1 // ignore-tidy-filelength
2 // This file almost exclusively consists of the definition of `Iterator`. We
3 // can't split that into multiple files.
5 use crate::cmp::{self, Ordering};
6 use crate::ops::{Add, Try};
8 use super::super::LoopState;
9 use super::super::{Chain, Cloned, Copied, Cycle, Enumerate, Filter, FilterMap, Fuse};
10 use super::super::{FlatMap, Flatten};
11 use super::super::{FromIterator, Product, Sum, Zip};
13 Inspect, Map, MapWhile, Peekable, Rev, Scan, Skip, SkipWhile, StepBy, Take, TakeWhile,
16 fn _assert_is_object_safe(_: &dyn Iterator<Item = ()>) {}
18 /// An interface for dealing with iterators.
20 /// This is the main iterator trait. For more about the concept of iterators
21 /// generally, please see the [module-level documentation]. In particular, you
22 /// may want to know how to [implement `Iterator`][impl].
24 /// [module-level documentation]: index.html
25 /// [impl]: index.html#implementing-iterator
26 #[stable(feature = "rust1", since = "1.0.0")]
27 #[rustc_on_unimplemented(
29 _Self = "[std::ops::Range<Idx>; 1]",
30 label = "if you meant to iterate between two values, remove the square brackets",
31 note = "`[start..end]` is an array of one `Range`; you might have meant to have a `Range` \
32 without the brackets: `start..end`"
35 _Self = "[std::ops::RangeFrom<Idx>; 1]",
36 label = "if you meant to iterate from a value onwards, remove the square brackets",
37 note = "`[start..]` is an array of one `RangeFrom`; you might have meant to have a \
38 `RangeFrom` without the brackets: `start..`, keeping in mind that iterating over an \
39 unbounded iterator will run forever unless you `break` or `return` from within the \
43 _Self = "[std::ops::RangeTo<Idx>; 1]",
44 label = "if you meant to iterate until a value, remove the square brackets and add a \
46 note = "`[..end]` is an array of one `RangeTo`; you might have meant to have a bounded \
47 `Range` without the brackets: `0..end`"
50 _Self = "[std::ops::RangeInclusive<Idx>; 1]",
51 label = "if you meant to iterate between two values, remove the square brackets",
52 note = "`[start..=end]` is an array of one `RangeInclusive`; you might have meant to have a \
53 `RangeInclusive` without the brackets: `start..=end`"
56 _Self = "[std::ops::RangeToInclusive<Idx>; 1]",
57 label = "if you meant to iterate until a value (including it), remove the square brackets \
58 and add a starting value",
59 note = "`[..=end]` is an array of one `RangeToInclusive`; you might have meant to have a \
60 bounded `RangeInclusive` without the brackets: `0..=end`"
63 _Self = "std::ops::RangeTo<Idx>",
64 label = "if you meant to iterate until a value, add a starting value",
65 note = "`..end` is a `RangeTo`, which cannot be iterated on; you might have meant to have a \
66 bounded `Range`: `0..end`"
69 _Self = "std::ops::RangeToInclusive<Idx>",
70 label = "if you meant to iterate until a value (including it), add a starting value",
71 note = "`..=end` is a `RangeToInclusive`, which cannot be iterated on; you might have meant \
72 to have a bounded `RangeInclusive`: `0..=end`"
76 label = "`{Self}` is not an iterator; try calling `.chars()` or `.bytes()`"
79 _Self = "std::string::String",
80 label = "`{Self}` is not an iterator; try calling `.chars()` or `.bytes()`"
84 label = "borrow the array with `&` or call `.iter()` on it to iterate over it",
85 note = "arrays are not iterators, but slices like the following are: `&[1, 2, 3]`"
89 note = "if you want to iterate between `start` until a value `end`, use the exclusive range \
90 syntax `start..end` or the inclusive range syntax `start..=end`"
92 label = "`{Self}` is not an iterator",
93 message = "`{Self}` is not an iterator"
96 #[must_use = "iterators are lazy and do nothing unless consumed"]
98 /// The type of the elements being iterated over.
99 #[stable(feature = "rust1", since = "1.0.0")]
102 /// Advances the iterator and returns the next value.
104 /// Returns [`None`] when iteration is finished. Individual iterator
105 /// implementations may choose to resume iteration, and so calling `next()`
106 /// again may or may not eventually start returning [`Some(Item)`] again at some
109 /// [`Some(Item)`]: Some
116 /// let a = [1, 2, 3];
118 /// let mut iter = a.iter();
120 /// // A call to next() returns the next value...
121 /// assert_eq!(Some(&1), iter.next());
122 /// assert_eq!(Some(&2), iter.next());
123 /// assert_eq!(Some(&3), iter.next());
125 /// // ... and then None once it's over.
126 /// assert_eq!(None, iter.next());
128 /// // More calls may or may not return `None`. Here, they always will.
129 /// assert_eq!(None, iter.next());
130 /// assert_eq!(None, iter.next());
132 #[cfg_attr(not(bootstrap), lang = "next")]
133 #[stable(feature = "rust1", since = "1.0.0")]
134 fn next(&mut self) -> Option<Self::Item>;
136 /// Returns the bounds on the remaining length of the iterator.
138 /// Specifically, `size_hint()` returns a tuple where the first element
139 /// is the lower bound, and the second element is the upper bound.
141 /// The second half of the tuple that is returned is an [`Option`]`<`[`usize`]`>`.
142 /// A [`None`] here means that either there is no known upper bound, or the
143 /// upper bound is larger than [`usize`].
145 /// # Implementation notes
147 /// It is not enforced that an iterator implementation yields the declared
148 /// number of elements. A buggy iterator may yield less than the lower bound
149 /// or more than the upper bound of elements.
151 /// `size_hint()` is primarily intended to be used for optimizations such as
152 /// reserving space for the elements of the iterator, but must not be
153 /// trusted to e.g., omit bounds checks in unsafe code. An incorrect
154 /// implementation of `size_hint()` should not lead to memory safety
157 /// That said, the implementation should provide a correct estimation,
158 /// because otherwise it would be a violation of the trait's protocol.
160 /// The default implementation returns `(0, `[`None`]`)` which is correct for any
163 /// [`usize`]: type@usize
170 /// let a = [1, 2, 3];
171 /// let iter = a.iter();
173 /// assert_eq!((3, Some(3)), iter.size_hint());
176 /// A more complex example:
179 /// // The even numbers from zero to ten.
180 /// let iter = (0..10).filter(|x| x % 2 == 0);
182 /// // We might iterate from zero to ten times. Knowing that it's five
183 /// // exactly wouldn't be possible without executing filter().
184 /// assert_eq!((0, Some(10)), iter.size_hint());
186 /// // Let's add five more numbers with chain()
187 /// let iter = (0..10).filter(|x| x % 2 == 0).chain(15..20);
189 /// // now both bounds are increased by five
190 /// assert_eq!((5, Some(15)), iter.size_hint());
193 /// Returning `None` for an upper bound:
196 /// // an infinite iterator has no upper bound
197 /// // and the maximum possible lower bound
200 /// assert_eq!((usize::MAX, None), iter.size_hint());
203 #[stable(feature = "rust1", since = "1.0.0")]
204 fn size_hint(&self) -> (usize, Option<usize>) {
208 /// Consumes the iterator, counting the number of iterations and returning it.
210 /// This method will call [`next`] repeatedly until [`None`] is encountered,
211 /// returning the number of times it saw [`Some`]. Note that [`next`] has to be
212 /// called at least once even if the iterator does not have any elements.
214 /// [`next`]: #tymethod.next
216 /// # Overflow Behavior
218 /// The method does no guarding against overflows, so counting elements of
219 /// an iterator with more than [`usize::MAX`] elements either produces the
220 /// wrong result or panics. If debug assertions are enabled, a panic is
225 /// This function might panic if the iterator has more than [`usize::MAX`]
228 /// [`usize::MAX`]: crate::usize::MAX
235 /// let a = [1, 2, 3];
236 /// assert_eq!(a.iter().count(), 3);
238 /// let a = [1, 2, 3, 4, 5];
239 /// assert_eq!(a.iter().count(), 5);
242 #[stable(feature = "rust1", since = "1.0.0")]
243 fn count(self) -> usize
248 fn add1<T>(count: usize, _: T) -> usize {
256 /// Consumes the iterator, returning the last element.
258 /// This method will evaluate the iterator until it returns [`None`]. While
259 /// doing so, it keeps track of the current element. After [`None`] is
260 /// returned, `last()` will then return the last element it saw.
267 /// let a = [1, 2, 3];
268 /// assert_eq!(a.iter().last(), Some(&3));
270 /// let a = [1, 2, 3, 4, 5];
271 /// assert_eq!(a.iter().last(), Some(&5));
274 #[stable(feature = "rust1", since = "1.0.0")]
275 fn last(self) -> Option<Self::Item>
280 fn some<T>(_: Option<T>, x: T) -> Option<T> {
284 self.fold(None, some)
287 /// Returns the `n`th element of the iterator.
289 /// Like most indexing operations, the count starts from zero, so `nth(0)`
290 /// returns the first value, `nth(1)` the second, and so on.
292 /// Note that all preceding elements, as well as the returned element, will be
293 /// consumed from the iterator. That means that the preceding elements will be
294 /// discarded, and also that calling `nth(0)` multiple times on the same iterator
295 /// will return different elements.
297 /// `nth()` will return [`None`] if `n` is greater than or equal to the length of the
305 /// let a = [1, 2, 3];
306 /// assert_eq!(a.iter().nth(1), Some(&2));
309 /// Calling `nth()` multiple times doesn't rewind the iterator:
312 /// let a = [1, 2, 3];
314 /// let mut iter = a.iter();
316 /// assert_eq!(iter.nth(1), Some(&2));
317 /// assert_eq!(iter.nth(1), None);
320 /// Returning `None` if there are less than `n + 1` elements:
323 /// let a = [1, 2, 3];
324 /// assert_eq!(a.iter().nth(10), None);
327 #[stable(feature = "rust1", since = "1.0.0")]
328 fn nth(&mut self, mut n: usize) -> Option<Self::Item> {
329 while let Some(x) = self.next() {
338 /// Creates an iterator starting at the same point, but stepping by
339 /// the given amount at each iteration.
341 /// Note 1: The first element of the iterator will always be returned,
342 /// regardless of the step given.
344 /// Note 2: The time at which ignored elements are pulled is not fixed.
345 /// `StepBy` behaves like the sequence `next(), nth(step-1), nth(step-1), …`,
346 /// but is also free to behave like the sequence
347 /// `advance_n_and_return_first(step), advance_n_and_return_first(step), …`
348 /// Which way is used may change for some iterators for performance reasons.
349 /// The second way will advance the iterator earlier and may consume more items.
351 /// `advance_n_and_return_first` is the equivalent of:
353 /// fn advance_n_and_return_first<I>(iter: &mut I, total_step: usize) -> Option<I::Item>
357 /// let next = iter.next();
358 /// if total_step > 1 {
359 /// iter.nth(total_step-2);
367 /// The method will panic if the given step is `0`.
374 /// let a = [0, 1, 2, 3, 4, 5];
375 /// let mut iter = a.iter().step_by(2);
377 /// assert_eq!(iter.next(), Some(&0));
378 /// assert_eq!(iter.next(), Some(&2));
379 /// assert_eq!(iter.next(), Some(&4));
380 /// assert_eq!(iter.next(), None);
383 #[stable(feature = "iterator_step_by", since = "1.28.0")]
384 fn step_by(self, step: usize) -> StepBy<Self>
388 StepBy::new(self, step)
391 /// Takes two iterators and creates a new iterator over both in sequence.
393 /// `chain()` will return a new iterator which will first iterate over
394 /// values from the first iterator and then over values from the second
397 /// In other words, it links two iterators together, in a chain. 🔗
399 /// [`once`] is commonly used to adapt a single value into a chain of
400 /// other kinds of iteration.
407 /// let a1 = [1, 2, 3];
408 /// let a2 = [4, 5, 6];
410 /// let mut iter = a1.iter().chain(a2.iter());
412 /// assert_eq!(iter.next(), Some(&1));
413 /// assert_eq!(iter.next(), Some(&2));
414 /// assert_eq!(iter.next(), Some(&3));
415 /// assert_eq!(iter.next(), Some(&4));
416 /// assert_eq!(iter.next(), Some(&5));
417 /// assert_eq!(iter.next(), Some(&6));
418 /// assert_eq!(iter.next(), None);
421 /// Since the argument to `chain()` uses [`IntoIterator`], we can pass
422 /// anything that can be converted into an [`Iterator`], not just an
423 /// [`Iterator`] itself. For example, slices (`&[T]`) implement
424 /// [`IntoIterator`], and so can be passed to `chain()` directly:
427 /// let s1 = &[1, 2, 3];
428 /// let s2 = &[4, 5, 6];
430 /// let mut iter = s1.iter().chain(s2);
432 /// assert_eq!(iter.next(), Some(&1));
433 /// assert_eq!(iter.next(), Some(&2));
434 /// assert_eq!(iter.next(), Some(&3));
435 /// assert_eq!(iter.next(), Some(&4));
436 /// assert_eq!(iter.next(), Some(&5));
437 /// assert_eq!(iter.next(), Some(&6));
438 /// assert_eq!(iter.next(), None);
441 /// If you work with Windows API, you may wish to convert [`OsStr`] to `Vec<u16>`:
445 /// fn os_str_to_utf16(s: &std::ffi::OsStr) -> Vec<u16> {
446 /// use std::os::windows::ffi::OsStrExt;
447 /// s.encode_wide().chain(std::iter::once(0)).collect()
451 /// [`once`]: fn.once.html
452 /// [`Iterator`]: trait.Iterator.html
453 /// [`IntoIterator`]: trait.IntoIterator.html
454 /// [`OsStr`]: ../../std/ffi/struct.OsStr.html
456 #[stable(feature = "rust1", since = "1.0.0")]
457 fn chain<U>(self, other: U) -> Chain<Self, U::IntoIter>
460 U: IntoIterator<Item = Self::Item>,
462 Chain::new(self, other.into_iter())
465 /// 'Zips up' two iterators into a single iterator of pairs.
467 /// `zip()` returns a new iterator that will iterate over two other
468 /// iterators, returning a tuple where the first element comes from the
469 /// first iterator, and the second element comes from the second iterator.
471 /// In other words, it zips two iterators together, into a single one.
473 /// If either iterator returns [`None`], [`next`] from the zipped iterator
474 /// will return [`None`]. If the first iterator returns [`None`], `zip` will
475 /// short-circuit and `next` will not be called on the second iterator.
482 /// let a1 = [1, 2, 3];
483 /// let a2 = [4, 5, 6];
485 /// let mut iter = a1.iter().zip(a2.iter());
487 /// assert_eq!(iter.next(), Some((&1, &4)));
488 /// assert_eq!(iter.next(), Some((&2, &5)));
489 /// assert_eq!(iter.next(), Some((&3, &6)));
490 /// assert_eq!(iter.next(), None);
493 /// Since the argument to `zip()` uses [`IntoIterator`], we can pass
494 /// anything that can be converted into an [`Iterator`], not just an
495 /// [`Iterator`] itself. For example, slices (`&[T]`) implement
496 /// [`IntoIterator`], and so can be passed to `zip()` directly:
498 /// [`IntoIterator`]: trait.IntoIterator.html
499 /// [`Iterator`]: trait.Iterator.html
502 /// let s1 = &[1, 2, 3];
503 /// let s2 = &[4, 5, 6];
505 /// let mut iter = s1.iter().zip(s2);
507 /// assert_eq!(iter.next(), Some((&1, &4)));
508 /// assert_eq!(iter.next(), Some((&2, &5)));
509 /// assert_eq!(iter.next(), Some((&3, &6)));
510 /// assert_eq!(iter.next(), None);
513 /// `zip()` is often used to zip an infinite iterator to a finite one.
514 /// This works because the finite iterator will eventually return [`None`],
515 /// ending the zipper. Zipping with `(0..)` can look a lot like [`enumerate`]:
518 /// let enumerate: Vec<_> = "foo".chars().enumerate().collect();
520 /// let zipper: Vec<_> = (0..).zip("foo".chars()).collect();
522 /// assert_eq!((0, 'f'), enumerate[0]);
523 /// assert_eq!((0, 'f'), zipper[0]);
525 /// assert_eq!((1, 'o'), enumerate[1]);
526 /// assert_eq!((1, 'o'), zipper[1]);
528 /// assert_eq!((2, 'o'), enumerate[2]);
529 /// assert_eq!((2, 'o'), zipper[2]);
532 /// [`enumerate`]: #method.enumerate
533 /// [`next`]: #tymethod.next
535 #[stable(feature = "rust1", since = "1.0.0")]
536 fn zip<U>(self, other: U) -> Zip<Self, U::IntoIter>
541 Zip::new(self, other.into_iter())
544 /// Takes a closure and creates an iterator which calls that closure on each
547 /// `map()` transforms one iterator into another, by means of its argument:
548 /// something that implements [`FnMut`]. It produces a new iterator which
549 /// calls this closure on each element of the original iterator.
551 /// If you are good at thinking in types, you can think of `map()` like this:
552 /// If you have an iterator that gives you elements of some type `A`, and
553 /// you want an iterator of some other type `B`, you can use `map()`,
554 /// passing a closure that takes an `A` and returns a `B`.
556 /// `map()` is conceptually similar to a [`for`] loop. However, as `map()` is
557 /// lazy, it is best used when you're already working with other iterators.
558 /// If you're doing some sort of looping for a side effect, it's considered
559 /// more idiomatic to use [`for`] than `map()`.
561 /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
562 /// [`FnMut`]: crate::ops::FnMut
569 /// let a = [1, 2, 3];
571 /// let mut iter = a.iter().map(|x| 2 * x);
573 /// assert_eq!(iter.next(), Some(2));
574 /// assert_eq!(iter.next(), Some(4));
575 /// assert_eq!(iter.next(), Some(6));
576 /// assert_eq!(iter.next(), None);
579 /// If you're doing some sort of side effect, prefer [`for`] to `map()`:
582 /// # #![allow(unused_must_use)]
583 /// // don't do this:
584 /// (0..5).map(|x| println!("{}", x));
586 /// // it won't even execute, as it is lazy. Rust will warn you about this.
588 /// // Instead, use for:
590 /// println!("{}", x);
594 #[stable(feature = "rust1", since = "1.0.0")]
595 fn map<B, F>(self, f: F) -> Map<Self, F>
598 F: FnMut(Self::Item) -> B,
603 /// Calls a closure on each element of an iterator.
605 /// This is equivalent to using a [`for`] loop on the iterator, although
606 /// `break` and `continue` are not possible from a closure. It's generally
607 /// more idiomatic to use a `for` loop, but `for_each` may be more legible
608 /// when processing items at the end of longer iterator chains. In some
609 /// cases `for_each` may also be faster than a loop, because it will use
610 /// internal iteration on adaptors like `Chain`.
612 /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
619 /// use std::sync::mpsc::channel;
621 /// let (tx, rx) = channel();
622 /// (0..5).map(|x| x * 2 + 1)
623 /// .for_each(move |x| tx.send(x).unwrap());
625 /// let v: Vec<_> = rx.iter().collect();
626 /// assert_eq!(v, vec![1, 3, 5, 7, 9]);
629 /// For such a small example, a `for` loop may be cleaner, but `for_each`
630 /// might be preferable to keep a functional style with longer iterators:
633 /// (0..5).flat_map(|x| x * 100 .. x * 110)
635 /// .filter(|&(i, x)| (i + x) % 3 == 0)
636 /// .for_each(|(i, x)| println!("{}:{}", i, x));
639 #[stable(feature = "iterator_for_each", since = "1.21.0")]
640 fn for_each<F>(self, f: F)
643 F: FnMut(Self::Item),
646 fn call<T>(mut f: impl FnMut(T)) -> impl FnMut((), T) {
647 move |(), item| f(item)
650 self.fold((), call(f));
653 /// Creates an iterator which uses a closure to determine if an element
654 /// should be yielded.
656 /// The closure must return `true` or `false`. `filter()` creates an
657 /// iterator which calls this closure on each element. If the closure
658 /// returns `true`, then the element is returned. If the closure returns
659 /// `false`, it will try again, and call the closure on the next element,
660 /// seeing if it passes the test.
667 /// let a = [0i32, 1, 2];
669 /// let mut iter = a.iter().filter(|x| x.is_positive());
671 /// assert_eq!(iter.next(), Some(&1));
672 /// assert_eq!(iter.next(), Some(&2));
673 /// assert_eq!(iter.next(), None);
676 /// Because the closure passed to `filter()` takes a reference, and many
677 /// iterators iterate over references, this leads to a possibly confusing
678 /// situation, where the type of the closure is a double reference:
681 /// let a = [0, 1, 2];
683 /// let mut iter = a.iter().filter(|x| **x > 1); // need two *s!
685 /// assert_eq!(iter.next(), Some(&2));
686 /// assert_eq!(iter.next(), None);
689 /// It's common to instead use destructuring on the argument to strip away
693 /// let a = [0, 1, 2];
695 /// let mut iter = a.iter().filter(|&x| *x > 1); // both & and *
697 /// assert_eq!(iter.next(), Some(&2));
698 /// assert_eq!(iter.next(), None);
704 /// let a = [0, 1, 2];
706 /// let mut iter = a.iter().filter(|&&x| x > 1); // two &s
708 /// assert_eq!(iter.next(), Some(&2));
709 /// assert_eq!(iter.next(), None);
714 /// Note that `iter.filter(f).next()` is equivalent to `iter.find(f)`.
716 #[stable(feature = "rust1", since = "1.0.0")]
717 fn filter<P>(self, predicate: P) -> Filter<Self, P>
720 P: FnMut(&Self::Item) -> bool,
722 Filter::new(self, predicate)
725 /// Creates an iterator that both filters and maps.
727 /// The closure must return an [`Option<T>`]. `filter_map` creates an
728 /// iterator which calls this closure on each element. If the closure
729 /// returns [`Some(element)`][`Some`], then that element is returned. If the
730 /// closure returns [`None`], it will try again, and call the closure on the
731 /// next element, seeing if it will return [`Some`].
733 /// Why `filter_map` and not just [`filter`] and [`map`]? The key is in this
736 /// [`filter`]: #method.filter
737 /// [`map`]: #method.map
739 /// > If the closure returns [`Some(element)`][`Some`], then that element is returned.
741 /// In other words, it removes the [`Option<T>`] layer automatically. If your
742 /// mapping is already returning an [`Option<T>`] and you want to skip over
743 /// [`None`]s, then `filter_map` is much, much nicer to use.
750 /// let a = ["1", "two", "NaN", "four", "5"];
752 /// let mut iter = a.iter().filter_map(|s| s.parse().ok());
754 /// assert_eq!(iter.next(), Some(1));
755 /// assert_eq!(iter.next(), Some(5));
756 /// assert_eq!(iter.next(), None);
759 /// Here's the same example, but with [`filter`] and [`map`]:
762 /// let a = ["1", "two", "NaN", "four", "5"];
763 /// let mut iter = a.iter().map(|s| s.parse()).filter(|s| s.is_ok()).map(|s| s.unwrap());
764 /// assert_eq!(iter.next(), Some(1));
765 /// assert_eq!(iter.next(), Some(5));
766 /// assert_eq!(iter.next(), None);
769 /// [`Option<T>`]: Option
771 #[stable(feature = "rust1", since = "1.0.0")]
772 fn filter_map<B, F>(self, f: F) -> FilterMap<Self, F>
775 F: FnMut(Self::Item) -> Option<B>,
777 FilterMap::new(self, f)
780 /// Creates an iterator which gives the current iteration count as well as
783 /// The iterator returned yields pairs `(i, val)`, where `i` is the
784 /// current index of iteration and `val` is the value returned by the
787 /// `enumerate()` keeps its count as a [`usize`]. If you want to count by a
788 /// different sized integer, the [`zip`] function provides similar
791 /// # Overflow Behavior
793 /// The method does no guarding against overflows, so enumerating more than
794 /// [`usize::MAX`] elements either produces the wrong result or panics. If
795 /// debug assertions are enabled, a panic is guaranteed.
799 /// The returned iterator might panic if the to-be-returned index would
800 /// overflow a [`usize`].
802 /// [`usize`]: type@usize
803 /// [`usize::MAX`]: crate::usize::MAX
804 /// [`zip`]: #method.zip
809 /// let a = ['a', 'b', 'c'];
811 /// let mut iter = a.iter().enumerate();
813 /// assert_eq!(iter.next(), Some((0, &'a')));
814 /// assert_eq!(iter.next(), Some((1, &'b')));
815 /// assert_eq!(iter.next(), Some((2, &'c')));
816 /// assert_eq!(iter.next(), None);
819 #[stable(feature = "rust1", since = "1.0.0")]
820 fn enumerate(self) -> Enumerate<Self>
827 /// Creates an iterator which can use `peek` to look at the next element of
828 /// the iterator without consuming it.
830 /// Adds a [`peek`] method to an iterator. See its documentation for
831 /// more information.
833 /// Note that the underlying iterator is still advanced when [`peek`] is
834 /// called for the first time: In order to retrieve the next element,
835 /// [`next`] is called on the underlying iterator, hence any side effects (i.e.
836 /// anything other than fetching the next value) of the [`next`] method
839 /// [`peek`]: crate::iter::Peekable::peek
840 /// [`next`]: #tymethod.next
847 /// let xs = [1, 2, 3];
849 /// let mut iter = xs.iter().peekable();
851 /// // peek() lets us see into the future
852 /// assert_eq!(iter.peek(), Some(&&1));
853 /// assert_eq!(iter.next(), Some(&1));
855 /// assert_eq!(iter.next(), Some(&2));
857 /// // we can peek() multiple times, the iterator won't advance
858 /// assert_eq!(iter.peek(), Some(&&3));
859 /// assert_eq!(iter.peek(), Some(&&3));
861 /// assert_eq!(iter.next(), Some(&3));
863 /// // after the iterator is finished, so is peek()
864 /// assert_eq!(iter.peek(), None);
865 /// assert_eq!(iter.next(), None);
868 #[stable(feature = "rust1", since = "1.0.0")]
869 fn peekable(self) -> Peekable<Self>
876 /// Creates an iterator that [`skip`]s elements based on a predicate.
878 /// [`skip`]: #method.skip
880 /// `skip_while()` takes a closure as an argument. It will call this
881 /// closure on each element of the iterator, and ignore elements
882 /// until it returns `false`.
884 /// After `false` is returned, `skip_while()`'s job is over, and the
885 /// rest of the elements are yielded.
892 /// let a = [-1i32, 0, 1];
894 /// let mut iter = a.iter().skip_while(|x| x.is_negative());
896 /// assert_eq!(iter.next(), Some(&0));
897 /// assert_eq!(iter.next(), Some(&1));
898 /// assert_eq!(iter.next(), None);
901 /// Because the closure passed to `skip_while()` takes a reference, and many
902 /// iterators iterate over references, this leads to a possibly confusing
903 /// situation, where the type of the closure is a double reference:
906 /// let a = [-1, 0, 1];
908 /// let mut iter = a.iter().skip_while(|x| **x < 0); // need two *s!
910 /// assert_eq!(iter.next(), Some(&0));
911 /// assert_eq!(iter.next(), Some(&1));
912 /// assert_eq!(iter.next(), None);
915 /// Stopping after an initial `false`:
918 /// let a = [-1, 0, 1, -2];
920 /// let mut iter = a.iter().skip_while(|x| **x < 0);
922 /// assert_eq!(iter.next(), Some(&0));
923 /// assert_eq!(iter.next(), Some(&1));
925 /// // while this would have been false, since we already got a false,
926 /// // skip_while() isn't used any more
927 /// assert_eq!(iter.next(), Some(&-2));
929 /// assert_eq!(iter.next(), None);
932 #[stable(feature = "rust1", since = "1.0.0")]
933 fn skip_while<P>(self, predicate: P) -> SkipWhile<Self, P>
936 P: FnMut(&Self::Item) -> bool,
938 SkipWhile::new(self, predicate)
941 /// Creates an iterator that yields elements based on a predicate.
943 /// `take_while()` takes a closure as an argument. It will call this
944 /// closure on each element of the iterator, and yield elements
945 /// while it returns `true`.
947 /// After `false` is returned, `take_while()`'s job is over, and the
948 /// rest of the elements are ignored.
955 /// let a = [-1i32, 0, 1];
957 /// let mut iter = a.iter().take_while(|x| x.is_negative());
959 /// assert_eq!(iter.next(), Some(&-1));
960 /// assert_eq!(iter.next(), None);
963 /// Because the closure passed to `take_while()` takes a reference, and many
964 /// iterators iterate over references, this leads to a possibly confusing
965 /// situation, where the type of the closure is a double reference:
968 /// let a = [-1, 0, 1];
970 /// let mut iter = a.iter().take_while(|x| **x < 0); // need two *s!
972 /// assert_eq!(iter.next(), Some(&-1));
973 /// assert_eq!(iter.next(), None);
976 /// Stopping after an initial `false`:
979 /// let a = [-1, 0, 1, -2];
981 /// let mut iter = a.iter().take_while(|x| **x < 0);
983 /// assert_eq!(iter.next(), Some(&-1));
985 /// // We have more elements that are less than zero, but since we already
986 /// // got a false, take_while() isn't used any more
987 /// assert_eq!(iter.next(), None);
990 /// Because `take_while()` needs to look at the value in order to see if it
991 /// should be included or not, consuming iterators will see that it is
995 /// let a = [1, 2, 3, 4];
996 /// let mut iter = a.iter();
998 /// let result: Vec<i32> = iter.by_ref()
999 /// .take_while(|n| **n != 3)
1003 /// assert_eq!(result, &[1, 2]);
1005 /// let result: Vec<i32> = iter.cloned().collect();
1007 /// assert_eq!(result, &[4]);
1010 /// The `3` is no longer there, because it was consumed in order to see if
1011 /// the iteration should stop, but wasn't placed back into the iterator.
1013 #[stable(feature = "rust1", since = "1.0.0")]
1014 fn take_while<P>(self, predicate: P) -> TakeWhile<Self, P>
1017 P: FnMut(&Self::Item) -> bool,
1019 TakeWhile::new(self, predicate)
1022 /// Creates an iterator that both yields elements based on a predicate and maps.
1024 /// `map_while()` takes a closure as an argument. It will call this
1025 /// closure on each element of the iterator, and yield elements
1026 /// while it returns [`Some(_)`][`Some`].
1033 /// #![feature(iter_map_while)]
1034 /// let a = [-1i32, 4, 0, 1];
1036 /// let mut iter = a.iter().map_while(|x| 16i32.checked_div(*x));
1038 /// assert_eq!(iter.next(), Some(-16));
1039 /// assert_eq!(iter.next(), Some(4));
1040 /// assert_eq!(iter.next(), None);
1043 /// Here's the same example, but with [`take_while`] and [`map`]:
1045 /// [`take_while`]: #method.take_while
1046 /// [`map`]: #method.map
1049 /// let a = [-1i32, 4, 0, 1];
1051 /// let mut iter = a.iter()
1052 /// .map(|x| 16i32.checked_div(*x))
1053 /// .take_while(|x| x.is_some())
1054 /// .map(|x| x.unwrap());
1056 /// assert_eq!(iter.next(), Some(-16));
1057 /// assert_eq!(iter.next(), Some(4));
1058 /// assert_eq!(iter.next(), None);
1061 /// Stopping after an initial [`None`]:
1064 /// #![feature(iter_map_while)]
1065 /// use std::convert::TryFrom;
1067 /// let a = [0, 1, 2, -3, 4, 5, -6];
1069 /// let iter = a.iter().map_while(|x| u32::try_from(*x).ok());
1070 /// let vec = iter.collect::<Vec<_>>();
1072 /// // We have more elements which could fit in u32 (4, 5), but `map_while` returned `None` for `-3`
1073 /// // (as the `predicate` returned `None`) and `collect` stops at the first `None` encountered.
1074 /// assert_eq!(vec, vec![0, 1, 2]);
1077 /// Because `map_while()` needs to look at the value in order to see if it
1078 /// should be included or not, consuming iterators will see that it is
1082 /// #![feature(iter_map_while)]
1083 /// use std::convert::TryFrom;
1085 /// let a = [1, 2, -3, 4];
1086 /// let mut iter = a.iter();
1088 /// let result: Vec<u32> = iter.by_ref()
1089 /// .map_while(|n| u32::try_from(*n).ok())
1092 /// assert_eq!(result, &[1, 2]);
1094 /// let result: Vec<i32> = iter.cloned().collect();
1096 /// assert_eq!(result, &[4]);
1099 /// The `-3` is no longer there, because it was consumed in order to see if
1100 /// the iteration should stop, but wasn't placed back into the iterator.
1102 /// Note that unlike [`take_while`] this iterator is **not** fused.
1103 /// It is also not specified what this iterator returns after the first` None` is returned.
1104 /// If you need fused iterator, use [`fuse`].
1106 /// [`fuse`]: #method.fuse
1108 #[unstable(feature = "iter_map_while", reason = "recently added", issue = "68537")]
1109 fn map_while<B, P>(self, predicate: P) -> MapWhile<Self, P>
1112 P: FnMut(Self::Item) -> Option<B>,
1114 MapWhile::new(self, predicate)
1117 /// Creates an iterator that skips the first `n` elements.
1119 /// After they have been consumed, the rest of the elements are yielded.
1120 /// Rather than overriding this method directly, instead override the `nth` method.
1127 /// let a = [1, 2, 3];
1129 /// let mut iter = a.iter().skip(2);
1131 /// assert_eq!(iter.next(), Some(&3));
1132 /// assert_eq!(iter.next(), None);
1135 #[stable(feature = "rust1", since = "1.0.0")]
1136 fn skip(self, n: usize) -> Skip<Self>
1143 /// Creates an iterator that yields its first `n` elements.
1150 /// let a = [1, 2, 3];
1152 /// let mut iter = a.iter().take(2);
1154 /// assert_eq!(iter.next(), Some(&1));
1155 /// assert_eq!(iter.next(), Some(&2));
1156 /// assert_eq!(iter.next(), None);
1159 /// `take()` is often used with an infinite iterator, to make it finite:
1162 /// let mut iter = (0..).take(3);
1164 /// assert_eq!(iter.next(), Some(0));
1165 /// assert_eq!(iter.next(), Some(1));
1166 /// assert_eq!(iter.next(), Some(2));
1167 /// assert_eq!(iter.next(), None);
1170 /// If less than `n` elements are available,
1171 /// `take` will limit itself to the size of the underlying iterator:
1174 /// let v = vec![1, 2];
1175 /// let mut iter = v.into_iter().take(5);
1176 /// assert_eq!(iter.next(), Some(1));
1177 /// assert_eq!(iter.next(), Some(2));
1178 /// assert_eq!(iter.next(), None);
1181 #[stable(feature = "rust1", since = "1.0.0")]
1182 fn take(self, n: usize) -> Take<Self>
1189 /// An iterator adaptor similar to [`fold`] that holds internal state and
1190 /// produces a new iterator.
1192 /// [`fold`]: #method.fold
1194 /// `scan()` takes two arguments: an initial value which seeds the internal
1195 /// state, and a closure with two arguments, the first being a mutable
1196 /// reference to the internal state and the second an iterator element.
1197 /// The closure can assign to the internal state to share state between
1200 /// On iteration, the closure will be applied to each element of the
1201 /// iterator and the return value from the closure, an [`Option`], is
1202 /// yielded by the iterator.
1209 /// let a = [1, 2, 3];
1211 /// let mut iter = a.iter().scan(1, |state, &x| {
1212 /// // each iteration, we'll multiply the state by the element
1213 /// *state = *state * x;
1215 /// // then, we'll yield the negation of the state
1219 /// assert_eq!(iter.next(), Some(-1));
1220 /// assert_eq!(iter.next(), Some(-2));
1221 /// assert_eq!(iter.next(), Some(-6));
1222 /// assert_eq!(iter.next(), None);
1225 #[stable(feature = "rust1", since = "1.0.0")]
1226 fn scan<St, B, F>(self, initial_state: St, f: F) -> Scan<Self, St, F>
1229 F: FnMut(&mut St, Self::Item) -> Option<B>,
1231 Scan::new(self, initial_state, f)
1234 /// Creates an iterator that works like map, but flattens nested structure.
1236 /// The [`map`] adapter is very useful, but only when the closure
1237 /// argument produces values. If it produces an iterator instead, there's
1238 /// an extra layer of indirection. `flat_map()` will remove this extra layer
1241 /// You can think of `flat_map(f)` as the semantic equivalent
1242 /// of [`map`]ping, and then [`flatten`]ing as in `map(f).flatten()`.
1244 /// Another way of thinking about `flat_map()`: [`map`]'s closure returns
1245 /// one item for each element, and `flat_map()`'s closure returns an
1246 /// iterator for each element.
1248 /// [`map`]: #method.map
1249 /// [`flatten`]: #method.flatten
1256 /// let words = ["alpha", "beta", "gamma"];
1258 /// // chars() returns an iterator
1259 /// let merged: String = words.iter()
1260 /// .flat_map(|s| s.chars())
1262 /// assert_eq!(merged, "alphabetagamma");
1265 #[stable(feature = "rust1", since = "1.0.0")]
1266 fn flat_map<U, F>(self, f: F) -> FlatMap<Self, U, F>
1270 F: FnMut(Self::Item) -> U,
1272 FlatMap::new(self, f)
1275 /// Creates an iterator that flattens nested structure.
1277 /// This is useful when you have an iterator of iterators or an iterator of
1278 /// things that can be turned into iterators and you want to remove one
1279 /// level of indirection.
1286 /// let data = vec![vec![1, 2, 3, 4], vec![5, 6]];
1287 /// let flattened = data.into_iter().flatten().collect::<Vec<u8>>();
1288 /// assert_eq!(flattened, &[1, 2, 3, 4, 5, 6]);
1291 /// Mapping and then flattening:
1294 /// let words = ["alpha", "beta", "gamma"];
1296 /// // chars() returns an iterator
1297 /// let merged: String = words.iter()
1298 /// .map(|s| s.chars())
1301 /// assert_eq!(merged, "alphabetagamma");
1304 /// You can also rewrite this in terms of [`flat_map()`], which is preferable
1305 /// in this case since it conveys intent more clearly:
1308 /// let words = ["alpha", "beta", "gamma"];
1310 /// // chars() returns an iterator
1311 /// let merged: String = words.iter()
1312 /// .flat_map(|s| s.chars())
1314 /// assert_eq!(merged, "alphabetagamma");
1317 /// Flattening once only removes one level of nesting:
1320 /// let d3 = [[[1, 2], [3, 4]], [[5, 6], [7, 8]]];
1322 /// let d2 = d3.iter().flatten().collect::<Vec<_>>();
1323 /// assert_eq!(d2, [&[1, 2], &[3, 4], &[5, 6], &[7, 8]]);
1325 /// let d1 = d3.iter().flatten().flatten().collect::<Vec<_>>();
1326 /// assert_eq!(d1, [&1, &2, &3, &4, &5, &6, &7, &8]);
1329 /// Here we see that `flatten()` does not perform a "deep" flatten.
1330 /// Instead, only one level of nesting is removed. That is, if you
1331 /// `flatten()` a three-dimensional array the result will be
1332 /// two-dimensional and not one-dimensional. To get a one-dimensional
1333 /// structure, you have to `flatten()` again.
1335 /// [`flat_map()`]: #method.flat_map
1337 #[stable(feature = "iterator_flatten", since = "1.29.0")]
1338 fn flatten(self) -> Flatten<Self>
1341 Self::Item: IntoIterator,
1346 /// Creates an iterator which ends after the first [`None`].
1348 /// After an iterator returns [`None`], future calls may or may not yield
1349 /// [`Some(T)`] again. `fuse()` adapts an iterator, ensuring that after a
1350 /// [`None`] is given, it will always return [`None`] forever.
1352 /// [`Some(T)`]: Some
1359 /// // an iterator which alternates between Some and None
1360 /// struct Alternate {
1364 /// impl Iterator for Alternate {
1365 /// type Item = i32;
1367 /// fn next(&mut self) -> Option<i32> {
1368 /// let val = self.state;
1369 /// self.state = self.state + 1;
1371 /// // if it's even, Some(i32), else None
1372 /// if val % 2 == 0 {
1380 /// let mut iter = Alternate { state: 0 };
1382 /// // we can see our iterator going back and forth
1383 /// assert_eq!(iter.next(), Some(0));
1384 /// assert_eq!(iter.next(), None);
1385 /// assert_eq!(iter.next(), Some(2));
1386 /// assert_eq!(iter.next(), None);
1388 /// // however, once we fuse it...
1389 /// let mut iter = iter.fuse();
1391 /// assert_eq!(iter.next(), Some(4));
1392 /// assert_eq!(iter.next(), None);
1394 /// // it will always return `None` after the first time.
1395 /// assert_eq!(iter.next(), None);
1396 /// assert_eq!(iter.next(), None);
1397 /// assert_eq!(iter.next(), None);
1400 #[stable(feature = "rust1", since = "1.0.0")]
1401 fn fuse(self) -> Fuse<Self>
1408 /// Does something with each element of an iterator, passing the value on.
1410 /// When using iterators, you'll often chain several of them together.
1411 /// While working on such code, you might want to check out what's
1412 /// happening at various parts in the pipeline. To do that, insert
1413 /// a call to `inspect()`.
1415 /// It's more common for `inspect()` to be used as a debugging tool than to
1416 /// exist in your final code, but applications may find it useful in certain
1417 /// situations when errors need to be logged before being discarded.
1424 /// let a = [1, 4, 2, 3];
1426 /// // this iterator sequence is complex.
1427 /// let sum = a.iter()
1429 /// .filter(|x| x % 2 == 0)
1430 /// .fold(0, |sum, i| sum + i);
1432 /// println!("{}", sum);
1434 /// // let's add some inspect() calls to investigate what's happening
1435 /// let sum = a.iter()
1437 /// .inspect(|x| println!("about to filter: {}", x))
1438 /// .filter(|x| x % 2 == 0)
1439 /// .inspect(|x| println!("made it through filter: {}", x))
1440 /// .fold(0, |sum, i| sum + i);
1442 /// println!("{}", sum);
1445 /// This will print:
1449 /// about to filter: 1
1450 /// about to filter: 4
1451 /// made it through filter: 4
1452 /// about to filter: 2
1453 /// made it through filter: 2
1454 /// about to filter: 3
1458 /// Logging errors before discarding them:
1461 /// let lines = ["1", "2", "a"];
1463 /// let sum: i32 = lines
1465 /// .map(|line| line.parse::<i32>())
1466 /// .inspect(|num| {
1467 /// if let Err(ref e) = *num {
1468 /// println!("Parsing error: {}", e);
1471 /// .filter_map(Result::ok)
1474 /// println!("Sum: {}", sum);
1477 /// This will print:
1480 /// Parsing error: invalid digit found in string
1484 #[stable(feature = "rust1", since = "1.0.0")]
1485 fn inspect<F>(self, f: F) -> Inspect<Self, F>
1488 F: FnMut(&Self::Item),
1490 Inspect::new(self, f)
1493 /// Borrows an iterator, rather than consuming it.
1495 /// This is useful to allow applying iterator adaptors while still
1496 /// retaining ownership of the original iterator.
1503 /// let a = [1, 2, 3];
1505 /// let iter = a.iter();
1507 /// let sum: i32 = iter.take(5).fold(0, |acc, i| acc + i);
1509 /// assert_eq!(sum, 6);
1511 /// // if we try to use iter again, it won't work. The following line
1512 /// // gives "error: use of moved value: `iter`
1513 /// // assert_eq!(iter.next(), None);
1515 /// // let's try that again
1516 /// let a = [1, 2, 3];
1518 /// let mut iter = a.iter();
1520 /// // instead, we add in a .by_ref()
1521 /// let sum: i32 = iter.by_ref().take(2).fold(0, |acc, i| acc + i);
1523 /// assert_eq!(sum, 3);
1525 /// // now this is just fine:
1526 /// assert_eq!(iter.next(), Some(&3));
1527 /// assert_eq!(iter.next(), None);
1529 #[stable(feature = "rust1", since = "1.0.0")]
1530 fn by_ref(&mut self) -> &mut Self
1537 /// Transforms an iterator into a collection.
1539 /// `collect()` can take anything iterable, and turn it into a relevant
1540 /// collection. This is one of the more powerful methods in the standard
1541 /// library, used in a variety of contexts.
1543 /// The most basic pattern in which `collect()` is used is to turn one
1544 /// collection into another. You take a collection, call [`iter`] on it,
1545 /// do a bunch of transformations, and then `collect()` at the end.
1547 /// `collect()` can also create instances of types that are not typical
1548 /// collections. For example, a [`String`] can be built from [`char`]s,
1549 /// and an iterator of [`Result<T, E>`][`Result`] items can be collected
1550 /// into `Result<Collection<T>, E>`. See the examples below for more.
1552 /// Because `collect()` is so general, it can cause problems with type
1553 /// inference. As such, `collect()` is one of the few times you'll see
1554 /// the syntax affectionately known as the 'turbofish': `::<>`. This
1555 /// helps the inference algorithm understand specifically which collection
1556 /// you're trying to collect into.
1563 /// let a = [1, 2, 3];
1565 /// let doubled: Vec<i32> = a.iter()
1566 /// .map(|&x| x * 2)
1569 /// assert_eq!(vec![2, 4, 6], doubled);
1572 /// Note that we needed the `: Vec<i32>` on the left-hand side. This is because
1573 /// we could collect into, for example, a [`VecDeque<T>`] instead:
1575 /// [`VecDeque<T>`]: ../../std/collections/struct.VecDeque.html
1578 /// use std::collections::VecDeque;
1580 /// let a = [1, 2, 3];
1582 /// let doubled: VecDeque<i32> = a.iter().map(|&x| x * 2).collect();
1584 /// assert_eq!(2, doubled[0]);
1585 /// assert_eq!(4, doubled[1]);
1586 /// assert_eq!(6, doubled[2]);
1589 /// Using the 'turbofish' instead of annotating `doubled`:
1592 /// let a = [1, 2, 3];
1594 /// let doubled = a.iter().map(|x| x * 2).collect::<Vec<i32>>();
1596 /// assert_eq!(vec![2, 4, 6], doubled);
1599 /// Because `collect()` only cares about what you're collecting into, you can
1600 /// still use a partial type hint, `_`, with the turbofish:
1603 /// let a = [1, 2, 3];
1605 /// let doubled = a.iter().map(|x| x * 2).collect::<Vec<_>>();
1607 /// assert_eq!(vec![2, 4, 6], doubled);
1610 /// Using `collect()` to make a [`String`]:
1613 /// let chars = ['g', 'd', 'k', 'k', 'n'];
1615 /// let hello: String = chars.iter()
1616 /// .map(|&x| x as u8)
1617 /// .map(|x| (x + 1) as char)
1620 /// assert_eq!("hello", hello);
1623 /// If you have a list of [`Result<T, E>`][`Result`]s, you can use `collect()` to
1624 /// see if any of them failed:
1627 /// let results = [Ok(1), Err("nope"), Ok(3), Err("bad")];
1629 /// let result: Result<Vec<_>, &str> = results.iter().cloned().collect();
1631 /// // gives us the first error
1632 /// assert_eq!(Err("nope"), result);
1634 /// let results = [Ok(1), Ok(3)];
1636 /// let result: Result<Vec<_>, &str> = results.iter().cloned().collect();
1638 /// // gives us the list of answers
1639 /// assert_eq!(Ok(vec![1, 3]), result);
1642 /// [`iter`]: #tymethod.next
1643 /// [`String`]: ../../std/string/struct.String.html
1644 /// [`char`]: type@char
1646 #[stable(feature = "rust1", since = "1.0.0")]
1647 #[must_use = "if you really need to exhaust the iterator, consider `.for_each(drop)` instead"]
1648 fn collect<B: FromIterator<Self::Item>>(self) -> B
1652 FromIterator::from_iter(self)
1655 /// Consumes an iterator, creating two collections from it.
1657 /// The predicate passed to `partition()` can return `true`, or `false`.
1658 /// `partition()` returns a pair, all of the elements for which it returned
1659 /// `true`, and all of the elements for which it returned `false`.
1661 /// See also [`is_partitioned()`] and [`partition_in_place()`].
1663 /// [`is_partitioned()`]: #method.is_partitioned
1664 /// [`partition_in_place()`]: #method.partition_in_place
1671 /// let a = [1, 2, 3];
1673 /// let (even, odd): (Vec<i32>, Vec<i32>) = a
1675 /// .partition(|&n| n % 2 == 0);
1677 /// assert_eq!(even, vec![2]);
1678 /// assert_eq!(odd, vec![1, 3]);
1680 #[stable(feature = "rust1", since = "1.0.0")]
1681 fn partition<B, F>(self, f: F) -> (B, B)
1684 B: Default + Extend<Self::Item>,
1685 F: FnMut(&Self::Item) -> bool,
1688 fn extend<'a, T, B: Extend<T>>(
1689 mut f: impl FnMut(&T) -> bool + 'a,
1692 ) -> impl FnMut((), T) + 'a {
1697 right.extend_one(x);
1702 let mut left: B = Default::default();
1703 let mut right: B = Default::default();
1705 self.fold((), extend(f, &mut left, &mut right));
1710 /// Reorders the elements of this iterator *in-place* according to the given predicate,
1711 /// such that all those that return `true` precede all those that return `false`.
1712 /// Returns the number of `true` elements found.
1714 /// The relative order of partitioned items is not maintained.
1716 /// See also [`is_partitioned()`] and [`partition()`].
1718 /// [`is_partitioned()`]: #method.is_partitioned
1719 /// [`partition()`]: #method.partition
1724 /// #![feature(iter_partition_in_place)]
1726 /// let mut a = [1, 2, 3, 4, 5, 6, 7];
1728 /// // Partition in-place between evens and odds
1729 /// let i = a.iter_mut().partition_in_place(|&n| n % 2 == 0);
1731 /// assert_eq!(i, 3);
1732 /// assert!(a[..i].iter().all(|&n| n % 2 == 0)); // evens
1733 /// assert!(a[i..].iter().all(|&n| n % 2 == 1)); // odds
1735 #[unstable(feature = "iter_partition_in_place", reason = "new API", issue = "62543")]
1736 fn partition_in_place<'a, T: 'a, P>(mut self, ref mut predicate: P) -> usize
1738 Self: Sized + DoubleEndedIterator<Item = &'a mut T>,
1739 P: FnMut(&T) -> bool,
1741 // FIXME: should we worry about the count overflowing? The only way to have more than
1742 // `usize::MAX` mutable references is with ZSTs, which aren't useful to partition...
1744 // These closure "factory" functions exist to avoid genericity in `Self`.
1748 predicate: &'a mut impl FnMut(&T) -> bool,
1749 true_count: &'a mut usize,
1750 ) -> impl FnMut(&&mut T) -> bool + 'a {
1752 let p = predicate(&**x);
1753 *true_count += p as usize;
1759 fn is_true<T>(predicate: &mut impl FnMut(&T) -> bool) -> impl FnMut(&&mut T) -> bool + '_ {
1760 move |x| predicate(&**x)
1763 // Repeatedly find the first `false` and swap it with the last `true`.
1764 let mut true_count = 0;
1765 while let Some(head) = self.find(is_false(predicate, &mut true_count)) {
1766 if let Some(tail) = self.rfind(is_true(predicate)) {
1767 crate::mem::swap(head, tail);
1776 /// Checks if the elements of this iterator are partitioned according to the given predicate,
1777 /// such that all those that return `true` precede all those that return `false`.
1779 /// See also [`partition()`] and [`partition_in_place()`].
1781 /// [`partition()`]: #method.partition
1782 /// [`partition_in_place()`]: #method.partition_in_place
1787 /// #![feature(iter_is_partitioned)]
1789 /// assert!("Iterator".chars().is_partitioned(char::is_uppercase));
1790 /// assert!(!"IntoIterator".chars().is_partitioned(char::is_uppercase));
1792 #[unstable(feature = "iter_is_partitioned", reason = "new API", issue = "62544")]
1793 fn is_partitioned<P>(mut self, mut predicate: P) -> bool
1796 P: FnMut(Self::Item) -> bool,
1798 // Either all items test `true`, or the first clause stops at `false`
1799 // and we check that there are no more `true` items after that.
1800 self.all(&mut predicate) || !self.any(predicate)
1803 /// An iterator method that applies a function as long as it returns
1804 /// successfully, producing a single, final value.
1806 /// `try_fold()` takes two arguments: an initial value, and a closure with
1807 /// two arguments: an 'accumulator', and an element. The closure either
1808 /// returns successfully, with the value that the accumulator should have
1809 /// for the next iteration, or it returns failure, with an error value that
1810 /// is propagated back to the caller immediately (short-circuiting).
1812 /// The initial value is the value the accumulator will have on the first
1813 /// call. If applying the closure succeeded against every element of the
1814 /// iterator, `try_fold()` returns the final accumulator as success.
1816 /// Folding is useful whenever you have a collection of something, and want
1817 /// to produce a single value from it.
1819 /// # Note to Implementors
1821 /// Several of the other (forward) methods have default implementations in
1822 /// terms of this one, so try to implement this explicitly if it can
1823 /// do something better than the default `for` loop implementation.
1825 /// In particular, try to have this call `try_fold()` on the internal parts
1826 /// from which this iterator is composed. If multiple calls are needed,
1827 /// the `?` operator may be convenient for chaining the accumulator value
1828 /// along, but beware any invariants that need to be upheld before those
1829 /// early returns. This is a `&mut self` method, so iteration needs to be
1830 /// resumable after hitting an error here.
1837 /// let a = [1, 2, 3];
1839 /// // the checked sum of all of the elements of the array
1840 /// let sum = a.iter().try_fold(0i8, |acc, &x| acc.checked_add(x));
1842 /// assert_eq!(sum, Some(6));
1845 /// Short-circuiting:
1848 /// let a = [10, 20, 30, 100, 40, 50];
1849 /// let mut it = a.iter();
1851 /// // This sum overflows when adding the 100 element
1852 /// let sum = it.try_fold(0i8, |acc, &x| acc.checked_add(x));
1853 /// assert_eq!(sum, None);
1855 /// // Because it short-circuited, the remaining elements are still
1856 /// // available through the iterator.
1857 /// assert_eq!(it.len(), 2);
1858 /// assert_eq!(it.next(), Some(&40));
1861 #[stable(feature = "iterator_try_fold", since = "1.27.0")]
1862 fn try_fold<B, F, R>(&mut self, init: B, mut f: F) -> R
1865 F: FnMut(B, Self::Item) -> R,
1868 let mut accum = init;
1869 while let Some(x) = self.next() {
1870 accum = f(accum, x)?;
1875 /// An iterator method that applies a fallible function to each item in the
1876 /// iterator, stopping at the first error and returning that error.
1878 /// This can also be thought of as the fallible form of [`for_each()`]
1879 /// or as the stateless version of [`try_fold()`].
1881 /// [`for_each()`]: #method.for_each
1882 /// [`try_fold()`]: #method.try_fold
1887 /// use std::fs::rename;
1888 /// use std::io::{stdout, Write};
1889 /// use std::path::Path;
1891 /// let data = ["no_tea.txt", "stale_bread.json", "torrential_rain.png"];
1893 /// let res = data.iter().try_for_each(|x| writeln!(stdout(), "{}", x));
1894 /// assert!(res.is_ok());
1896 /// let mut it = data.iter().cloned();
1897 /// let res = it.try_for_each(|x| rename(x, Path::new(x).with_extension("old")));
1898 /// assert!(res.is_err());
1899 /// // It short-circuited, so the remaining items are still in the iterator:
1900 /// assert_eq!(it.next(), Some("stale_bread.json"));
1903 #[stable(feature = "iterator_try_fold", since = "1.27.0")]
1904 fn try_for_each<F, R>(&mut self, f: F) -> R
1907 F: FnMut(Self::Item) -> R,
1911 fn call<T, R>(mut f: impl FnMut(T) -> R) -> impl FnMut((), T) -> R {
1915 self.try_fold((), call(f))
1918 /// An iterator method that applies a function, producing a single, final value.
1920 /// `fold()` takes two arguments: an initial value, and a closure with two
1921 /// arguments: an 'accumulator', and an element. The closure returns the value that
1922 /// the accumulator should have for the next iteration.
1924 /// The initial value is the value the accumulator will have on the first
1927 /// After applying this closure to every element of the iterator, `fold()`
1928 /// returns the accumulator.
1930 /// This operation is sometimes called 'reduce' or 'inject'.
1932 /// Folding is useful whenever you have a collection of something, and want
1933 /// to produce a single value from it.
1935 /// Note: `fold()`, and similar methods that traverse the entire iterator,
1936 /// may not terminate for infinite iterators, even on traits for which a
1937 /// result is determinable in finite time.
1939 /// # Note to Implementors
1941 /// Several of the other (forward) methods have default implementations in
1942 /// terms of this one, so try to implement this explicitly if it can
1943 /// do something better than the default `for` loop implementation.
1945 /// In particular, try to have this call `fold()` on the internal parts
1946 /// from which this iterator is composed.
1953 /// let a = [1, 2, 3];
1955 /// // the sum of all of the elements of the array
1956 /// let sum = a.iter().fold(0, |acc, x| acc + x);
1958 /// assert_eq!(sum, 6);
1961 /// Let's walk through each step of the iteration here:
1963 /// | element | acc | x | result |
1964 /// |---------|-----|---|--------|
1966 /// | 1 | 0 | 1 | 1 |
1967 /// | 2 | 1 | 2 | 3 |
1968 /// | 3 | 3 | 3 | 6 |
1970 /// And so, our final result, `6`.
1972 /// It's common for people who haven't used iterators a lot to
1973 /// use a `for` loop with a list of things to build up a result. Those
1974 /// can be turned into `fold()`s:
1976 /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
1979 /// let numbers = [1, 2, 3, 4, 5];
1981 /// let mut result = 0;
1984 /// for i in &numbers {
1985 /// result = result + i;
1989 /// let result2 = numbers.iter().fold(0, |acc, &x| acc + x);
1991 /// // they're the same
1992 /// assert_eq!(result, result2);
1995 #[stable(feature = "rust1", since = "1.0.0")]
1996 fn fold<B, F>(mut self, init: B, mut f: F) -> B
1999 F: FnMut(B, Self::Item) -> B,
2001 let mut accum = init;
2002 while let Some(x) = self.next() {
2003 accum = f(accum, x);
2008 /// The same as [`fold()`](#method.fold), but uses the first element in the
2009 /// iterator as the initial value, folding every subsequent element into it.
2010 /// If the iterator is empty, return `None`; otherwise, return the result
2015 /// Find the maximum value:
2018 /// #![feature(iterator_fold_self)]
2020 /// fn find_max<I>(iter: I) -> Option<I::Item>
2021 /// where I: Iterator,
2024 /// iter.fold_first(|a, b| {
2025 /// if a >= b { a } else { b }
2028 /// let a = [10, 20, 5, -23, 0];
2029 /// let b: [u32; 0] = [];
2031 /// assert_eq!(find_max(a.iter()), Some(&20));
2032 /// assert_eq!(find_max(b.iter()), None);
2035 #[unstable(feature = "iterator_fold_self", issue = "68125")]
2036 fn fold_first<F>(mut self, f: F) -> Option<Self::Item>
2039 F: FnMut(Self::Item, Self::Item) -> Self::Item,
2041 let first = self.next()?;
2042 Some(self.fold(first, f))
2045 /// Tests if every element of the iterator matches a predicate.
2047 /// `all()` takes a closure that returns `true` or `false`. It applies
2048 /// this closure to each element of the iterator, and if they all return
2049 /// `true`, then so does `all()`. If any of them return `false`, it
2050 /// returns `false`.
2052 /// `all()` is short-circuiting; in other words, it will stop processing
2053 /// as soon as it finds a `false`, given that no matter what else happens,
2054 /// the result will also be `false`.
2056 /// An empty iterator returns `true`.
2063 /// let a = [1, 2, 3];
2065 /// assert!(a.iter().all(|&x| x > 0));
2067 /// assert!(!a.iter().all(|&x| x > 2));
2070 /// Stopping at the first `false`:
2073 /// let a = [1, 2, 3];
2075 /// let mut iter = a.iter();
2077 /// assert!(!iter.all(|&x| x != 2));
2079 /// // we can still use `iter`, as there are more elements.
2080 /// assert_eq!(iter.next(), Some(&3));
2083 #[stable(feature = "rust1", since = "1.0.0")]
2084 fn all<F>(&mut self, f: F) -> bool
2087 F: FnMut(Self::Item) -> bool,
2090 fn check<T>(mut f: impl FnMut(T) -> bool) -> impl FnMut((), T) -> LoopState<(), ()> {
2092 if f(x) { LoopState::Continue(()) } else { LoopState::Break(()) }
2095 self.try_fold((), check(f)) == LoopState::Continue(())
2098 /// Tests if any element of the iterator matches a predicate.
2100 /// `any()` takes a closure that returns `true` or `false`. It applies
2101 /// this closure to each element of the iterator, and if any of them return
2102 /// `true`, then so does `any()`. If they all return `false`, it
2103 /// returns `false`.
2105 /// `any()` is short-circuiting; in other words, it will stop processing
2106 /// as soon as it finds a `true`, given that no matter what else happens,
2107 /// the result will also be `true`.
2109 /// An empty iterator returns `false`.
2116 /// let a = [1, 2, 3];
2118 /// assert!(a.iter().any(|&x| x > 0));
2120 /// assert!(!a.iter().any(|&x| x > 5));
2123 /// Stopping at the first `true`:
2126 /// let a = [1, 2, 3];
2128 /// let mut iter = a.iter();
2130 /// assert!(iter.any(|&x| x != 2));
2132 /// // we can still use `iter`, as there are more elements.
2133 /// assert_eq!(iter.next(), Some(&2));
2136 #[stable(feature = "rust1", since = "1.0.0")]
2137 fn any<F>(&mut self, f: F) -> bool
2140 F: FnMut(Self::Item) -> bool,
2143 fn check<T>(mut f: impl FnMut(T) -> bool) -> impl FnMut((), T) -> LoopState<(), ()> {
2145 if f(x) { LoopState::Break(()) } else { LoopState::Continue(()) }
2149 self.try_fold((), check(f)) == LoopState::Break(())
2152 /// Searches for an element of an iterator that satisfies a predicate.
2154 /// `find()` takes a closure that returns `true` or `false`. It applies
2155 /// this closure to each element of the iterator, and if any of them return
2156 /// `true`, then `find()` returns [`Some(element)`]. If they all return
2157 /// `false`, it returns [`None`].
2159 /// `find()` is short-circuiting; in other words, it will stop processing
2160 /// as soon as the closure returns `true`.
2162 /// Because `find()` takes a reference, and many iterators iterate over
2163 /// references, this leads to a possibly confusing situation where the
2164 /// argument is a double reference. You can see this effect in the
2165 /// examples below, with `&&x`.
2167 /// [`Some(element)`]: Some
2174 /// let a = [1, 2, 3];
2176 /// assert_eq!(a.iter().find(|&&x| x == 2), Some(&2));
2178 /// assert_eq!(a.iter().find(|&&x| x == 5), None);
2181 /// Stopping at the first `true`:
2184 /// let a = [1, 2, 3];
2186 /// let mut iter = a.iter();
2188 /// assert_eq!(iter.find(|&&x| x == 2), Some(&2));
2190 /// // we can still use `iter`, as there are more elements.
2191 /// assert_eq!(iter.next(), Some(&3));
2194 /// Note that `iter.find(f)` is equivalent to `iter.filter(f).next()`.
2196 #[stable(feature = "rust1", since = "1.0.0")]
2197 fn find<P>(&mut self, predicate: P) -> Option<Self::Item>
2200 P: FnMut(&Self::Item) -> bool,
2204 mut predicate: impl FnMut(&T) -> bool,
2205 ) -> impl FnMut((), T) -> LoopState<(), T> {
2207 if predicate(&x) { LoopState::Break(x) } else { LoopState::Continue(()) }
2211 self.try_fold((), check(predicate)).break_value()
2214 /// Applies function to the elements of iterator and returns
2215 /// the first non-none result.
2217 /// `iter.find_map(f)` is equivalent to `iter.filter_map(f).next()`.
2223 /// let a = ["lol", "NaN", "2", "5"];
2225 /// let first_number = a.iter().find_map(|s| s.parse().ok());
2227 /// assert_eq!(first_number, Some(2));
2230 #[stable(feature = "iterator_find_map", since = "1.30.0")]
2231 fn find_map<B, F>(&mut self, f: F) -> Option<B>
2234 F: FnMut(Self::Item) -> Option<B>,
2237 fn check<T, B>(mut f: impl FnMut(T) -> Option<B>) -> impl FnMut((), T) -> LoopState<(), B> {
2238 move |(), x| match f(x) {
2239 Some(x) => LoopState::Break(x),
2240 None => LoopState::Continue(()),
2244 self.try_fold((), check(f)).break_value()
2247 /// Applies function to the elements of iterator and returns
2248 /// the first true result or the first error.
2253 /// #![feature(try_find)]
2255 /// let a = ["1", "2", "lol", "NaN", "5"];
2257 /// let is_my_num = |s: &str, search: i32| -> Result<bool, std::num::ParseIntError> {
2258 /// Ok(s.parse::<i32>()? == search)
2261 /// let result = a.iter().try_find(|&&s| is_my_num(s, 2));
2262 /// assert_eq!(result, Ok(Some(&"2")));
2264 /// let result = a.iter().try_find(|&&s| is_my_num(s, 5));
2265 /// assert!(result.is_err());
2268 #[unstable(feature = "try_find", reason = "new API", issue = "63178")]
2269 fn try_find<F, R>(&mut self, f: F) -> Result<Option<Self::Item>, R::Error>
2272 F: FnMut(&Self::Item) -> R,
2276 fn check<F, T, R>(mut f: F) -> impl FnMut((), T) -> LoopState<(), Result<T, R::Error>>
2281 move |(), x| match f(&x).into_result() {
2282 Ok(false) => LoopState::Continue(()),
2283 Ok(true) => LoopState::Break(Ok(x)),
2284 Err(x) => LoopState::Break(Err(x)),
2288 self.try_fold((), check(f)).break_value().transpose()
2291 /// Searches for an element in an iterator, returning its index.
2293 /// `position()` takes a closure that returns `true` or `false`. It applies
2294 /// this closure to each element of the iterator, and if one of them
2295 /// returns `true`, then `position()` returns [`Some(index)`]. If all of
2296 /// them return `false`, it returns [`None`].
2298 /// `position()` is short-circuiting; in other words, it will stop
2299 /// processing as soon as it finds a `true`.
2301 /// # Overflow Behavior
2303 /// The method does no guarding against overflows, so if there are more
2304 /// than [`usize::MAX`] non-matching elements, it either produces the wrong
2305 /// result or panics. If debug assertions are enabled, a panic is
2310 /// This function might panic if the iterator has more than `usize::MAX`
2311 /// non-matching elements.
2313 /// [`Some(index)`]: Some
2314 /// [`usize::MAX`]: crate::usize::MAX
2321 /// let a = [1, 2, 3];
2323 /// assert_eq!(a.iter().position(|&x| x == 2), Some(1));
2325 /// assert_eq!(a.iter().position(|&x| x == 5), None);
2328 /// Stopping at the first `true`:
2331 /// let a = [1, 2, 3, 4];
2333 /// let mut iter = a.iter();
2335 /// assert_eq!(iter.position(|&x| x >= 2), Some(1));
2337 /// // we can still use `iter`, as there are more elements.
2338 /// assert_eq!(iter.next(), Some(&3));
2340 /// // The returned index depends on iterator state
2341 /// assert_eq!(iter.position(|&x| x == 4), Some(0));
2345 #[stable(feature = "rust1", since = "1.0.0")]
2346 fn position<P>(&mut self, predicate: P) -> Option<usize>
2349 P: FnMut(Self::Item) -> bool,
2353 mut predicate: impl FnMut(T) -> bool,
2354 ) -> impl FnMut(usize, T) -> LoopState<usize, usize> {
2355 // The addition might panic on overflow
2357 if predicate(x) { LoopState::Break(i) } else { LoopState::Continue(Add::add(i, 1)) }
2361 self.try_fold(0, check(predicate)).break_value()
2364 /// Searches for an element in an iterator from the right, returning its
2367 /// `rposition()` takes a closure that returns `true` or `false`. It applies
2368 /// this closure to each element of the iterator, starting from the end,
2369 /// and if one of them returns `true`, then `rposition()` returns
2370 /// [`Some(index)`]. If all of them return `false`, it returns [`None`].
2372 /// `rposition()` is short-circuiting; in other words, it will stop
2373 /// processing as soon as it finds a `true`.
2375 /// [`Some(index)`]: Some
2382 /// let a = [1, 2, 3];
2384 /// assert_eq!(a.iter().rposition(|&x| x == 3), Some(2));
2386 /// assert_eq!(a.iter().rposition(|&x| x == 5), None);
2389 /// Stopping at the first `true`:
2392 /// let a = [1, 2, 3];
2394 /// let mut iter = a.iter();
2396 /// assert_eq!(iter.rposition(|&x| x == 2), Some(1));
2398 /// // we can still use `iter`, as there are more elements.
2399 /// assert_eq!(iter.next(), Some(&1));
2402 #[stable(feature = "rust1", since = "1.0.0")]
2403 fn rposition<P>(&mut self, predicate: P) -> Option<usize>
2405 P: FnMut(Self::Item) -> bool,
2406 Self: Sized + ExactSizeIterator + DoubleEndedIterator,
2408 // No need for an overflow check here, because `ExactSizeIterator`
2409 // implies that the number of elements fits into a `usize`.
2412 mut predicate: impl FnMut(T) -> bool,
2413 ) -> impl FnMut(usize, T) -> LoopState<usize, usize> {
2416 if predicate(x) { LoopState::Break(i) } else { LoopState::Continue(i) }
2421 self.try_rfold(n, check(predicate)).break_value()
2424 /// Returns the maximum element of an iterator.
2426 /// If several elements are equally maximum, the last element is
2427 /// returned. If the iterator is empty, [`None`] is returned.
2434 /// let a = [1, 2, 3];
2435 /// let b: Vec<u32> = Vec::new();
2437 /// assert_eq!(a.iter().max(), Some(&3));
2438 /// assert_eq!(b.iter().max(), None);
2441 #[stable(feature = "rust1", since = "1.0.0")]
2442 fn max(self) -> Option<Self::Item>
2447 self.max_by(Ord::cmp)
2450 /// Returns the minimum element of an iterator.
2452 /// If several elements are equally minimum, the first element is
2453 /// returned. If the iterator is empty, [`None`] is returned.
2460 /// let a = [1, 2, 3];
2461 /// let b: Vec<u32> = Vec::new();
2463 /// assert_eq!(a.iter().min(), Some(&1));
2464 /// assert_eq!(b.iter().min(), None);
2467 #[stable(feature = "rust1", since = "1.0.0")]
2468 fn min(self) -> Option<Self::Item>
2473 self.min_by(Ord::cmp)
2476 /// Returns the element that gives the maximum value from the
2477 /// specified function.
2479 /// If several elements are equally maximum, the last element is
2480 /// returned. If the iterator is empty, [`None`] is returned.
2485 /// let a = [-3_i32, 0, 1, 5, -10];
2486 /// assert_eq!(*a.iter().max_by_key(|x| x.abs()).unwrap(), -10);
2489 #[stable(feature = "iter_cmp_by_key", since = "1.6.0")]
2490 fn max_by_key<B: Ord, F>(self, f: F) -> Option<Self::Item>
2493 F: FnMut(&Self::Item) -> B,
2496 fn key<T, B>(mut f: impl FnMut(&T) -> B) -> impl FnMut(T) -> (B, T) {
2501 fn compare<T, B: Ord>((x_p, _): &(B, T), (y_p, _): &(B, T)) -> Ordering {
2505 let (_, x) = self.map(key(f)).max_by(compare)?;
2509 /// Returns the element that gives the maximum value with respect to the
2510 /// specified comparison function.
2512 /// If several elements are equally maximum, the last element is
2513 /// returned. If the iterator is empty, [`None`] is returned.
2518 /// let a = [-3_i32, 0, 1, 5, -10];
2519 /// assert_eq!(*a.iter().max_by(|x, y| x.cmp(y)).unwrap(), 5);
2522 #[stable(feature = "iter_max_by", since = "1.15.0")]
2523 fn max_by<F>(self, compare: F) -> Option<Self::Item>
2526 F: FnMut(&Self::Item, &Self::Item) -> Ordering,
2529 fn fold<T>(mut compare: impl FnMut(&T, &T) -> Ordering) -> impl FnMut(T, T) -> T {
2530 move |x, y| cmp::max_by(x, y, &mut compare)
2533 self.fold_first(fold(compare))
2536 /// Returns the element that gives the minimum value from the
2537 /// specified function.
2539 /// If several elements are equally minimum, the first element is
2540 /// returned. If the iterator is empty, [`None`] is returned.
2545 /// let a = [-3_i32, 0, 1, 5, -10];
2546 /// assert_eq!(*a.iter().min_by_key(|x| x.abs()).unwrap(), 0);
2549 #[stable(feature = "iter_cmp_by_key", since = "1.6.0")]
2550 fn min_by_key<B: Ord, F>(self, f: F) -> Option<Self::Item>
2553 F: FnMut(&Self::Item) -> B,
2556 fn key<T, B>(mut f: impl FnMut(&T) -> B) -> impl FnMut(T) -> (B, T) {
2561 fn compare<T, B: Ord>((x_p, _): &(B, T), (y_p, _): &(B, T)) -> Ordering {
2565 let (_, x) = self.map(key(f)).min_by(compare)?;
2569 /// Returns the element that gives the minimum value with respect to the
2570 /// specified comparison function.
2572 /// If several elements are equally minimum, the first element is
2573 /// returned. If the iterator is empty, [`None`] is returned.
2578 /// let a = [-3_i32, 0, 1, 5, -10];
2579 /// assert_eq!(*a.iter().min_by(|x, y| x.cmp(y)).unwrap(), -10);
2582 #[stable(feature = "iter_min_by", since = "1.15.0")]
2583 fn min_by<F>(self, compare: F) -> Option<Self::Item>
2586 F: FnMut(&Self::Item, &Self::Item) -> Ordering,
2589 fn fold<T>(mut compare: impl FnMut(&T, &T) -> Ordering) -> impl FnMut(T, T) -> T {
2590 move |x, y| cmp::min_by(x, y, &mut compare)
2593 self.fold_first(fold(compare))
2596 /// Reverses an iterator's direction.
2598 /// Usually, iterators iterate from left to right. After using `rev()`,
2599 /// an iterator will instead iterate from right to left.
2601 /// This is only possible if the iterator has an end, so `rev()` only
2602 /// works on [`DoubleEndedIterator`]s.
2604 /// [`DoubleEndedIterator`]: trait.DoubleEndedIterator.html
2609 /// let a = [1, 2, 3];
2611 /// let mut iter = a.iter().rev();
2613 /// assert_eq!(iter.next(), Some(&3));
2614 /// assert_eq!(iter.next(), Some(&2));
2615 /// assert_eq!(iter.next(), Some(&1));
2617 /// assert_eq!(iter.next(), None);
2620 #[stable(feature = "rust1", since = "1.0.0")]
2621 fn rev(self) -> Rev<Self>
2623 Self: Sized + DoubleEndedIterator,
2628 /// Converts an iterator of pairs into a pair of containers.
2630 /// `unzip()` consumes an entire iterator of pairs, producing two
2631 /// collections: one from the left elements of the pairs, and one
2632 /// from the right elements.
2634 /// This function is, in some sense, the opposite of [`zip`].
2636 /// [`zip`]: #method.zip
2643 /// let a = [(1, 2), (3, 4)];
2645 /// let (left, right): (Vec<_>, Vec<_>) = a.iter().cloned().unzip();
2647 /// assert_eq!(left, [1, 3]);
2648 /// assert_eq!(right, [2, 4]);
2650 #[stable(feature = "rust1", since = "1.0.0")]
2651 fn unzip<A, B, FromA, FromB>(self) -> (FromA, FromB)
2653 FromA: Default + Extend<A>,
2654 FromB: Default + Extend<B>,
2655 Self: Sized + Iterator<Item = (A, B)>,
2657 fn extend<'a, A, B>(
2658 ts: &'a mut impl Extend<A>,
2659 us: &'a mut impl Extend<B>,
2660 ) -> impl FnMut((), (A, B)) + 'a {
2667 let mut ts: FromA = Default::default();
2668 let mut us: FromB = Default::default();
2670 let (lower_bound, _) = self.size_hint();
2671 if lower_bound > 0 {
2672 ts.extend_reserve(lower_bound);
2673 us.extend_reserve(lower_bound);
2676 self.fold((), extend(&mut ts, &mut us));
2681 /// Creates an iterator which copies all of its elements.
2683 /// This is useful when you have an iterator over `&T`, but you need an
2684 /// iterator over `T`.
2691 /// let a = [1, 2, 3];
2693 /// let v_copied: Vec<_> = a.iter().copied().collect();
2695 /// // copied is the same as .map(|&x| x)
2696 /// let v_map: Vec<_> = a.iter().map(|&x| x).collect();
2698 /// assert_eq!(v_copied, vec![1, 2, 3]);
2699 /// assert_eq!(v_map, vec![1, 2, 3]);
2701 #[stable(feature = "iter_copied", since = "1.36.0")]
2702 fn copied<'a, T: 'a>(self) -> Copied<Self>
2704 Self: Sized + Iterator<Item = &'a T>,
2710 /// Creates an iterator which [`clone`]s all of its elements.
2712 /// This is useful when you have an iterator over `&T`, but you need an
2713 /// iterator over `T`.
2715 /// [`clone`]: crate::clone::Clone::clone
2722 /// let a = [1, 2, 3];
2724 /// let v_cloned: Vec<_> = a.iter().cloned().collect();
2726 /// // cloned is the same as .map(|&x| x), for integers
2727 /// let v_map: Vec<_> = a.iter().map(|&x| x).collect();
2729 /// assert_eq!(v_cloned, vec![1, 2, 3]);
2730 /// assert_eq!(v_map, vec![1, 2, 3]);
2732 #[stable(feature = "rust1", since = "1.0.0")]
2733 fn cloned<'a, T: 'a>(self) -> Cloned<Self>
2735 Self: Sized + Iterator<Item = &'a T>,
2741 /// Repeats an iterator endlessly.
2743 /// Instead of stopping at [`None`], the iterator will instead start again,
2744 /// from the beginning. After iterating again, it will start at the
2745 /// beginning again. And again. And again. Forever.
2752 /// let a = [1, 2, 3];
2754 /// let mut it = a.iter().cycle();
2756 /// assert_eq!(it.next(), Some(&1));
2757 /// assert_eq!(it.next(), Some(&2));
2758 /// assert_eq!(it.next(), Some(&3));
2759 /// assert_eq!(it.next(), Some(&1));
2760 /// assert_eq!(it.next(), Some(&2));
2761 /// assert_eq!(it.next(), Some(&3));
2762 /// assert_eq!(it.next(), Some(&1));
2764 #[stable(feature = "rust1", since = "1.0.0")]
2766 fn cycle(self) -> Cycle<Self>
2768 Self: Sized + Clone,
2773 /// Sums the elements of an iterator.
2775 /// Takes each element, adds them together, and returns the result.
2777 /// An empty iterator returns the zero value of the type.
2781 /// When calling `sum()` and a primitive integer type is being returned, this
2782 /// method will panic if the computation overflows and debug assertions are
2790 /// let a = [1, 2, 3];
2791 /// let sum: i32 = a.iter().sum();
2793 /// assert_eq!(sum, 6);
2795 #[stable(feature = "iter_arith", since = "1.11.0")]
2796 fn sum<S>(self) -> S
2804 /// Iterates over the entire iterator, multiplying all the elements
2806 /// An empty iterator returns the one value of the type.
2810 /// When calling `product()` and a primitive integer type is being returned,
2811 /// method will panic if the computation overflows and debug assertions are
2817 /// fn factorial(n: u32) -> u32 {
2818 /// (1..=n).product()
2820 /// assert_eq!(factorial(0), 1);
2821 /// assert_eq!(factorial(1), 1);
2822 /// assert_eq!(factorial(5), 120);
2824 #[stable(feature = "iter_arith", since = "1.11.0")]
2825 fn product<P>(self) -> P
2828 P: Product<Self::Item>,
2830 Product::product(self)
2833 /// Lexicographically compares the elements of this `Iterator` with those
2839 /// use std::cmp::Ordering;
2841 /// assert_eq!([1].iter().cmp([1].iter()), Ordering::Equal);
2842 /// assert_eq!([1].iter().cmp([1, 2].iter()), Ordering::Less);
2843 /// assert_eq!([1, 2].iter().cmp([1].iter()), Ordering::Greater);
2845 #[stable(feature = "iter_order", since = "1.5.0")]
2846 fn cmp<I>(self, other: I) -> Ordering
2848 I: IntoIterator<Item = Self::Item>,
2852 self.cmp_by(other, |x, y| x.cmp(&y))
2855 /// Lexicographically compares the elements of this `Iterator` with those
2856 /// of another with respect to the specified comparison function.
2863 /// #![feature(iter_order_by)]
2865 /// use std::cmp::Ordering;
2867 /// let xs = [1, 2, 3, 4];
2868 /// let ys = [1, 4, 9, 16];
2870 /// assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| x.cmp(&y)), Ordering::Less);
2871 /// assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| (x * x).cmp(&y)), Ordering::Equal);
2872 /// assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| (2 * x).cmp(&y)), Ordering::Greater);
2874 #[unstable(feature = "iter_order_by", issue = "64295")]
2875 fn cmp_by<I, F>(mut self, other: I, mut cmp: F) -> Ordering
2879 F: FnMut(Self::Item, I::Item) -> Ordering,
2881 let mut other = other.into_iter();
2884 let x = match self.next() {
2886 if other.next().is_none() {
2887 return Ordering::Equal;
2889 return Ordering::Less;
2895 let y = match other.next() {
2896 None => return Ordering::Greater,
2901 Ordering::Equal => (),
2902 non_eq => return non_eq,
2907 /// Lexicographically compares the elements of this `Iterator` with those
2913 /// use std::cmp::Ordering;
2915 /// assert_eq!([1.].iter().partial_cmp([1.].iter()), Some(Ordering::Equal));
2916 /// assert_eq!([1.].iter().partial_cmp([1., 2.].iter()), Some(Ordering::Less));
2917 /// assert_eq!([1., 2.].iter().partial_cmp([1.].iter()), Some(Ordering::Greater));
2919 /// assert_eq!([f64::NAN].iter().partial_cmp([1.].iter()), None);
2921 #[stable(feature = "iter_order", since = "1.5.0")]
2922 fn partial_cmp<I>(self, other: I) -> Option<Ordering>
2925 Self::Item: PartialOrd<I::Item>,
2928 self.partial_cmp_by(other, |x, y| x.partial_cmp(&y))
2931 /// Lexicographically compares the elements of this `Iterator` with those
2932 /// of another with respect to the specified comparison function.
2939 /// #![feature(iter_order_by)]
2941 /// use std::cmp::Ordering;
2943 /// let xs = [1.0, 2.0, 3.0, 4.0];
2944 /// let ys = [1.0, 4.0, 9.0, 16.0];
2947 /// xs.iter().partial_cmp_by(&ys, |&x, &y| x.partial_cmp(&y)),
2948 /// Some(Ordering::Less)
2951 /// xs.iter().partial_cmp_by(&ys, |&x, &y| (x * x).partial_cmp(&y)),
2952 /// Some(Ordering::Equal)
2955 /// xs.iter().partial_cmp_by(&ys, |&x, &y| (2.0 * x).partial_cmp(&y)),
2956 /// Some(Ordering::Greater)
2959 #[unstable(feature = "iter_order_by", issue = "64295")]
2960 fn partial_cmp_by<I, F>(mut self, other: I, mut partial_cmp: F) -> Option<Ordering>
2964 F: FnMut(Self::Item, I::Item) -> Option<Ordering>,
2966 let mut other = other.into_iter();
2969 let x = match self.next() {
2971 if other.next().is_none() {
2972 return Some(Ordering::Equal);
2974 return Some(Ordering::Less);
2980 let y = match other.next() {
2981 None => return Some(Ordering::Greater),
2985 match partial_cmp(x, y) {
2986 Some(Ordering::Equal) => (),
2987 non_eq => return non_eq,
2992 /// Determines if the elements of this `Iterator` are equal to those of
2998 /// assert_eq!([1].iter().eq([1].iter()), true);
2999 /// assert_eq!([1].iter().eq([1, 2].iter()), false);
3001 #[stable(feature = "iter_order", since = "1.5.0")]
3002 fn eq<I>(self, other: I) -> bool
3005 Self::Item: PartialEq<I::Item>,
3008 self.eq_by(other, |x, y| x == y)
3011 /// Determines if the elements of this `Iterator` are equal to those of
3012 /// another with respect to the specified equality function.
3019 /// #![feature(iter_order_by)]
3021 /// let xs = [1, 2, 3, 4];
3022 /// let ys = [1, 4, 9, 16];
3024 /// assert!(xs.iter().eq_by(&ys, |&x, &y| x * x == y));
3026 #[unstable(feature = "iter_order_by", issue = "64295")]
3027 fn eq_by<I, F>(mut self, other: I, mut eq: F) -> bool
3031 F: FnMut(Self::Item, I::Item) -> bool,
3033 let mut other = other.into_iter();
3036 let x = match self.next() {
3037 None => return other.next().is_none(),
3041 let y = match other.next() {
3042 None => return false,
3052 /// Determines if the elements of this `Iterator` are unequal to those of
3058 /// assert_eq!([1].iter().ne([1].iter()), false);
3059 /// assert_eq!([1].iter().ne([1, 2].iter()), true);
3061 #[stable(feature = "iter_order", since = "1.5.0")]
3062 fn ne<I>(self, other: I) -> bool
3065 Self::Item: PartialEq<I::Item>,
3071 /// Determines if the elements of this `Iterator` are lexicographically
3072 /// less than those of another.
3077 /// assert_eq!([1].iter().lt([1].iter()), false);
3078 /// assert_eq!([1].iter().lt([1, 2].iter()), true);
3079 /// assert_eq!([1, 2].iter().lt([1].iter()), false);
3081 #[stable(feature = "iter_order", since = "1.5.0")]
3082 fn lt<I>(self, other: I) -> bool
3085 Self::Item: PartialOrd<I::Item>,
3088 self.partial_cmp(other) == Some(Ordering::Less)
3091 /// Determines if the elements of this `Iterator` are lexicographically
3092 /// less or equal to those of another.
3097 /// assert_eq!([1].iter().le([1].iter()), true);
3098 /// assert_eq!([1].iter().le([1, 2].iter()), true);
3099 /// assert_eq!([1, 2].iter().le([1].iter()), false);
3101 #[stable(feature = "iter_order", since = "1.5.0")]
3102 fn le<I>(self, other: I) -> bool
3105 Self::Item: PartialOrd<I::Item>,
3108 matches!(self.partial_cmp(other), Some(Ordering::Less | Ordering::Equal))
3111 /// Determines if the elements of this `Iterator` are lexicographically
3112 /// greater than those of another.
3117 /// assert_eq!([1].iter().gt([1].iter()), false);
3118 /// assert_eq!([1].iter().gt([1, 2].iter()), false);
3119 /// assert_eq!([1, 2].iter().gt([1].iter()), true);
3121 #[stable(feature = "iter_order", since = "1.5.0")]
3122 fn gt<I>(self, other: I) -> bool
3125 Self::Item: PartialOrd<I::Item>,
3128 self.partial_cmp(other) == Some(Ordering::Greater)
3131 /// Determines if the elements of this `Iterator` are lexicographically
3132 /// greater than or equal to those of another.
3137 /// assert_eq!([1].iter().ge([1].iter()), true);
3138 /// assert_eq!([1].iter().ge([1, 2].iter()), false);
3139 /// assert_eq!([1, 2].iter().ge([1].iter()), true);
3141 #[stable(feature = "iter_order", since = "1.5.0")]
3142 fn ge<I>(self, other: I) -> bool
3145 Self::Item: PartialOrd<I::Item>,
3148 matches!(self.partial_cmp(other), Some(Ordering::Greater | Ordering::Equal))
3151 /// Checks if the elements of this iterator are sorted.
3153 /// That is, for each element `a` and its following element `b`, `a <= b` must hold. If the
3154 /// iterator yields exactly zero or one element, `true` is returned.
3156 /// Note that if `Self::Item` is only `PartialOrd`, but not `Ord`, the above definition
3157 /// implies that this function returns `false` if any two consecutive items are not
3163 /// #![feature(is_sorted)]
3165 /// assert!([1, 2, 2, 9].iter().is_sorted());
3166 /// assert!(![1, 3, 2, 4].iter().is_sorted());
3167 /// assert!([0].iter().is_sorted());
3168 /// assert!(std::iter::empty::<i32>().is_sorted());
3169 /// assert!(![0.0, 1.0, f32::NAN].iter().is_sorted());
3172 #[unstable(feature = "is_sorted", reason = "new API", issue = "53485")]
3173 fn is_sorted(self) -> bool
3176 Self::Item: PartialOrd,
3178 self.is_sorted_by(PartialOrd::partial_cmp)
3181 /// Checks if the elements of this iterator are sorted using the given comparator function.
3183 /// Instead of using `PartialOrd::partial_cmp`, this function uses the given `compare`
3184 /// function to determine the ordering of two elements. Apart from that, it's equivalent to
3185 /// [`is_sorted`]; see its documentation for more information.
3190 /// #![feature(is_sorted)]
3192 /// assert!([1, 2, 2, 9].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
3193 /// assert!(![1, 3, 2, 4].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
3194 /// assert!([0].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
3195 /// assert!(std::iter::empty::<i32>().is_sorted_by(|a, b| a.partial_cmp(b)));
3196 /// assert!(![0.0, 1.0, f32::NAN].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
3199 /// [`is_sorted`]: #method.is_sorted
3200 #[unstable(feature = "is_sorted", reason = "new API", issue = "53485")]
3201 fn is_sorted_by<F>(mut self, mut compare: F) -> bool
3204 F: FnMut(&Self::Item, &Self::Item) -> Option<Ordering>,
3206 let mut last = match self.next() {
3208 None => return true,
3211 while let Some(curr) = self.next() {
3212 if let Some(Ordering::Greater) | None = compare(&last, &curr) {
3221 /// Checks if the elements of this iterator are sorted using the given key extraction
3224 /// Instead of comparing the iterator's elements directly, this function compares the keys of
3225 /// the elements, as determined by `f`. Apart from that, it's equivalent to [`is_sorted`]; see
3226 /// its documentation for more information.
3228 /// [`is_sorted`]: #method.is_sorted
3233 /// #![feature(is_sorted)]
3235 /// assert!(["c", "bb", "aaa"].iter().is_sorted_by_key(|s| s.len()));
3236 /// assert!(![-2i32, -1, 0, 3].iter().is_sorted_by_key(|n| n.abs()));
3239 #[unstable(feature = "is_sorted", reason = "new API", issue = "53485")]
3240 fn is_sorted_by_key<F, K>(self, f: F) -> bool
3243 F: FnMut(Self::Item) -> K,
3246 self.map(f).is_sorted()
3250 #[stable(feature = "rust1", since = "1.0.0")]
3251 impl<I: Iterator + ?Sized> Iterator for &mut I {
3252 type Item = I::Item;
3253 fn next(&mut self) -> Option<I::Item> {
3256 fn size_hint(&self) -> (usize, Option<usize>) {
3257 (**self).size_hint()
3259 fn nth(&mut self, n: usize) -> Option<Self::Item> {