 /// should be included or not, consuming iterators will see that it is
1168 /// #![feature(iter_map_while)]
1169 /// use std::convert::TryFrom;
1171 /// let a = [1, 2, -3, 4];
1172 /// let mut iter = a.iter();
1174 /// let result: Vec<u32> = iter.by_ref()
1175 /// .map_while(|n| u32::try_from(*n).ok())
1178 /// assert_eq!(result, &[1, 2]);
1180 /// let result: Vec<i32> = iter.cloned().collect();
1182 /// assert_eq!(result, &[4]);
1185 /// The `-3` is no longer there, because it was consumed in order to see if
1186 /// the iteration should stop, but wasn't placed back into the iterator.
1188 /// Note that unlike [`take_while`] this iterator is **not** fused.
1189 /// It is also not specified what this iterator returns after the first [`None`] is returned.
1190 /// If you need fused iterator, use [`fuse`].
1192 /// [`fuse`]: Iterator::fuse
1194 #[unstable(feature = "iter_map_while", reason = "recently added", issue = "68537")]
1195 fn map_while<B, P>(self, predicate: P) -> MapWhile<Self, P>
1198 P: FnMut(Self::Item) -> Option<B>,
1200 MapWhile::new(self, predicate)
1203 /// Creates an iterator that skips the first `n` elements.
1205 /// `skip(n)` skips elements until `n` elements are skipped or the end of the
1206 /// iterator is reached (whichever happens first). After that, all the remaining
1207 /// elements are yielded. In particular, if the original iterator is too short,
1208 /// then the returned iterator is empty.
1210 /// Rather than overriding this method directly, instead override the `nth` method.
1217 /// let a = [1, 2, 3];
1219 /// let mut iter = a.iter().skip(2);
1221 /// assert_eq!(iter.next(), Some(&3));
1222 /// assert_eq!(iter.next(), None);
1225 #[stable(feature = "rust1", since = "1.0.0")]
1226 fn skip(self, n: usize) -> Skip<Self>
1233 /// Creates an iterator that yields the first `n` elements, or fewer
1234 /// if the underlying iterator ends sooner.
1236 /// `take(n)` yields elements until `n` elements are yielded or the end of
1237 /// the iterator is reached (whichever happens first).
1238 /// The returned iterator is a prefix of length `n` if the original iterator
1239 /// contains at least `n` elements, otherwise it contains all of the
1240 /// (fewer than `n`) elements of the original iterator.
1247 /// let a = [1, 2, 3];
1249 /// let mut iter = a.iter().take(2);
1251 /// assert_eq!(iter.next(), Some(&1));
1252 /// assert_eq!(iter.next(), Some(&2));
1253 /// assert_eq!(iter.next(), None);
1256 /// `take()` is often used with an infinite iterator, to make it finite:
1259 /// let mut iter = (0..).take(3);
1261 /// assert_eq!(iter.next(), Some(0));
1262 /// assert_eq!(iter.next(), Some(1));
1263 /// assert_eq!(iter.next(), Some(2));
1264 /// assert_eq!(iter.next(), None);
1267 /// If less than `n` elements are available,
1268 /// `take` will limit itself to the size of the underlying iterator:
1271 /// let v = vec![1, 2];
1272 /// let mut iter = v.into_iter().take(5);
1273 /// assert_eq!(iter.next(), Some(1));
1274 /// assert_eq!(iter.next(), Some(2));
1275 /// assert_eq!(iter.next(), None);
1278 #[stable(feature = "rust1", since = "1.0.0")]
1279 fn take(self, n: usize) -> Take<Self>
1286 /// An iterator adapter similar to [`fold`] that holds internal state and
1287 /// produces a new iterator.
1289 /// [`fold`]: Iterator::fold
1291 /// `scan()` takes two arguments: an initial value which seeds the internal
1292 /// state, and a closure with two arguments, the first being a mutable
1293 /// reference to the internal state and the second an iterator element.
1294 /// The closure can assign to the internal state to share state between
1297 /// On iteration, the closure will be applied to each element of the
1298 /// iterator and the return value from the closure, an [`Option`], is
1299 /// yielded by the iterator.
1306 /// let a = [1, 2, 3];
1308 /// let mut iter = a.iter().scan(1, |state, &x| {
1309 /// // each iteration, we'll multiply the state by the element
1310 /// *state = *state * x;
1312 /// // then, we'll yield the negation of the state
1316 /// assert_eq!(iter.next(), Some(-1));
1317 /// assert_eq!(iter.next(), Some(-2));
1318 /// assert_eq!(iter.next(), Some(-6));
1319 /// assert_eq!(iter.next(), None);
1322 #[stable(feature = "rust1", since = "1.0.0")]
1323 fn scan<St, B, F>(self, initial_state: St, f: F) -> Scan<Self, St, F>
1326 F: FnMut(&mut St, Self::Item) -> Option<B>,
1328 Scan::new(self, initial_state, f)
1331 /// Creates an iterator that works like map, but flattens nested structure.
1333 /// The [`map`] adapter is very useful, but only when the closure
1334 /// argument produces values. If it produces an iterator instead, there's
1335 /// an extra layer of indirection. `flat_map()` will remove this extra layer
1338 /// You can think of `flat_map(f)` as the semantic equivalent
1339 /// of [`map`]ping, and then [`flatten`]ing as in `map(f).flatten()`.
1341 /// Another way of thinking about `flat_map()`: [`map`]'s closure returns
1342 /// one item for each element, and `flat_map()`'s closure returns an
1343 /// iterator for each element.
1345 /// [`map`]: Iterator::map
1346 /// [`flatten`]: Iterator::flatten
1353 /// let words = ["alpha", "beta", "gamma"];
1355 /// // chars() returns an iterator
1356 /// let merged: String = words.iter()
1357 /// .flat_map(|s| s.chars())
1359 /// assert_eq!(merged, "alphabetagamma");
1362 #[stable(feature = "rust1", since = "1.0.0")]
1363 fn flat_map<U, F>(self, f: F) -> FlatMap<Self, U, F>
1367 F: FnMut(Self::Item) -> U,
1369 FlatMap::new(self, f)
1372 /// Creates an iterator that flattens nested structure.
1374 /// This is useful when you have an iterator of iterators or an iterator of
1375 /// things that can be turned into iterators and you want to remove one
1376 /// level of indirection.
1383 /// let data = vec![vec![1, 2, 3, 4], vec![5, 6]];
1384 /// let flattened = data.into_iter().flatten().collect::<Vec<u8>>();
1385 /// assert_eq!(flattened, &[1, 2, 3, 4, 5, 6]);
1388 /// Mapping and then flattening:
1391 /// let words = ["alpha", "beta", "gamma"];
1393 /// // chars() returns an iterator
1394 /// let merged: String = words.iter()
1395 /// .map(|s| s.chars())
1398 /// assert_eq!(merged, "alphabetagamma");
1401 /// You can also rewrite this in terms of [`flat_map()`], which is preferable
1402 /// in this case since it conveys intent more clearly:
1405 /// let words = ["alpha", "beta", "gamma"];
1407 /// // chars() returns an iterator
1408 /// let merged: String = words.iter()
1409 /// .flat_map(|s| s.chars())
1411 /// assert_eq!(merged, "alphabetagamma");
1414 /// Flattening only removes one level of nesting at a time:
1417 /// let d3 = [[[1, 2], [3, 4]], [[5, 6], [7, 8]]];
1419 /// let d2 = d3.iter().flatten().collect::<Vec<_>>();
1420 /// assert_eq!(d2, [&[1, 2], &[3, 4], &[5, 6], &[7, 8]]);
1422 /// let d1 = d3.iter().flatten().flatten().collect::<Vec<_>>();
1423 /// assert_eq!(d1, [&1, &2, &3, &4, &5, &6, &7, &8]);
1426 /// Here we see that `flatten()` does not perform a "deep" flatten.
1427 /// Instead, only one level of nesting is removed. That is, if you
1428 /// `flatten()` a three-dimensional array, the result will be
1429 /// two-dimensional and not one-dimensional. To get a one-dimensional
1430 /// structure, you have to `flatten()` again.
1432 /// [`flat_map()`]: Iterator::flat_map
1434 #[stable(feature = "iterator_flatten", since = "1.29.0")]
1435 fn flatten(self) -> Flatten<Self>
1438 Self::Item: IntoIterator,
1443 /// Creates an iterator which ends after the first [`None`].
1445 /// After an iterator returns [`None`], future calls may or may not yield
1446 /// [`Some(T)`] again. `fuse()` adapts an iterator, ensuring that after a
1447 /// [`None`] is given, it will always return [`None`] forever.
1449 /// Note that the [`Fuse`] wrapper is a no-op on iterators that implement
1450 /// the [`FusedIterator`] trait. `fuse()` may therefore behave incorrectly
1451 /// if the [`FusedIterator`] trait is improperly implemented.
1453 /// [`Some(T)`]: Some
1454 /// [`FusedIterator`]: crate::iter::FusedIterator
1461 /// // an iterator which alternates between Some and None
1462 /// struct Alternate {
1466 /// impl Iterator for Alternate {
1467 /// type Item = i32;
1469 /// fn next(&mut self) -> Option<i32> {
1470 /// let val = self.state;
1471 /// self.state = self.state + 1;
1473 /// // if it's even, Some(i32), else None
1474 /// if val % 2 == 0 {
1482 /// let mut iter = Alternate { state: 0 };
1484 /// // we can see our iterator going back and forth
1485 /// assert_eq!(iter.next(), Some(0));
1486 /// assert_eq!(iter.next(), None);
1487 /// assert_eq!(iter.next(), Some(2));
1488 /// assert_eq!(iter.next(), None);
1490 /// // however, once we fuse it...
1491 /// let mut iter = iter.fuse();
1493 /// assert_eq!(iter.next(), Some(4));
1494 /// assert_eq!(iter.next(), None);
1496 /// // it will always return `None` after the first time.
1497 /// assert_eq!(iter.next(), None);
1498 /// assert_eq!(iter.next(), None);
1499 /// assert_eq!(iter.next(), None);
1502 #[stable(feature = "rust1", since = "1.0.0")]
1503 fn fuse(self) -> Fuse<Self>
1510 /// Does something with each element of an iterator, passing the value on.
1512 /// When using iterators, you'll often chain several of them together.
1513 /// While working on such code, you might want to check out what's
1514 /// happening at various parts in the pipeline. To do that, insert
1515 /// a call to `inspect()`.
1517 /// It's more common for `inspect()` to be used as a debugging tool than to
1518 /// exist in your final code, but applications may find it useful in certain
1519 /// situations when errors need to be logged before being discarded.
1526 /// let a = [1, 4, 2, 3];
1528 /// // this iterator sequence is complex.
1529 /// let sum = a.iter()
1531 /// .filter(|x| x % 2 == 0)
1532 /// .fold(0, |sum, i| sum + i);
1534 /// println!("{}", sum);
1536 /// // let's add some inspect() calls to investigate what's happening
1537 /// let sum = a.iter()
1539 /// .inspect(|x| println!("about to filter: {}", x))
1540 /// .filter(|x| x % 2 == 0)
1541 /// .inspect(|x| println!("made it through filter: {}", x))
1542 /// .fold(0, |sum, i| sum + i);
1544 /// println!("{}", sum);
1547 /// This will print:
1551 /// about to filter: 1
1552 /// about to filter: 4
1553 /// made it through filter: 4
1554 /// about to filter: 2
1555 /// made it through filter: 2
1556 /// about to filter: 3
1560 /// Logging errors before discarding them:
1563 /// let lines = ["1", "2", "a"];
1565 /// let sum: i32 = lines
1567 /// .map(|line| line.parse::<i32>())
1568 /// .inspect(|num| {
1569 /// if let Err(ref e) = *num {
1570 /// println!("Parsing error: {}", e);
1573 /// .filter_map(Result::ok)
1576 /// println!("Sum: {}", sum);
1579 /// This will print:
1582 /// Parsing error: invalid digit found in string
1586 #[stable(feature = "rust1", since = "1.0.0")]
1587 fn inspect<F>(self, f: F) -> Inspect<Self, F>
1590 F: FnMut(&Self::Item),
1592 Inspect::new(self, f)
1595 /// Borrows an iterator, rather than consuming it.
1597 /// This is useful to allow applying iterator adapters while still
1598 /// retaining ownership of the original iterator.
1605 /// let mut words = vec!["hello", "world", "of", "Rust"].into_iter();
1607 /// // Take the first two words.
1608 /// let hello_world: Vec<_> = words.by_ref().take(2).collect();
1609 /// assert_eq!(hello_world, vec!["hello", "world"]);
1611 /// // Collect the rest of the words.
1612 /// // We can only do this because we used `by_ref` earlier.
1613 /// let of_rust: Vec<_> = words.collect();
1614 /// assert_eq!(of_rust, vec!["of", "Rust"]);
1616 #[stable(feature = "rust1", since = "1.0.0")]
1617 fn by_ref(&mut self) -> &mut Self
1624 /// Transforms an iterator into a collection.
1626 /// `collect()` can take anything iterable, and turn it into a relevant
1627 /// collection. This is one of the more powerful methods in the standard
1628 /// library, used in a variety of contexts.
1630 /// The most basic pattern in which `collect()` is used is to turn one
1631 /// collection into another. You take a collection, call [`iter`] on it,
1632 /// do a bunch of transformations, and then `collect()` at the end.
1634 /// `collect()` can also create instances of types that are not typical
1635 /// collections. For example, a [`String`] can be built from [`char`]s,
1636 /// and an iterator of [`Result<T, E>`][`Result`] items can be collected
1637 /// into `Result<Collection<T>, E>`. See the examples below for more.
1639 /// Because `collect()` is so general, it can cause problems with type
1640 /// inference. As such, `collect()` is one of the few times you'll see
1641 /// the syntax affectionately known as the 'turbofish': `::<>`. This
1642 /// helps the inference algorithm understand specifically which collection
1643 /// you're trying to collect into.
1650 /// let a = [1, 2, 3];
1652 /// let doubled: Vec<i32> = a.iter()
1653 /// .map(|&x| x * 2)
1656 /// assert_eq!(vec![2, 4, 6], doubled);
1659 /// Note that we needed the `: Vec<i32>` on the left-hand side. This is because
1660 /// we could collect into, for example, a [`VecDeque<T>`] instead:
1662 /// [`VecDeque<T>`]: ../../std/collections/struct.VecDeque.html
1665 /// use std::collections::VecDeque;
1667 /// let a = [1, 2, 3];
1669 /// let doubled: VecDeque<i32> = a.iter().map(|&x| x * 2).collect();
1671 /// assert_eq!(2, doubled[0]);
1672 /// assert_eq!(4, doubled[1]);
1673 /// assert_eq!(6, doubled[2]);
1676 /// Using the 'turbofish' instead of annotating `doubled`:
1679 /// let a = [1, 2, 3];
1681 /// let doubled = a.iter().map(|x| x * 2).collect::<Vec<i32>>();
1683 /// assert_eq!(vec![2, 4, 6], doubled);
1686 /// Because `collect()` only cares about what you're collecting into, you can
1687 /// still use a partial type hint, `_`, with the turbofish:
1690 /// let a = [1, 2, 3];
1692 /// let doubled = a.iter().map(|x| x * 2).collect::<Vec<_>>();
1694 /// assert_eq!(vec![2, 4, 6], doubled);
1697 /// Using `collect()` to make a [`String`]:
1700 /// let chars = ['g', 'd', 'k', 'k', 'n'];
1702 /// let hello: String = chars.iter()
1703 /// .map(|&x| x as u8)
1704 /// .map(|x| (x + 1) as char)
1707 /// assert_eq!("hello", hello);
1710 /// If you have a list of [`Result<T, E>`][`Result`]s, you can use `collect()` to
1711 /// see if any of them failed:
1714 /// let results = [Ok(1), Err("nope"), Ok(3), Err("bad")];
1716 /// let result: Result<Vec<_>, &str> = results.iter().cloned().collect();
1718 /// // gives us the first error
1719 /// assert_eq!(Err("nope"), result);
1721 /// let results = [Ok(1), Ok(3)];
1723 /// let result: Result<Vec<_>, &str> = results.iter().cloned().collect();
1725 /// // gives us the list of answers
1726 /// assert_eq!(Ok(vec![1, 3]), result);
1729 /// [`iter`]: Iterator::next
1730 /// [`String`]: ../../std/string/struct.String.html
1731 /// [`char`]: type@char
1733 #[stable(feature = "rust1", since = "1.0.0")]
1734 #[must_use = "if you really need to exhaust the iterator, consider `.for_each(drop)` instead"]
1735 fn collect<B: FromIterator<Self::Item>>(self) -> B
1739 FromIterator::from_iter(self)
1742 /// Consumes an iterator, creating two collections from it.
1744 /// The predicate passed to `partition()` can return `true`, or `false`.
1745 /// `partition()` returns a pair, all of the elements for which it returned
1746 /// `true`, and all of the elements for which it returned `false`.
1748 /// See also [`is_partitioned()`] and [`partition_in_place()`].
1750 /// [`is_partitioned()`]: Iterator::is_partitioned
1751 /// [`partition_in_place()`]: Iterator::partition_in_place
1758 /// let a = [1, 2, 3];
1760 /// let (even, odd): (Vec<i32>, Vec<i32>) = a
1762 /// .partition(|&n| n % 2 == 0);
1764 /// assert_eq!(even, vec![2]);
1765 /// assert_eq!(odd, vec![1, 3]);
1767 #[stable(feature = "rust1", since = "1.0.0")]
1768 fn partition<B, F>(self, f: F) -> (B, B)
1771 B: Default + Extend<Self::Item>,
1772 F: FnMut(&Self::Item) -> bool,
1775 fn extend<'a, T, B: Extend<T>>(
1776 mut f: impl FnMut(&T) -> bool + 'a,
1779 ) -> impl FnMut((), T) + 'a {
1784 right.extend_one(x);
1789 let mut left: B = Default::default();
1790 let mut right: B = Default::default();
1792 self.fold((), extend(f, &mut left, &mut right));
1797 /// Reorders the elements of this iterator *in-place* according to the given predicate,
1798 /// such that all those that return `true` precede all those that return `false`.
1799 /// Returns the number of `true` elements found.
1801 /// The relative order of partitioned items is not maintained.
1803 /// # Current implementation
1804 /// Current algorithms tries finding the first element for which the predicate evaluates
1805 /// to false, and the last element for which it evaluates to true and repeatedly swaps them.
1807 /// Time Complexity: *O*(*N*)
1809 /// See also [`is_partitioned()`] and [`partition()`].
1811 /// [`is_partitioned()`]: Iterator::is_partitioned
1812 /// [`partition()`]: Iterator::partition
1817 /// #![feature(iter_partition_in_place)]
1819 /// let mut a = [1, 2, 3, 4, 5, 6, 7];
1821 /// // Partition in-place between evens and odds
1822 /// let i = a.iter_mut().partition_in_place(|&n| n % 2 == 0);
1824 /// assert_eq!(i, 3);
1825 /// assert!(a[..i].iter().all(|&n| n % 2 == 0)); // evens
1826 /// assert!(a[i..].iter().all(|&n| n % 2 == 1)); // odds
1828 #[unstable(feature = "iter_partition_in_place", reason = "new API", issue = "62543")]
1829 fn partition_in_place<'a, T: 'a, P>(mut self, ref mut predicate: P) -> usize
1831 Self: Sized + DoubleEndedIterator<Item = &'a mut T>,
1832 P: FnMut(&T) -> bool,
1834 // FIXME: should we worry about the count overflowing? The only way to have more than
1835 // `usize::MAX` mutable references is with ZSTs, which aren't useful to partition...
1837 // These closure "factory" functions exist to avoid genericity in `Self`.
1841 predicate: &'a mut impl FnMut(&T) -> bool,
1842 true_count: &'a mut usize,
1843 ) -> impl FnMut(&&mut T) -> bool + 'a {
1845 let p = predicate(&**x);
1846 *true_count += p as usize;
1852 fn is_true<T>(predicate: &mut impl FnMut(&T) -> bool) -> impl FnMut(&&mut T) -> bool + '_ {
1853 move |x| predicate(&**x)
1856 // Repeatedly find the first `false` and swap it with the last `true`.
1857 let mut true_count = 0;
1858 while let Some(head) = self.find(is_false(predicate, &mut true_count)) {
1859 if let Some(tail) = self.rfind(is_true(predicate)) {
1860 crate::mem::swap(head, tail);
1869 /// Checks if the elements of this iterator are partitioned according to the given predicate,
1870 /// such that all those that return `true` precede all those that return `false`.
1872 /// See also [`partition()`] and [`partition_in_place()`].
1874 /// [`partition()`]: Iterator::partition
1875 /// [`partition_in_place()`]: Iterator::partition_in_place
1880 /// #![feature(iter_is_partitioned)]
1882 /// assert!("Iterator".chars().is_partitioned(char::is_uppercase));
1883 /// assert!(!"IntoIterator".chars().is_partitioned(char::is_uppercase));
1885 #[unstable(feature = "iter_is_partitioned", reason = "new API", issue = "62544")]
1886 fn is_partitioned<P>(mut self, mut predicate: P) -> bool
1889 P: FnMut(Self::Item) -> bool,
1891 // Either all items test `true`, or the first clause stops at `false`
1892 // and we check that there are no more `true` items after that.
1893 self.all(&mut predicate) || !self.any(predicate)
1896 /// An iterator method that applies a function as long as it returns
1897 /// successfully, producing a single, final value.
1899 /// `try_fold()` takes two arguments: an initial value, and a closure with
1900 /// two arguments: an 'accumulator', and an element. The closure either
1901 /// returns successfully, with the value that the accumulator should have
1902 /// for the next iteration, or it returns failure, with an error value that
1903 /// is propagated back to the caller immediately (short-circuiting).
1905 /// The initial value is the value the accumulator will have on the first
1906 /// call. If applying the closure succeeded against every element of the
1907 /// iterator, `try_fold()` returns the final accumulator as success.
1909 /// Folding is useful whenever you have a collection of something, and want
1910 /// to produce a single value from it.
1912 /// # Note to Implementors
1914 /// Several of the other (forward) methods have default implementations in
1915 /// terms of this one, so try to implement this explicitly if it can
1916 /// do something better than the default `for` loop implementation.
1918 /// In particular, try to have this call `try_fold()` on the internal parts
1919 /// from which this iterator is composed. If multiple calls are needed,
1920 /// the `?` operator may be convenient for chaining the accumulator value
1921 /// along, but beware any invariants that need to be upheld before those
1922 /// early returns. This is a `&mut self` method, so iteration needs to be
1923 /// resumable after hitting an error here.
1930 /// let a = [1, 2, 3];
1932 /// // the checked sum of all of the elements of the array
1933 /// let sum = a.iter().try_fold(0i8, |acc, &x| acc.checked_add(x));
1935 /// assert_eq!(sum, Some(6));
1938 /// Short-circuiting:
1941 /// let a = [10, 20, 30, 100, 40, 50];
1942 /// let mut it = a.iter();
1944 /// // This sum overflows when adding the 100 element
1945 /// let sum = it.try_fold(0i8, |acc, &x| acc.checked_add(x));
1946 /// assert_eq!(sum, None);
1948 /// // Because it short-circuited, the remaining elements are still
1949 /// // available through the iterator.
1950 /// assert_eq!(it.len(), 2);
1951 /// assert_eq!(it.next(), Some(&40));
1954 /// While you cannot `break` from a closure, the [`ControlFlow`] type allows
1958 /// use std::ops::ControlFlow;
1960 /// let triangular = (1..30).try_fold(0_i8, |prev, x| {
1961 /// if let Some(next) = prev.checked_add(x) {
1962 /// ControlFlow::Continue(next)
1964 /// ControlFlow::Break(prev)
1967 /// assert_eq!(triangular, ControlFlow::Break(120));
1969 /// let triangular = (1..30).try_fold(0_u64, |prev, x| {
1970 /// if let Some(next) = prev.checked_add(x) {
1971 /// ControlFlow::Continue(next)
1973 /// ControlFlow::Break(prev)
1976 /// assert_eq!(triangular, ControlFlow::Continue(435));
1979 #[stable(feature = "iterator_try_fold", since = "1.27.0")]
1980 fn try_fold<B, F, R>(&mut self, init: B, mut f: F) -> R
1983 F: FnMut(B, Self::Item) -> R,
1986 let mut accum = init;
1987 while let Some(x) = self.next() {
1988 accum = f(accum, x)?;
1993 /// An iterator method that applies a fallible function to each item in the
1994 /// iterator, stopping at the first error and returning that error.
1996 /// This can also be thought of as the fallible form of [`for_each()`]
1997 /// or as the stateless version of [`try_fold()`].
1999 /// [`for_each()`]: Iterator::for_each
2000 /// [`try_fold()`]: Iterator::try_fold
2005 /// use std::fs::rename;
2006 /// use std::io::{stdout, Write};
2007 /// use std::path::Path;
2009 /// let data = ["no_tea.txt", "stale_bread.json", "torrential_rain.png"];
2011 /// let res = data.iter().try_for_each(|x| writeln!(stdout(), "{}", x));
2012 /// assert!(res.is_ok());
2014 /// let mut it = data.iter().cloned();
2015 /// let res = it.try_for_each(|x| rename(x, Path::new(x).with_extension("old")));
2016 /// assert!(res.is_err());
2017 /// // It short-circuited, so the remaining items are still in the iterator:
2018 /// assert_eq!(it.next(), Some("stale_bread.json"));
2021 /// The [`ControlFlow`] type can be used with this method for the situations
2022 /// in which you'd use `break` and `continue` in a normal loop:
2025 /// use std::ops::ControlFlow;
2027 /// let r = (2..100).try_for_each(|x| {
2028 /// if 323 % x == 0 {
2029 /// return ControlFlow::Break(x)
2032 /// ControlFlow::Continue(())
2034 /// assert_eq!(r, ControlFlow::Break(17));
2037 #[stable(feature = "iterator_try_fold", since = "1.27.0")]
2038 fn try_for_each<F, R>(&mut self, f: F) -> R
2041 F: FnMut(Self::Item) -> R,
2042 R: Try<Output = ()>,
2045 fn call<T, R>(mut f: impl FnMut(T) -> R) -> impl FnMut((), T) -> R {
2049 self.try_fold((), call(f))
2052 /// Folds every element into an accumulator by applying an operation,
2053 /// returning the final result.
2055 /// `fold()` takes two arguments: an initial value, and a closure with two
2056 /// arguments: an 'accumulator', and an element. The closure returns the value that
2057 /// the accumulator should have for the next iteration.
2059 /// The initial value is the value the accumulator will have on the first
2062 /// After applying this closure to every element of the iterator, `fold()`
2063 /// returns the accumulator.
2065 /// This operation is sometimes called 'reduce' or 'inject'.
2067 /// Folding is useful whenever you have a collection of something, and want
2068 /// to produce a single value from it.
2070 /// Note: `fold()`, and similar methods that traverse the entire iterator,
2071 /// might not terminate for infinite iterators, even on traits for which a
2072 /// result is determinable in finite time.
2074 /// Note: [`reduce()`] can be used to use the first element as the initial
2075 /// value, if the accumulator type and item type is the same.
2077 /// Note: `fold()` combines elements in a *left-associative* fashion. For associative
2078 /// operators like `+`, the order the elements are combined in is not important, but for non-associative
2079 /// operators like `-` the order will affect the final result.
2080 /// For a *right-associative* version of `fold()`, see [`DoubleEndedIterator::rfold()`].
2082 /// # Note to Implementors
2084 /// Several of the other (forward) methods have default implementations in
2085 /// terms of this one, so try to implement this explicitly if it can
2086 /// do something better than the default `for` loop implementation.
2088 /// In particular, try to have this call `fold()` on the internal parts
2089 /// from which this iterator is composed.
2096 /// let a = [1, 2, 3];
2098 /// // the sum of all of the elements of the array
2099 /// let sum = a.iter().fold(0, |acc, x| acc + x);
2101 /// assert_eq!(sum, 6);
2104 /// Let's walk through each step of the iteration here:
2106 /// | element | acc | x | result |
2107 /// |---------|-----|---|--------|
2109 /// | 1 | 0 | 1 | 1 |
2110 /// | 2 | 1 | 2 | 3 |
2111 /// | 3 | 3 | 3 | 6 |
2113 /// And so, our final result, `6`.
2115 /// This example demonstrates the left-associative nature of `fold()`:
2116 /// it builds a string, starting with an initial value
2117 /// and continuing with each element from the front until the back:
2120 /// let numbers = [1, 2, 3, 4, 5];
2122 /// let zero = "0".to_string();
2124 /// let result = numbers.iter().fold(zero, |acc, &x| {
2125 /// format!("({} + {})", acc, x)
2128 /// assert_eq!(result, "(((((0 + 1) + 2) + 3) + 4) + 5)");
2130 /// It's common for people who haven't used iterators a lot to
2131 /// use a `for` loop with a list of things to build up a result. Those
2132 /// can be turned into `fold()`s:
2134 /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
2137 /// let numbers = [1, 2, 3, 4, 5];
2139 /// let mut result = 0;
2142 /// for i in &numbers {
2143 /// result = result + i;
2147 /// let result2 = numbers.iter().fold(0, |acc, &x| acc + x);
2149 /// // they're the same
2150 /// assert_eq!(result, result2);
2153 /// [`reduce()`]: Iterator::reduce
2154 #[doc(alias = "inject", alias = "foldl")]
2156 #[stable(feature = "rust1", since = "1.0.0")]
2157 fn fold<B, F>(mut self, init: B, mut f: F) -> B
2160 F: FnMut(B, Self::Item) -> B,
2162 let mut accum = init;
2163 while let Some(x) = self.next() {
2164 accum = f(accum, x);
2169 /// Reduces the elements to a single one, by repeatedly applying a reducing
2172 /// If the iterator is empty, returns [`None`]; otherwise, returns the
2173 /// result of the reduction.
2175 /// The reducing function is a closure with two arguments: an 'accumulator', and an element.
2176 /// For iterators with at least one element, this is the same as [`fold()`]
2177 /// with the first element of the iterator as the initial accumulator value, folding
2178 /// every subsequent element into it.
2180 /// [`fold()`]: Iterator::fold
2184 /// Find the maximum value:
2187 /// fn find_max<I>(iter: I) -> Option<I::Item>
2188 /// where I: Iterator,
2191 /// iter.reduce(|accum, item| {
2192 /// if accum >= item { accum } else { item }
2195 /// let a = [10, 20, 5, -23, 0];
2196 /// let b: [u32; 0] = [];
2198 /// assert_eq!(find_max(a.iter()), Some(&20));
2199 /// assert_eq!(find_max(b.iter()), None);
2202 #[stable(feature = "iterator_fold_self", since = "1.51.0")]
2203 fn reduce<F>(mut self, f: F) -> Option<Self::Item>
2206 F: FnMut(Self::Item, Self::Item) -> Self::Item,
2208 let first = self.next()?;
2209 Some(self.fold(first, f))
2212 /// Tests if every element of the iterator matches a predicate.
2214 /// `all()` takes a closure that returns `true` or `false`. It applies
2215 /// this closure to each element of the iterator, and if they all return
2216 /// `true`, then so does `all()`. If any of them return `false`, it
2217 /// returns `false`.
2219 /// `all()` is short-circuiting; in other words, it will stop processing
2220 /// as soon as it finds a `false`, given that no matter what else happens,
2221 /// the result will also be `false`.
2223 /// An empty iterator returns `true`.
2230 /// let a = [1, 2, 3];
2232 /// assert!(a.iter().all(|&x| x > 0));
2234 /// assert!(!a.iter().all(|&x| x > 2));
2237 /// Stopping at the first `false`:
2240 /// let a = [1, 2, 3];
2242 /// let mut iter = a.iter();
2244 /// assert!(!iter.all(|&x| x != 2));
2246 /// // we can still use `iter`, as there are more elements.
2247 /// assert_eq!(iter.next(), Some(&3));
2250 #[stable(feature = "rust1", since = "1.0.0")]
2251 fn all<F>(&mut self, f: F) -> bool
2254 F: FnMut(Self::Item) -> bool,
2257 fn check<T>(mut f: impl FnMut(T) -> bool) -> impl FnMut((), T) -> ControlFlow<()> {
2259 if f(x) { ControlFlow::CONTINUE } else { ControlFlow::BREAK }
2262 self.try_fold((), check(f)) == ControlFlow::CONTINUE
2265 /// Tests if any element of the iterator matches a predicate.
2267 /// `any()` takes a closure that returns `true` or `false`. It applies
2268 /// this closure to each element of the iterator, and if any of them return
2269 /// `true`, then so does `any()`. If they all return `false`, it
2270 /// returns `false`.
2272 /// `any()` is short-circuiting; in other words, it will stop processing
2273 /// as soon as it finds a `true`, given that no matter what else happens,
2274 /// the result will also be `true`.
2276 /// An empty iterator returns `false`.
2283 /// let a = [1, 2, 3];
2285 /// assert!(a.iter().any(|&x| x > 0));
2287 /// assert!(!a.iter().any(|&x| x > 5));
2290 /// Stopping at the first `true`:
2293 /// let a = [1, 2, 3];
2295 /// let mut iter = a.iter();
2297 /// assert!(iter.any(|&x| x != 2));
2299 /// // we can still use `iter`, as there are more elements.
2300 /// assert_eq!(iter.next(), Some(&2));
2303 #[stable(feature = "rust1", since = "1.0.0")]
2304 fn any<F>(&mut self, f: F) -> bool
2307 F: FnMut(Self::Item) -> bool,
2310 fn check<T>(mut f: impl FnMut(T) -> bool) -> impl FnMut((), T) -> ControlFlow<()> {
2312 if f(x) { ControlFlow::BREAK } else { ControlFlow::CONTINUE }
2316 self.try_fold((), check(f)) == ControlFlow::BREAK
2319 /// Searches for an element of an iterator that satisfies a predicate.
2321 /// `find()` takes a closure that returns `true` or `false`. It applies
2322 /// this closure to each element of the iterator, and if any of them return
2323 /// `true`, then `find()` returns [`Some(element)`]. If they all return
2324 /// `false`, it returns [`None`].
2326 /// `find()` is short-circuiting; in other words, it will stop processing
2327 /// as soon as the closure returns `true`.
2329 /// Because `find()` takes a reference, and many iterators iterate over
2330 /// references, this leads to a possibly confusing situation where the
2331 /// argument is a double reference. You can see this effect in the
2332 /// examples below, with `&&x`.
2334 /// [`Some(element)`]: Some
2341 /// let a = [1, 2, 3];
2343 /// assert_eq!(a.iter().find(|&&x| x == 2), Some(&2));
2345 /// assert_eq!(a.iter().find(|&&x| x == 5), None);
2348 /// Stopping at the first `true`:
2351 /// let a = [1, 2, 3];
2353 /// let mut iter = a.iter();
2355 /// assert_eq!(iter.find(|&&x| x == 2), Some(&2));
2357 /// // we can still use `iter`, as there are more elements.
2358 /// assert_eq!(iter.next(), Some(&3));
2361 /// Note that `iter.find(f)` is equivalent to `iter.filter(f).next()`.
2363 #[stable(feature = "rust1", since = "1.0.0")]
2364 fn find<P>(&mut self, predicate: P) -> Option<Self::Item>
2367 P: FnMut(&Self::Item) -> bool,
2370 fn check<T>(mut predicate: impl FnMut(&T) -> bool) -> impl FnMut((), T) -> ControlFlow<T> {
2372 if predicate(&x) { ControlFlow::Break(x) } else { ControlFlow::CONTINUE }
2376 self.try_fold((), check(predicate)).break_value()
2379 /// Applies function to the elements of iterator and returns
2380 /// the first non-none result.
2382 /// `iter.find_map(f)` is equivalent to `iter.filter_map(f).next()`.
2387 /// let a = ["lol", "NaN", "2", "5"];
2389 /// let first_number = a.iter().find_map(|s| s.parse().ok());
2391 /// assert_eq!(first_number, Some(2));
2394 #[stable(feature = "iterator_find_map", since = "1.30.0")]
2395 fn find_map<B, F>(&mut self, f: F) -> Option<B>
2398 F: FnMut(Self::Item) -> Option<B>,
2401 fn check<T, B>(mut f: impl FnMut(T) -> Option<B>) -> impl FnMut((), T) -> ControlFlow<B> {
2402 move |(), x| match f(x) {
2403 Some(x) => ControlFlow::Break(x),
2404 None => ControlFlow::CONTINUE,
2408 self.try_fold((), check(f)).break_value()
2411 /// Applies function to the elements of iterator and returns
2412 /// the first true result or the first error.
2417 /// #![feature(try_find)]
2419 /// let a = ["1", "2", "lol", "NaN", "5"];
2421 /// let is_my_num = |s: &str, search: i32| -> Result<bool, std::num::ParseIntError> {
2422 /// Ok(s.parse::<i32>()? == search)
2425 /// let result = a.iter().try_find(|&&s| is_my_num(s, 2));
2426 /// assert_eq!(result, Ok(Some(&"2")));
2428 /// let result = a.iter().try_find(|&&s| is_my_num(s, 5));
2429 /// assert!(result.is_err());
2432 #[unstable(feature = "try_find", reason = "new API", issue = "63178")]
2433 fn try_find<F, R, E>(&mut self, f: F) -> Result<Option<Self::Item>, E>
2436 F: FnMut(&Self::Item) -> R,
2437 R: Try<Output = bool>,
2438 // FIXME: This bound is rather strange, but means minimal breakage on nightly.
2439 // See #85115 for the issue tracking a holistic solution for this and try_map.
2440 R: Try<Residual = Result<crate::convert::Infallible, E>>,
2443 fn check<F, T, R, E>(mut f: F) -> impl FnMut((), T) -> ControlFlow<Result<T, E>>
2446 R: Try<Output = bool>,
2447 R: Try<Residual = Result<crate::convert::Infallible, E>>,
2449 move |(), x| match f(&x).branch() {
2450 ControlFlow::Continue(false) => ControlFlow::CONTINUE,
2451 ControlFlow::Continue(true) => ControlFlow::Break(Ok(x)),
2452 ControlFlow::Break(Err(x)) => ControlFlow::Break(Err(x)),
2456 self.try_fold((), check(f)).break_value().transpose()
2459 /// Searches for an element in an iterator, returning its index.
2461 /// `position()` takes a closure that returns `true` or `false`. It applies
2462 /// this closure to each element of the iterator, and if one of them
2463 /// returns `true`, then `position()` returns [`Some(index)`]. If all of
2464 /// them return `false`, it returns [`None`].
2466 /// `position()` is short-circuiting; in other words, it will stop
2467 /// processing as soon as it finds a `true`.
2469 /// # Overflow Behavior
2471 /// The method does no guarding against overflows, so if there are more
2472 /// than [`usize::MAX`] non-matching elements, it either produces the wrong
2473 /// result or panics. If debug assertions are enabled, a panic is
2478 /// This function might panic if the iterator has more than `usize::MAX`
2479 /// non-matching elements.
2481 /// [`Some(index)`]: Some
2488 /// let a = [1, 2, 3];
2490 /// assert_eq!(a.iter().position(|&x| x == 2), Some(1));
2492 /// assert_eq!(a.iter().position(|&x| x == 5), None);
2495 /// Stopping at the first `true`:
2498 /// let a = [1, 2, 3, 4];
2500 /// let mut iter = a.iter();
2502 /// assert_eq!(iter.position(|&x| x >= 2), Some(1));
2504 /// // we can still use `iter`, as there are more elements.
2505 /// assert_eq!(iter.next(), Some(&3));
2507 /// // The returned index depends on iterator state
2508 /// assert_eq!(iter.position(|&x| x == 4), Some(0));
2512 #[stable(feature = "rust1", since = "1.0.0")]
2513 fn position<P>(&mut self, predicate: P) -> Option<usize>
2516 P: FnMut(Self::Item) -> bool,
2520 mut predicate: impl FnMut(T) -> bool,
2521 ) -> impl FnMut(usize, T) -> ControlFlow<usize, usize> {
2522 #[rustc_inherit_overflow_checks]
2524 if predicate(x) { ControlFlow::Break(i) } else { ControlFlow::Continue(i + 1) }
2528 self.try_fold(0, check(predicate)).break_value()
2531 /// Searches for an element in an iterator from the right, returning its
2534 /// `rposition()` takes a closure that returns `true` or `false`. It applies
2535 /// this closure to each element of the iterator, starting from the end,
2536 /// and if one of them returns `true`, then `rposition()` returns
2537 /// [`Some(index)`]. If all of them return `false`, it returns [`None`].
2539 /// `rposition()` is short-circuiting; in other words, it will stop
2540 /// processing as soon as it finds a `true`.
2542 /// [`Some(index)`]: Some
2549 /// let a = [1, 2, 3];
2551 /// assert_eq!(a.iter().rposition(|&x| x == 3), Some(2));
2553 /// assert_eq!(a.iter().rposition(|&x| x == 5), None);
2556 /// Stopping at the first `true`:
2559 /// let a = [1, 2, 3];
2561 /// let mut iter = a.iter();
2563 /// assert_eq!(iter.rposition(|&x| x == 2), Some(1));
2565 /// // we can still use `iter`, as there are more elements.
2566 /// assert_eq!(iter.next(), Some(&1));
2569 #[stable(feature = "rust1", since = "1.0.0")]
2570 fn rposition<P>(&mut self, predicate: P) -> Option<usize>
2572 P: FnMut(Self::Item) -> bool,
2573 Self: Sized + ExactSizeIterator + DoubleEndedIterator,
2575 // No need for an overflow check here, because `ExactSizeIterator`
2576 // implies that the number of elements fits into a `usize`.
2579 mut predicate: impl FnMut(T) -> bool,
2580 ) -> impl FnMut(usize, T) -> ControlFlow<usize, usize> {
2583 if predicate(x) { ControlFlow::Break(i) } else { ControlFlow::Continue(i) }
2588 self.try_rfold(n, check(predicate)).break_value()
2591 /// Returns the maximum element of an iterator.
2593 /// If several elements are equally maximum, the last element is
2594 /// returned. If the iterator is empty, [`None`] is returned.
2596 /// Note that [`f32`]/[`f64`] doesn't implement [`Ord`] due to NaN being
2597 /// incomparable. You can work around this by using [`Iterator::reduce`]:
2600 /// vec![2.4, f32::NAN, 1.3]
2602 /// .reduce(f32::max)
2613 /// let a = [1, 2, 3];
2614 /// let b: Vec<u32> = Vec::new();
2616 /// assert_eq!(a.iter().max(), Some(&3));
2617 /// assert_eq!(b.iter().max(), None);
2620 #[stable(feature = "rust1", since = "1.0.0")]
2621 fn max(self) -> Option<Self::Item>
2626 self.max_by(Ord::cmp)
2629 /// Returns the minimum element of an iterator.
2631 /// If several elements are equally minimum, the first element is returned.
2632 /// If the iterator is empty, [`None`] is returned.
2634 /// Note that [`f32`]/[`f64`] doesn't implement [`Ord`] due to NaN being
2635 /// incomparable. You can work around this by using [`Iterator::reduce`]:
2638 /// vec![2.4, f32::NAN, 1.3]
2640 /// .reduce(f32::min)
2651 /// let a = [1, 2, 3];
2652 /// let b: Vec<u32> = Vec::new();
2654 /// assert_eq!(a.iter().min(), Some(&1));
2655 /// assert_eq!(b.iter().min(), None);
2658 #[stable(feature = "rust1", since = "1.0.0")]
2659 fn min(self) -> Option<Self::Item>
2664 self.min_by(Ord::cmp)
2667 /// Returns the element that gives the maximum value from the
2668 /// specified function.
2670 /// If several elements are equally maximum, the last element is
2671 /// returned. If the iterator is empty, [`None`] is returned.
2676 /// let a = [-3_i32, 0, 1, 5, -10];
2677 /// assert_eq!(*a.iter().max_by_key(|x| x.abs()).unwrap(), -10);
2680 #[stable(feature = "iter_cmp_by_key", since = "1.6.0")]
2681 fn max_by_key<B: Ord, F>(self, f: F) -> Option<Self::Item>
2684 F: FnMut(&Self::Item) -> B,
2687 fn key<T, B>(mut f: impl FnMut(&T) -> B) -> impl FnMut(T) -> (B, T) {
2692 fn compare<T, B: Ord>((x_p, _): &(B, T), (y_p, _): &(B, T)) -> Ordering {
2696 let (_, x) = self.map(key(f)).max_by(compare)?;
2700 /// Returns the element that gives the maximum value with respect to the
2701 /// specified comparison function.
2703 /// If several elements are equally maximum, the last element is
2704 /// returned. If the iterator is empty, [`None`] is returned.
2709 /// let a = [-3_i32, 0, 1, 5, -10];
2710 /// assert_eq!(*a.iter().max_by(|x, y| x.cmp(y)).unwrap(), 5);
2713 #[stable(feature = "iter_max_by", since = "1.15.0")]
2714 fn max_by<F>(self, compare: F) -> Option<Self::Item>
2717 F: FnMut(&Self::Item, &Self::Item) -> Ordering,
2720 fn fold<T>(mut compare: impl FnMut(&T, &T) -> Ordering) -> impl FnMut(T, T) -> T {
2721 move |x, y| cmp::max_by(x, y, &mut compare)
2724 self.reduce(fold(compare))
2727 /// Returns the element that gives the minimum value from the
2728 /// specified function.
2730 /// If several elements are equally minimum, the first element is
2731 /// returned. If the iterator is empty, [`None`] is returned.
2736 /// let a = [-3_i32, 0, 1, 5, -10];
2737 /// assert_eq!(*a.iter().min_by_key(|x| x.abs()).unwrap(), 0);
2740 #[stable(feature = "iter_cmp_by_key", since = "1.6.0")]
2741 fn min_by_key<B: Ord, F>(self, f: F) -> Option<Self::Item>
2744 F: FnMut(&Self::Item) -> B,
2747 fn key<T, B>(mut f: impl FnMut(&T) -> B) -> impl FnMut(T) -> (B, T) {
2752 fn compare<T, B: Ord>((x_p, _): &(B, T), (y_p, _): &(B, T)) -> Ordering {
2756 let (_, x) = self.map(key(f)).min_by(compare)?;
2760 /// Returns the element that gives the minimum value with respect to the
2761 /// specified comparison function.
2763 /// If several elements are equally minimum, the first element is
2764 /// returned. If the iterator is empty, [`None`] is returned.
2769 /// let a = [-3_i32, 0, 1, 5, -10];
2770 /// assert_eq!(*a.iter().min_by(|x, y| x.cmp(y)).unwrap(), -10);
2773 #[stable(feature = "iter_min_by", since = "1.15.0")]
2774 fn min_by<F>(self, compare: F) -> Option<Self::Item>
2777 F: FnMut(&Self::Item, &Self::Item) -> Ordering,
2780 fn fold<T>(mut compare: impl FnMut(&T, &T) -> Ordering) -> impl FnMut(T, T) -> T {
2781 move |x, y| cmp::min_by(x, y, &mut compare)
2784 self.reduce(fold(compare))
2787 /// Reverses an iterator's direction.
2789 /// Usually, iterators iterate from left to right. After using `rev()`,
2790 /// an iterator will instead iterate from right to left.
2792 /// This is only possible if the iterator has an end, so `rev()` only
2793 /// works on [`DoubleEndedIterator`]s.
2798 /// let a = [1, 2, 3];
2800 /// let mut iter = a.iter().rev();
2802 /// assert_eq!(iter.next(), Some(&3));
2803 /// assert_eq!(iter.next(), Some(&2));
2804 /// assert_eq!(iter.next(), Some(&1));
2806 /// assert_eq!(iter.next(), None);
2809 #[doc(alias = "reverse")]
2810 #[stable(feature = "rust1", since = "1.0.0")]
2811 fn rev(self) -> Rev<Self>
2813 Self: Sized + DoubleEndedIterator,
2818 /// Converts an iterator of pairs into a pair of containers.
2820 /// `unzip()` consumes an entire iterator of pairs, producing two
2821 /// collections: one from the left elements of the pairs, and one
2822 /// from the right elements.
2824 /// This function is, in some sense, the opposite of [`zip`].
2826 /// [`zip`]: Iterator::zip
2833 /// let a = [(1, 2), (3, 4)];
2835 /// let (left, right): (Vec<_>, Vec<_>) = a.iter().cloned().unzip();
2837 /// assert_eq!(left, [1, 3]);
2838 /// assert_eq!(right, [2, 4]);
2840 /// // you can also unzip multiple nested tuples at once
2841 /// let a = [(1, (2, 3)), (4, (5, 6))];
2843 /// let (x, (y, z)): (Vec<_>, (Vec<_>, Vec<_>)) = a.iter().cloned().unzip();
2844 /// assert_eq!(x, [1, 4]);
2845 /// assert_eq!(y, [2, 5]);
2846 /// assert_eq!(z, [3, 6]);
2848 #[stable(feature = "rust1", since = "1.0.0")]
2849 fn unzip<A, B, FromA, FromB>(self) -> (FromA, FromB)
2851 FromA: Default + Extend<A>,
2852 FromB: Default + Extend<B>,
2853 Self: Sized + Iterator<Item = (A, B)>,
2855 let mut unzipped: (FromA, FromB) = Default::default();
2856 unzipped.extend(self);
2860 /// Creates an iterator which copies all of its elements.
2862 /// This is useful when you have an iterator over `&T`, but you need an
2863 /// iterator over `T`.
2870 /// let a = [1, 2, 3];
2872 /// let v_copied: Vec<_> = a.iter().copied().collect();
2874 /// // copied is the same as .map(|&x| x)
2875 /// let v_map: Vec<_> = a.iter().map(|&x| x).collect();
2877 /// assert_eq!(v_copied, vec![1, 2, 3]);
2878 /// assert_eq!(v_map, vec![1, 2, 3]);
2880 #[stable(feature = "iter_copied", since = "1.36.0")]
2881 fn copied<'a, T: 'a>(self) -> Copied<Self>
2883 Self: Sized + Iterator<Item = &'a T>,
2889 /// Creates an iterator which [`clone`]s all of its elements.
2891 /// This is useful when you have an iterator over `&T`, but you need an
2892 /// iterator over `T`.
2894 /// [`clone`]: Clone::clone
2901 /// let a = [1, 2, 3];
2903 /// let v_cloned: Vec<_> = a.iter().cloned().collect();
2905 /// // cloned is the same as .map(|&x| x), for integers
2906 /// let v_map: Vec<_> = a.iter().map(|&x| x).collect();
2908 /// assert_eq!(v_cloned, vec![1, 2, 3]);
2909 /// assert_eq!(v_map, vec![1, 2, 3]);
2911 #[stable(feature = "rust1", since = "1.0.0")]
2912 fn cloned<'a, T: 'a>(self) -> Cloned<Self>
2914 Self: Sized + Iterator<Item = &'a T>,
2920 /// Repeats an iterator endlessly.
2922 /// Instead of stopping at [`None`], the iterator will instead start again,
2923 /// from the beginning. After iterating again, it will start at the
2924 /// beginning again. And again. And again. Forever.
2931 /// let a = [1, 2, 3];
2933 /// let mut it = a.iter().cycle();
2935 /// assert_eq!(it.next(), Some(&1));
2936 /// assert_eq!(it.next(), Some(&2));
2937 /// assert_eq!(it.next(), Some(&3));
2938 /// assert_eq!(it.next(), Some(&1));
2939 /// assert_eq!(it.next(), Some(&2));
2940 /// assert_eq!(it.next(), Some(&3));
2941 /// assert_eq!(it.next(), Some(&1));
2943 #[stable(feature = "rust1", since = "1.0.0")]
2945 fn cycle(self) -> Cycle<Self>
2947 Self: Sized + Clone,
2952 /// Sums the elements of an iterator.
2954 /// Takes each element, adds them together, and returns the result.
2956 /// An empty iterator returns the zero value of the type.
2960 /// When calling `sum()` and a primitive integer type is being returned, this
2961 /// method will panic if the computation overflows and debug assertions are
2969 /// let a = [1, 2, 3];
2970 /// let sum: i32 = a.iter().sum();
2972 /// assert_eq!(sum, 6);
2974 #[stable(feature = "iter_arith", since = "1.11.0")]
2975 fn sum<S>(self) -> S
2983 /// Iterates over the entire iterator, multiplying all the elements
2985 /// An empty iterator returns the one value of the type.
2989 /// When calling `product()` and a primitive integer type is being returned,
2990 /// method will panic if the computation overflows and debug assertions are
2996 /// fn factorial(n: u32) -> u32 {
2997 /// (1..=n).product()
2999 /// assert_eq!(factorial(0), 1);
3000 /// assert_eq!(factorial(1), 1);
3001 /// assert_eq!(factorial(5), 120);
3003 #[stable(feature = "iter_arith", since = "1.11.0")]
3004 fn product<P>(self) -> P
3007 P: Product<Self::Item>,
3009 Product::product(self)
3012 /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
3018 /// use std::cmp::Ordering;
3020 /// assert_eq!([1].iter().cmp([1].iter()), Ordering::Equal);
3021 /// assert_eq!([1].iter().cmp([1, 2].iter()), Ordering::Less);
3022 /// assert_eq!([1, 2].iter().cmp([1].iter()), Ordering::Greater);
3024 #[stable(feature = "iter_order", since = "1.5.0")]
3025 fn cmp<I>(self, other: I) -> Ordering
3027 I: IntoIterator<Item = Self::Item>,
3031 self.cmp_by(other, |x, y| x.cmp(&y))
3034 /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
3035 /// of another with respect to the specified comparison function.
3042 /// #![feature(iter_order_by)]
3044 /// use std::cmp::Ordering;
3046 /// let xs = [1, 2, 3, 4];
3047 /// let ys = [1, 4, 9, 16];
3049 /// assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| x.cmp(&y)), Ordering::Less);
3050 /// assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| (x * x).cmp(&y)), Ordering::Equal);
3051 /// assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| (2 * x).cmp(&y)), Ordering::Greater);
3053 #[unstable(feature = "iter_order_by", issue = "64295")]
3054 fn cmp_by<I, F>(mut self, other: I, mut cmp: F) -> Ordering
3058 F: FnMut(Self::Item, I::Item) -> Ordering,
3060 let mut other = other.into_iter();
3063 let x = match self.next() {
3065 if other.next().is_none() {
3066 return Ordering::Equal;
3068 return Ordering::Less;
3074 let y = match other.next() {
3075 None => return Ordering::Greater,
3080 Ordering::Equal => (),
3081 non_eq => return non_eq,
3086 /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
3092 /// use std::cmp::Ordering;
3094 /// assert_eq!([1.].iter().partial_cmp([1.].iter()), Some(Ordering::Equal));
3095 /// assert_eq!([1.].iter().partial_cmp([1., 2.].iter()), Some(Ordering::Less));
3096 /// assert_eq!([1., 2.].iter().partial_cmp([1.].iter()), Some(Ordering::Greater));
3098 /// assert_eq!([f64::NAN].iter().partial_cmp([1.].iter()), None);
3100 #[stable(feature = "iter_order", since = "1.5.0")]
3101 fn partial_cmp<I>(self, other: I) -> Option<Ordering>
3104 Self::Item: PartialOrd<I::Item>,
3107 self.partial_cmp_by(other, |x, y| x.partial_cmp(&y))
3110 /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
3111 /// of another with respect to the specified comparison function.
3118 /// #![feature(iter_order_by)]
3120 /// use std::cmp::Ordering;
3122 /// let xs = [1.0, 2.0, 3.0, 4.0];
3123 /// let ys = [1.0, 4.0, 9.0, 16.0];
3126 /// xs.iter().partial_cmp_by(&ys, |&x, &y| x.partial_cmp(&y)),
3127 /// Some(Ordering::Less)
3130 /// xs.iter().partial_cmp_by(&ys, |&x, &y| (x * x).partial_cmp(&y)),
3131 /// Some(Ordering::Equal)
3134 /// xs.iter().partial_cmp_by(&ys, |&x, &y| (2.0 * x).partial_cmp(&y)),
3135 /// Some(Ordering::Greater)
3138 #[unstable(feature = "iter_order_by", issue = "64295")]
3139 fn partial_cmp_by<I, F>(mut self, other: I, mut partial_cmp: F) -> Option<Ordering>
3143 F: FnMut(Self::Item, I::Item) -> Option<Ordering>,
3145 let mut other = other.into_iter();
3148 let x = match self.next() {
3150 if other.next().is_none() {
3151 return Some(Ordering::Equal);
3153 return Some(Ordering::Less);
3159 let y = match other.next() {
3160 None => return Some(Ordering::Greater),
3164 match partial_cmp(x, y) {
3165 Some(Ordering::Equal) => (),
3166 non_eq => return non_eq,
3171 /// Determines if the elements of this [`Iterator`] are equal to those of
3177 /// assert_eq!([1].iter().eq([1].iter()), true);
3178 /// assert_eq!([1].iter().eq([1, 2].iter()), false);
3180 #[stable(feature = "iter_order", since = "1.5.0")]
3181 fn eq<I>(self, other: I) -> bool
3184 Self::Item: PartialEq<I::Item>,
3187 self.eq_by(other, |x, y| x == y)
3190 /// Determines if the elements of this [`Iterator`] are equal to those of
3191 /// another with respect to the specified equality function.
3198 /// #![feature(iter_order_by)]
3200 /// let xs = [1, 2, 3, 4];
3201 /// let ys = [1, 4, 9, 16];
3203 /// assert!(xs.iter().eq_by(&ys, |&x, &y| x * x == y));
3205 #[unstable(feature = "iter_order_by", issue = "64295")]
3206 fn eq_by<I, F>(mut self, other: I, mut eq: F) -> bool
3210 F: FnMut(Self::Item, I::Item) -> bool,
3212 let mut other = other.into_iter();
3215 let x = match self.next() {
3216 None => return other.next().is_none(),
3220 let y = match other.next() {
3221 None => return false,
3231 /// Determines if the elements of this [`Iterator`] are unequal to those of
3237 /// assert_eq!([1].iter().ne([1].iter()), false);
3238 /// assert_eq!([1].iter().ne([1, 2].iter()), true);
3240 #[stable(feature = "iter_order", since = "1.5.0")]
3241 fn ne<I>(self, other: I) -> bool
3244 Self::Item: PartialEq<I::Item>,
3250 /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
3251 /// less than those of another.
3256 /// assert_eq!([1].iter().lt([1].iter()), false);
3257 /// assert_eq!([1].iter().lt([1, 2].iter()), true);
3258 /// assert_eq!([1, 2].iter().lt([1].iter()), false);
3259 /// assert_eq!([1, 2].iter().lt([1, 2].iter()), false);
3261 #[stable(feature = "iter_order", since = "1.5.0")]
3262 fn lt<I>(self, other: I) -> bool
3265 Self::Item: PartialOrd<I::Item>,
3268 self.partial_cmp(other) == Some(Ordering::Less)
3271 /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
3272 /// less or equal to those of another.
3277 /// assert_eq!([1].iter().le([1].iter()), true);
3278 /// assert_eq!([1].iter().le([1, 2].iter()), true);
3279 /// assert_eq!([1, 2].iter().le([1].iter()), false);
3280 /// assert_eq!([1, 2].iter().le([1, 2].iter()), true);
3282 #[stable(feature = "iter_order", since = "1.5.0")]
3283 fn le<I>(self, other: I) -> bool
3286 Self::Item: PartialOrd<I::Item>,
3289 matches!(self.partial_cmp(other), Some(Ordering::Less | Ordering::Equal))
3292 /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
3293 /// greater than those of another.
3298 /// assert_eq!([1].iter().gt([1].iter()), false);
3299 /// assert_eq!([1].iter().gt([1, 2].iter()), false);
3300 /// assert_eq!([1, 2].iter().gt([1].iter()), true);
3301 /// assert_eq!([1, 2].iter().gt([1, 2].iter()), false);
3303 #[stable(feature = "iter_order", since = "1.5.0")]
3304 fn gt<I>(self, other: I) -> bool
3307 Self::Item: PartialOrd<I::Item>,
3310 self.partial_cmp(other) == Some(Ordering::Greater)
3313 /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
3314 /// greater than or equal to those of another.
3319 /// assert_eq!([1].iter().ge([1].iter()), true);
3320 /// assert_eq!([1].iter().ge([1, 2].iter()), false);
3321 /// assert_eq!([1, 2].iter().ge([1].iter()), true);
3322 /// assert_eq!([1, 2].iter().ge([1, 2].iter()), true);
3324 #[stable(feature = "iter_order", since = "1.5.0")]
3325 fn ge<I>(self, other: I) -> bool
3328 Self::Item: PartialOrd<I::Item>,
3331 matches!(self.partial_cmp(other), Some(Ordering::Greater | Ordering::Equal))
3334 /// Checks if the elements of this iterator are sorted.
3336 /// That is, for each element `a` and its following element `b`, `a <= b` must hold. If the
3337 /// iterator yields exactly zero or one element, `true` is returned.
3339 /// Note that if `Self::Item` is only `PartialOrd`, but not `Ord`, the above definition
3340 /// implies that this function returns `false` if any two consecutive items are not
3346 /// #![feature(is_sorted)]
3348 /// assert!([1, 2, 2, 9].iter().is_sorted());
3349 /// assert!(![1, 3, 2, 4].iter().is_sorted());
3350 /// assert!([0].iter().is_sorted());
3351 /// assert!(std::iter::empty::<i32>().is_sorted());
3352 /// assert!(![0.0, 1.0, f32::NAN].iter().is_sorted());
3355 #[unstable(feature = "is_sorted", reason = "new API", issue = "53485")]
3356 fn is_sorted(self) -> bool
3359 Self::Item: PartialOrd,
3361 self.is_sorted_by(PartialOrd::partial_cmp)
3364 /// Checks if the elements of this iterator are sorted using the given comparator function.
3366 /// Instead of using `PartialOrd::partial_cmp`, this function uses the given `compare`
3367 /// function to determine the ordering of two elements. Apart from that, it's equivalent to
3368 /// [`is_sorted`]; see its documentation for more information.
3373 /// #![feature(is_sorted)]
3375 /// assert!([1, 2, 2, 9].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
3376 /// assert!(![1, 3, 2, 4].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
3377 /// assert!([0].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
3378 /// assert!(std::iter::empty::<i32>().is_sorted_by(|a, b| a.partial_cmp(b)));
3379 /// assert!(![0.0, 1.0, f32::NAN].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
3382 /// [`is_sorted`]: Iterator::is_sorted
3383 #[unstable(feature = "is_sorted", reason = "new API", issue = "53485")]
3384 fn is_sorted_by<F>(mut self, compare: F) -> bool
3387 F: FnMut(&Self::Item, &Self::Item) -> Option<Ordering>,
3392 mut compare: impl FnMut(&T, &T) -> Option<Ordering> + 'a,
3393 ) -> impl FnMut(T) -> bool + 'a {
3395 if let Some(Ordering::Greater) | None = compare(&last, &curr) {
3403 let mut last = match self.next() {
3405 None => return true,
3408 self.all(check(&mut last, compare))
3411 /// Checks if the elements of this iterator are sorted using the given key extraction
3414 /// Instead of comparing the iterator's elements directly, this function compares the keys of
3415 /// the elements, as determined by `f`. Apart from that, it's equivalent to [`is_sorted`]; see
3416 /// its documentation for more information.
3418 /// [`is_sorted`]: Iterator::is_sorted
3423 /// #![feature(is_sorted)]
3425 /// assert!(["c", "bb", "aaa"].iter().is_sorted_by_key(|s| s.len()));
3426 /// assert!(![-2i32, -1, 0, 3].iter().is_sorted_by_key(|n| n.abs()));
3429 #[unstable(feature = "is_sorted", reason = "new API", issue = "53485")]
3430 fn is_sorted_by_key<F, K>(self, f: F) -> bool
3433 F: FnMut(Self::Item) -> K,
3436 self.map(f).is_sorted()
3439 /// See [TrustedRandomAccess][super::super::TrustedRandomAccess]
3440 // The unusual name is to avoid name collisions in method resolution
3444 #[unstable(feature = "trusted_random_access", issue = "none")]
3445 unsafe fn __iterator_get_unchecked(&mut self, _idx: usize) -> Self::Item
3447 Self: TrustedRandomAccessNoCoerce,
3449 unreachable!("Always specialized");
3453 #[stable(feature = "rust1", since = "1.0.0")]
3454 impl<I: Iterator + ?Sized> Iterator for &mut I {
3455 type Item = I::Item;
3456 fn next(&mut self) -> Option<I::Item> {
3459 fn size_hint(&self) -> (usize, Option<usize>) {
3460 (**self).size_hint()
3462 fn advance_by(&mut self, n: usize) -> Result<(), usize> {
3463 (**self).advance_by(n)
3465 fn nth(&mut self, n: usize) -> Option<Self::Item> {