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::{ControlFlow, Try};
8 use super::super::TrustedRandomAccess;
9 use super::super::{Chain, Cloned, Copied, Cycle, Enumerate, Filter, FilterMap, Fuse};
10 use super::super::{FlatMap, Flatten};
11 use super::super::{FromIterator, Intersperse, IntersperseWith, 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]: crate::iter
25 /// [impl]: crate::iter#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 = "arrays do not yet implement `IntoIterator`; try using `std::array::IntoIter::new(arr)`",
85 note = "see <https://github.com/rust-lang/rust/pull/65819> for more details"
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"
95 #[cfg_attr(bootstrap, doc(spotlight))]
96 #[cfg_attr(not(bootstrap), doc(notable_trait))]
97 #[rustc_diagnostic_item = "Iterator"]
98 #[must_use = "iterators are lazy and do nothing unless consumed"]
100 /// The type of the elements being iterated over.
101 #[stable(feature = "rust1", since = "1.0.0")]
104 /// Advances the iterator and returns the next value.
106 /// Returns [`None`] when iteration is finished. Individual iterator
107 /// implementations may choose to resume iteration, and so calling `next()`
108 /// again may or may not eventually start returning [`Some(Item)`] again at some
111 /// [`Some(Item)`]: Some
118 /// let a = [1, 2, 3];
120 /// let mut iter = a.iter();
122 /// // A call to next() returns the next value...
123 /// assert_eq!(Some(&1), iter.next());
124 /// assert_eq!(Some(&2), iter.next());
125 /// assert_eq!(Some(&3), iter.next());
127 /// // ... and then None once it's over.
128 /// assert_eq!(None, iter.next());
130 /// // More calls may or may not return `None`. Here, they always will.
131 /// assert_eq!(None, iter.next());
132 /// assert_eq!(None, iter.next());
135 #[stable(feature = "rust1", since = "1.0.0")]
136 fn next(&mut self) -> Option<Self::Item>;
138 /// Returns the bounds on the remaining length of the iterator.
140 /// Specifically, `size_hint()` returns a tuple where the first element
141 /// is the lower bound, and the second element is the upper bound.
143 /// The second half of the tuple that is returned is an [`Option`]`<`[`usize`]`>`.
144 /// A [`None`] here means that either there is no known upper bound, or the
145 /// upper bound is larger than [`usize`].
147 /// # Implementation notes
149 /// It is not enforced that an iterator implementation yields the declared
150 /// number of elements. A buggy iterator may yield less than the lower bound
151 /// or more than the upper bound of elements.
153 /// `size_hint()` is primarily intended to be used for optimizations such as
154 /// reserving space for the elements of the iterator, but must not be
155 /// trusted to e.g., omit bounds checks in unsafe code. An incorrect
156 /// implementation of `size_hint()` should not lead to memory safety
159 /// That said, the implementation should provide a correct estimation,
160 /// because otherwise it would be a violation of the trait's protocol.
162 /// The default implementation returns `(0, `[`None`]`)` which is correct for any
165 /// [`usize`]: type@usize
172 /// let a = [1, 2, 3];
173 /// let iter = a.iter();
175 /// assert_eq!((3, Some(3)), iter.size_hint());
178 /// A more complex example:
181 /// // The even numbers from zero to ten.
182 /// let iter = (0..10).filter(|x| x % 2 == 0);
184 /// // We might iterate from zero to ten times. Knowing that it's five
185 /// // exactly wouldn't be possible without executing filter().
186 /// assert_eq!((0, Some(10)), iter.size_hint());
188 /// // Let's add five more numbers with chain()
189 /// let iter = (0..10).filter(|x| x % 2 == 0).chain(15..20);
191 /// // now both bounds are increased by five
192 /// assert_eq!((5, Some(15)), iter.size_hint());
195 /// Returning `None` for an upper bound:
198 /// // an infinite iterator has no upper bound
199 /// // and the maximum possible lower bound
202 /// assert_eq!((usize::MAX, None), iter.size_hint());
205 #[stable(feature = "rust1", since = "1.0.0")]
206 fn size_hint(&self) -> (usize, Option<usize>) {
210 /// Consumes the iterator, counting the number of iterations and returning it.
212 /// This method will call [`next`] repeatedly until [`None`] is encountered,
213 /// returning the number of times it saw [`Some`]. Note that [`next`] has to be
214 /// called at least once even if the iterator does not have any elements.
216 /// [`next`]: Iterator::next
218 /// # Overflow Behavior
220 /// The method does no guarding against overflows, so counting elements of
221 /// an iterator with more than [`usize::MAX`] elements either produces the
222 /// wrong result or panics. If debug assertions are enabled, a panic is
227 /// This function might panic if the iterator has more than [`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
249 #[rustc_inherit_overflow_checks]
250 |count, _| count + 1,
254 /// Consumes the iterator, returning the last element.
256 /// This method will evaluate the iterator until it returns [`None`]. While
257 /// doing so, it keeps track of the current element. After [`None`] is
258 /// returned, `last()` will then return the last element it saw.
265 /// let a = [1, 2, 3];
266 /// assert_eq!(a.iter().last(), Some(&3));
268 /// let a = [1, 2, 3, 4, 5];
269 /// assert_eq!(a.iter().last(), Some(&5));
272 #[stable(feature = "rust1", since = "1.0.0")]
273 fn last(self) -> Option<Self::Item>
278 fn some<T>(_: Option<T>, x: T) -> Option<T> {
282 self.fold(None, some)
285 /// Advances the iterator by `n` elements.
287 /// This method will eagerly skip `n` elements by calling [`next`] up to `n`
288 /// times until [`None`] is encountered.
290 /// `advance_by(n)` will return [`Ok(())`][Ok] if the iterator successfully advances by
291 /// `n` elements, or [`Err(k)`][Err] if [`None`] is encountered, where `k` is the number
292 /// of elements the iterator is advanced by before running out of elements (i.e. the
293 /// length of the iterator). Note that `k` is always less than `n`.
295 /// Calling `advance_by(0)` does not consume any elements and always returns [`Ok(())`][Ok].
297 /// [`next`]: Iterator::next
304 /// #![feature(iter_advance_by)]
306 /// let a = [1, 2, 3, 4];
307 /// let mut iter = a.iter();
309 /// assert_eq!(iter.advance_by(2), Ok(()));
310 /// assert_eq!(iter.next(), Some(&3));
311 /// assert_eq!(iter.advance_by(0), Ok(()));
312 /// assert_eq!(iter.advance_by(100), Err(1)); // only `&4` was skipped
315 #[unstable(feature = "iter_advance_by", reason = "recently added", issue = "77404")]
316 fn advance_by(&mut self, n: usize) -> Result<(), usize> {
318 self.next().ok_or(i)?;
323 /// Returns the `n`th element of the iterator.
325 /// Like most indexing operations, the count starts from zero, so `nth(0)`
326 /// returns the first value, `nth(1)` the second, and so on.
328 /// Note that all preceding elements, as well as the returned element, will be
329 /// consumed from the iterator. That means that the preceding elements will be
330 /// discarded, and also that calling `nth(0)` multiple times on the same iterator
331 /// will return different elements.
333 /// `nth()` will return [`None`] if `n` is greater than or equal to the length of the
341 /// let a = [1, 2, 3];
342 /// assert_eq!(a.iter().nth(1), Some(&2));
345 /// Calling `nth()` multiple times doesn't rewind the iterator:
348 /// let a = [1, 2, 3];
350 /// let mut iter = a.iter();
352 /// assert_eq!(iter.nth(1), Some(&2));
353 /// assert_eq!(iter.nth(1), None);
356 /// Returning `None` if there are less than `n + 1` elements:
359 /// let a = [1, 2, 3];
360 /// assert_eq!(a.iter().nth(10), None);
363 #[stable(feature = "rust1", since = "1.0.0")]
364 fn nth(&mut self, n: usize) -> Option<Self::Item> {
365 self.advance_by(n).ok()?;
369 /// Creates an iterator starting at the same point, but stepping by
370 /// the given amount at each iteration.
372 /// Note 1: The first element of the iterator will always be returned,
373 /// regardless of the step given.
375 /// Note 2: The time at which ignored elements are pulled is not fixed.
376 /// `StepBy` behaves like the sequence `next(), nth(step-1), nth(step-1), …`,
377 /// but is also free to behave like the sequence
378 /// `advance_n_and_return_first(step), advance_n_and_return_first(step), …`
379 /// Which way is used may change for some iterators for performance reasons.
380 /// The second way will advance the iterator earlier and may consume more items.
382 /// `advance_n_and_return_first` is the equivalent of:
384 /// fn advance_n_and_return_first<I>(iter: &mut I, total_step: usize) -> Option<I::Item>
388 /// let next = iter.next();
389 /// if total_step > 1 {
390 /// iter.nth(total_step-2);
398 /// The method will panic if the given step is `0`.
405 /// let a = [0, 1, 2, 3, 4, 5];
406 /// let mut iter = a.iter().step_by(2);
408 /// assert_eq!(iter.next(), Some(&0));
409 /// assert_eq!(iter.next(), Some(&2));
410 /// assert_eq!(iter.next(), Some(&4));
411 /// assert_eq!(iter.next(), None);
414 #[stable(feature = "iterator_step_by", since = "1.28.0")]
415 fn step_by(self, step: usize) -> StepBy<Self>
419 StepBy::new(self, step)
422 /// Takes two iterators and creates a new iterator over both in sequence.
424 /// `chain()` will return a new iterator which will first iterate over
425 /// values from the first iterator and then over values from the second
428 /// In other words, it links two iterators together, in a chain. 🔗
430 /// [`once`] is commonly used to adapt a single value into a chain of
431 /// other kinds of iteration.
438 /// let a1 = [1, 2, 3];
439 /// let a2 = [4, 5, 6];
441 /// let mut iter = a1.iter().chain(a2.iter());
443 /// assert_eq!(iter.next(), Some(&1));
444 /// assert_eq!(iter.next(), Some(&2));
445 /// assert_eq!(iter.next(), Some(&3));
446 /// assert_eq!(iter.next(), Some(&4));
447 /// assert_eq!(iter.next(), Some(&5));
448 /// assert_eq!(iter.next(), Some(&6));
449 /// assert_eq!(iter.next(), None);
452 /// Since the argument to `chain()` uses [`IntoIterator`], we can pass
453 /// anything that can be converted into an [`Iterator`], not just an
454 /// [`Iterator`] itself. For example, slices (`&[T]`) implement
455 /// [`IntoIterator`], and so can be passed to `chain()` directly:
458 /// let s1 = &[1, 2, 3];
459 /// let s2 = &[4, 5, 6];
461 /// let mut iter = s1.iter().chain(s2);
463 /// assert_eq!(iter.next(), Some(&1));
464 /// assert_eq!(iter.next(), Some(&2));
465 /// assert_eq!(iter.next(), Some(&3));
466 /// assert_eq!(iter.next(), Some(&4));
467 /// assert_eq!(iter.next(), Some(&5));
468 /// assert_eq!(iter.next(), Some(&6));
469 /// assert_eq!(iter.next(), None);
472 /// If you work with Windows API, you may wish to convert [`OsStr`] to `Vec<u16>`:
476 /// fn os_str_to_utf16(s: &std::ffi::OsStr) -> Vec<u16> {
477 /// use std::os::windows::ffi::OsStrExt;
478 /// s.encode_wide().chain(std::iter::once(0)).collect()
482 /// [`once`]: crate::iter::once
483 /// [`OsStr`]: ../../std/ffi/struct.OsStr.html
485 #[stable(feature = "rust1", since = "1.0.0")]
486 fn chain<U>(self, other: U) -> Chain<Self, U::IntoIter>
489 U: IntoIterator<Item = Self::Item>,
491 Chain::new(self, other.into_iter())
494 /// 'Zips up' two iterators into a single iterator of pairs.
496 /// `zip()` returns a new iterator that will iterate over two other
497 /// iterators, returning a tuple where the first element comes from the
498 /// first iterator, and the second element comes from the second iterator.
500 /// In other words, it zips two iterators together, into a single one.
502 /// If either iterator returns [`None`], [`next`] from the zipped iterator
503 /// will return [`None`]. If the first iterator returns [`None`], `zip` will
504 /// short-circuit and `next` will not be called on the second iterator.
511 /// let a1 = [1, 2, 3];
512 /// let a2 = [4, 5, 6];
514 /// let mut iter = a1.iter().zip(a2.iter());
516 /// assert_eq!(iter.next(), Some((&1, &4)));
517 /// assert_eq!(iter.next(), Some((&2, &5)));
518 /// assert_eq!(iter.next(), Some((&3, &6)));
519 /// assert_eq!(iter.next(), None);
522 /// Since the argument to `zip()` uses [`IntoIterator`], we can pass
523 /// anything that can be converted into an [`Iterator`], not just an
524 /// [`Iterator`] itself. For example, slices (`&[T]`) implement
525 /// [`IntoIterator`], and so can be passed to `zip()` directly:
528 /// let s1 = &[1, 2, 3];
529 /// let s2 = &[4, 5, 6];
531 /// let mut iter = s1.iter().zip(s2);
533 /// assert_eq!(iter.next(), Some((&1, &4)));
534 /// assert_eq!(iter.next(), Some((&2, &5)));
535 /// assert_eq!(iter.next(), Some((&3, &6)));
536 /// assert_eq!(iter.next(), None);
539 /// `zip()` is often used to zip an infinite iterator to a finite one.
540 /// This works because the finite iterator will eventually return [`None`],
541 /// ending the zipper. Zipping with `(0..)` can look a lot like [`enumerate`]:
544 /// let enumerate: Vec<_> = "foo".chars().enumerate().collect();
546 /// let zipper: Vec<_> = (0..).zip("foo".chars()).collect();
548 /// assert_eq!((0, 'f'), enumerate[0]);
549 /// assert_eq!((0, 'f'), zipper[0]);
551 /// assert_eq!((1, 'o'), enumerate[1]);
552 /// assert_eq!((1, 'o'), zipper[1]);
554 /// assert_eq!((2, 'o'), enumerate[2]);
555 /// assert_eq!((2, 'o'), zipper[2]);
558 /// [`enumerate`]: Iterator::enumerate
559 /// [`next`]: Iterator::next
561 #[stable(feature = "rust1", since = "1.0.0")]
562 fn zip<U>(self, other: U) -> Zip<Self, U::IntoIter>
567 Zip::new(self, other.into_iter())
570 /// Creates a new iterator which places a copy of `separator` between adjacent
571 /// items of the original iterator.
573 /// In case `separator` does not implement [`Clone`] or needs to be
574 /// computed every time, use [`intersperse_with`].
581 /// #![feature(iter_intersperse)]
583 /// let mut a = [0, 1, 2].iter().intersperse(&100);
584 /// assert_eq!(a.next(), Some(&0)); // The first element from `a`.
585 /// assert_eq!(a.next(), Some(&100)); // The separator.
586 /// assert_eq!(a.next(), Some(&1)); // The next element from `a`.
587 /// assert_eq!(a.next(), Some(&100)); // The separator.
588 /// assert_eq!(a.next(), Some(&2)); // The last element from `a`.
589 /// assert_eq!(a.next(), None); // The iterator is finished.
592 /// `intersperse` can be very useful to join an iterator's items using a common element:
594 /// #![feature(iter_intersperse)]
596 /// let hello = ["Hello", "World", "!"].iter().copied().intersperse(" ").collect::<String>();
597 /// assert_eq!(hello, "Hello World !");
600 /// [`Clone`]: crate::clone::Clone
601 /// [`intersperse_with`]: Iterator::intersperse_with
603 #[unstable(feature = "iter_intersperse", reason = "recently added", issue = "79524")]
604 fn intersperse(self, separator: Self::Item) -> Intersperse<Self>
609 Intersperse::new(self, separator)
612 /// Creates a new iterator which places an item generated by `separator`
613 /// between adjacent items of the original iterator.
615 /// The closure will be called exactly once each time an item is placed
616 /// between two adjacent items from the underlying iterator; specifically,
617 /// the closure is not called if the underlying iterator yields less than
618 /// two items and after the last item is yielded.
620 /// If the iterator's item implements [`Clone`], it may be easier to use
628 /// #![feature(iter_intersperse)]
630 /// #[derive(PartialEq, Debug)]
631 /// struct NotClone(usize);
633 /// let v = vec![NotClone(0), NotClone(1), NotClone(2)];
634 /// let mut it = v.into_iter().intersperse_with(|| NotClone(99));
636 /// assert_eq!(it.next(), Some(NotClone(0))); // The first element from `v`.
637 /// assert_eq!(it.next(), Some(NotClone(99))); // The separator.
638 /// assert_eq!(it.next(), Some(NotClone(1))); // The next element from `v`.
639 /// assert_eq!(it.next(), Some(NotClone(99))); // The separator.
640 /// assert_eq!(it.next(), Some(NotClone(2))); // The last element from from `v`.
641 /// assert_eq!(it.next(), None); // The iterator is finished.
644 /// `intersperse_with` can be used in situations where the separator needs
647 /// #![feature(iter_intersperse)]
649 /// let src = ["Hello", "to", "all", "people", "!!"].iter().copied();
651 /// // The closure mutably borrows its context to generate an item.
652 /// let mut happy_emojis = [" ❤️ ", " 😀 "].iter().copied();
653 /// let separator = || happy_emojis.next().unwrap_or(" 🦀 ");
655 /// let result = src.intersperse_with(separator).collect::<String>();
656 /// assert_eq!(result, "Hello ❤️ to 😀 all 🦀 people 🦀 !!");
658 /// [`Clone`]: crate::clone::Clone
659 /// [`intersperse`]: Iterator::intersperse
661 #[unstable(feature = "iter_intersperse", reason = "recently added", issue = "79524")]
662 fn intersperse_with<G>(self, separator: G) -> IntersperseWith<Self, G>
665 G: FnMut() -> Self::Item,
667 IntersperseWith::new(self, separator)
670 /// Takes a closure and creates an iterator which calls that closure on each
673 /// `map()` transforms one iterator into another, by means of its argument:
674 /// something that implements [`FnMut`]. It produces a new iterator which
675 /// calls this closure on each element of the original iterator.
677 /// If you are good at thinking in types, you can think of `map()` like this:
678 /// If you have an iterator that gives you elements of some type `A`, and
679 /// you want an iterator of some other type `B`, you can use `map()`,
680 /// passing a closure that takes an `A` and returns a `B`.
682 /// `map()` is conceptually similar to a [`for`] loop. However, as `map()` is
683 /// lazy, it is best used when you're already working with other iterators.
684 /// If you're doing some sort of looping for a side effect, it's considered
685 /// more idiomatic to use [`for`] than `map()`.
687 /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
688 /// [`FnMut`]: crate::ops::FnMut
695 /// let a = [1, 2, 3];
697 /// let mut iter = a.iter().map(|x| 2 * x);
699 /// assert_eq!(iter.next(), Some(2));
700 /// assert_eq!(iter.next(), Some(4));
701 /// assert_eq!(iter.next(), Some(6));
702 /// assert_eq!(iter.next(), None);
705 /// If you're doing some sort of side effect, prefer [`for`] to `map()`:
708 /// # #![allow(unused_must_use)]
709 /// // don't do this:
710 /// (0..5).map(|x| println!("{}", x));
712 /// // it won't even execute, as it is lazy. Rust will warn you about this.
714 /// // Instead, use for:
716 /// println!("{}", x);
720 #[stable(feature = "rust1", since = "1.0.0")]
721 fn map<B, F>(self, f: F) -> Map<Self, F>
724 F: FnMut(Self::Item) -> B,
729 /// Calls a closure on each element of an iterator.
731 /// This is equivalent to using a [`for`] loop on the iterator, although
732 /// `break` and `continue` are not possible from a closure. It's generally
733 /// more idiomatic to use a `for` loop, but `for_each` may be more legible
734 /// when processing items at the end of longer iterator chains. In some
735 /// cases `for_each` may also be faster than a loop, because it will use
736 /// internal iteration on adaptors like `Chain`.
738 /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
745 /// use std::sync::mpsc::channel;
747 /// let (tx, rx) = channel();
748 /// (0..5).map(|x| x * 2 + 1)
749 /// .for_each(move |x| tx.send(x).unwrap());
751 /// let v: Vec<_> = rx.iter().collect();
752 /// assert_eq!(v, vec![1, 3, 5, 7, 9]);
755 /// For such a small example, a `for` loop may be cleaner, but `for_each`
756 /// might be preferable to keep a functional style with longer iterators:
759 /// (0..5).flat_map(|x| x * 100 .. x * 110)
761 /// .filter(|&(i, x)| (i + x) % 3 == 0)
762 /// .for_each(|(i, x)| println!("{}:{}", i, x));
765 #[stable(feature = "iterator_for_each", since = "1.21.0")]
766 fn for_each<F>(self, f: F)
769 F: FnMut(Self::Item),
772 fn call<T>(mut f: impl FnMut(T)) -> impl FnMut((), T) {
773 move |(), item| f(item)
776 self.fold((), call(f));
779 /// Creates an iterator which uses a closure to determine if an element
780 /// should be yielded.
782 /// Given an element the closure must return `true` or `false`. The returned
783 /// iterator will yield only the elements for which the closure returns
791 /// let a = [0i32, 1, 2];
793 /// let mut iter = a.iter().filter(|x| x.is_positive());
795 /// assert_eq!(iter.next(), Some(&1));
796 /// assert_eq!(iter.next(), Some(&2));
797 /// assert_eq!(iter.next(), None);
800 /// Because the closure passed to `filter()` takes a reference, and many
801 /// iterators iterate over references, this leads to a possibly confusing
802 /// situation, where the type of the closure is a double reference:
805 /// let a = [0, 1, 2];
807 /// let mut iter = a.iter().filter(|x| **x > 1); // need two *s!
809 /// assert_eq!(iter.next(), Some(&2));
810 /// assert_eq!(iter.next(), None);
813 /// It's common to instead use destructuring on the argument to strip away
817 /// let a = [0, 1, 2];
819 /// let mut iter = a.iter().filter(|&x| *x > 1); // both & and *
821 /// assert_eq!(iter.next(), Some(&2));
822 /// assert_eq!(iter.next(), None);
828 /// let a = [0, 1, 2];
830 /// let mut iter = a.iter().filter(|&&x| x > 1); // two &s
832 /// assert_eq!(iter.next(), Some(&2));
833 /// assert_eq!(iter.next(), None);
838 /// Note that `iter.filter(f).next()` is equivalent to `iter.find(f)`.
840 #[stable(feature = "rust1", since = "1.0.0")]
841 fn filter<P>(self, predicate: P) -> Filter<Self, P>
844 P: FnMut(&Self::Item) -> bool,
846 Filter::new(self, predicate)
849 /// Creates an iterator that both filters and maps.
851 /// The returned iterator yields only the `value`s for which the supplied
852 /// closure returns `Some(value)`.
854 /// `filter_map` can be used to make chains of [`filter`] and [`map`] more
855 /// concise. The example below shows how a `map().filter().map()` can be
856 /// shortened to a single call to `filter_map`.
858 /// [`filter`]: Iterator::filter
859 /// [`map`]: Iterator::map
866 /// let a = ["1", "two", "NaN", "four", "5"];
868 /// let mut iter = a.iter().filter_map(|s| s.parse().ok());
870 /// assert_eq!(iter.next(), Some(1));
871 /// assert_eq!(iter.next(), Some(5));
872 /// assert_eq!(iter.next(), None);
875 /// Here's the same example, but with [`filter`] and [`map`]:
878 /// let a = ["1", "two", "NaN", "four", "5"];
879 /// let mut iter = a.iter().map(|s| s.parse()).filter(|s| s.is_ok()).map(|s| s.unwrap());
880 /// assert_eq!(iter.next(), Some(1));
881 /// assert_eq!(iter.next(), Some(5));
882 /// assert_eq!(iter.next(), None);
885 #[stable(feature = "rust1", since = "1.0.0")]
886 fn filter_map<B, F>(self, f: F) -> FilterMap<Self, F>
889 F: FnMut(Self::Item) -> Option<B>,
891 FilterMap::new(self, f)
894 /// Creates an iterator which gives the current iteration count as well as
897 /// The iterator returned yields pairs `(i, val)`, where `i` is the
898 /// current index of iteration and `val` is the value returned by the
901 /// `enumerate()` keeps its count as a [`usize`]. If you want to count by a
902 /// different sized integer, the [`zip`] function provides similar
905 /// # Overflow Behavior
907 /// The method does no guarding against overflows, so enumerating more than
908 /// [`usize::MAX`] elements either produces the wrong result or panics. If
909 /// debug assertions are enabled, a panic is guaranteed.
913 /// The returned iterator might panic if the to-be-returned index would
914 /// overflow a [`usize`].
916 /// [`usize`]: type@usize
917 /// [`zip`]: Iterator::zip
922 /// let a = ['a', 'b', 'c'];
924 /// let mut iter = a.iter().enumerate();
926 /// assert_eq!(iter.next(), Some((0, &'a')));
927 /// assert_eq!(iter.next(), Some((1, &'b')));
928 /// assert_eq!(iter.next(), Some((2, &'c')));
929 /// assert_eq!(iter.next(), None);
932 #[stable(feature = "rust1", since = "1.0.0")]
933 fn enumerate(self) -> Enumerate<Self>
940 /// Creates an iterator which can use [`peek`] to look at the next element of
941 /// the iterator without consuming it.
943 /// Adds a [`peek`] method to an iterator. See its documentation for
944 /// more information.
946 /// Note that the underlying iterator is still advanced when [`peek`] is
947 /// called for the first time: In order to retrieve the next element,
948 /// [`next`] is called on the underlying iterator, hence any side effects (i.e.
949 /// anything other than fetching the next value) of the [`next`] method
952 /// [`peek`]: Peekable::peek
953 /// [`next`]: Iterator::next
960 /// let xs = [1, 2, 3];
962 /// let mut iter = xs.iter().peekable();
964 /// // peek() lets us see into the future
965 /// assert_eq!(iter.peek(), Some(&&1));
966 /// assert_eq!(iter.next(), Some(&1));
968 /// assert_eq!(iter.next(), Some(&2));
970 /// // we can peek() multiple times, the iterator won't advance
971 /// assert_eq!(iter.peek(), Some(&&3));
972 /// assert_eq!(iter.peek(), Some(&&3));
974 /// assert_eq!(iter.next(), Some(&3));
976 /// // after the iterator is finished, so is peek()
977 /// assert_eq!(iter.peek(), None);
978 /// assert_eq!(iter.next(), None);
981 #[stable(feature = "rust1", since = "1.0.0")]
982 fn peekable(self) -> Peekable<Self>
989 /// Creates an iterator that [`skip`]s elements based on a predicate.
991 /// [`skip`]: Iterator::skip
993 /// `skip_while()` takes a closure as an argument. It will call this
994 /// closure on each element of the iterator, and ignore elements
995 /// until it returns `false`.
997 /// After `false` is returned, `skip_while()`'s job is over, and the
998 /// rest of the elements are yielded.
1005 /// let a = [-1i32, 0, 1];
1007 /// let mut iter = a.iter().skip_while(|x| x.is_negative());
1009 /// assert_eq!(iter.next(), Some(&0));
1010 /// assert_eq!(iter.next(), Some(&1));
1011 /// assert_eq!(iter.next(), None);
1014 /// Because the closure passed to `skip_while()` takes a reference, and many
1015 /// iterators iterate over references, this leads to a possibly confusing
1016 /// situation, where the type of the closure argument is a double reference:
1019 /// let a = [-1, 0, 1];
1021 /// let mut iter = a.iter().skip_while(|x| **x < 0); // need two *s!
1023 /// assert_eq!(iter.next(), Some(&0));
1024 /// assert_eq!(iter.next(), Some(&1));
1025 /// assert_eq!(iter.next(), None);
1028 /// Stopping after an initial `false`:
1031 /// let a = [-1, 0, 1, -2];
1033 /// let mut iter = a.iter().skip_while(|x| **x < 0);
1035 /// assert_eq!(iter.next(), Some(&0));
1036 /// assert_eq!(iter.next(), Some(&1));
1038 /// // while this would have been false, since we already got a false,
1039 /// // skip_while() isn't used any more
1040 /// assert_eq!(iter.next(), Some(&-2));
1042 /// assert_eq!(iter.next(), None);
1045 #[stable(feature = "rust1", since = "1.0.0")]
1046 fn skip_while<P>(self, predicate: P) -> SkipWhile<Self, P>
1049 P: FnMut(&Self::Item) -> bool,
1051 SkipWhile::new(self, predicate)
1054 /// Creates an iterator that yields elements based on a predicate.
1056 /// `take_while()` takes a closure as an argument. It will call this
1057 /// closure on each element of the iterator, and yield elements
1058 /// while it returns `true`.
1060 /// After `false` is returned, `take_while()`'s job is over, and the
1061 /// rest of the elements are ignored.
1068 /// let a = [-1i32, 0, 1];
1070 /// let mut iter = a.iter().take_while(|x| x.is_negative());
1072 /// assert_eq!(iter.next(), Some(&-1));
1073 /// assert_eq!(iter.next(), None);
1076 /// Because the closure passed to `take_while()` takes a reference, and many
1077 /// iterators iterate over references, this leads to a possibly confusing
1078 /// situation, where the type of the closure is a double reference:
1081 /// let a = [-1, 0, 1];
1083 /// let mut iter = a.iter().take_while(|x| **x < 0); // need two *s!
1085 /// assert_eq!(iter.next(), Some(&-1));
1086 /// assert_eq!(iter.next(), None);
1089 /// Stopping after an initial `false`:
1092 /// let a = [-1, 0, 1, -2];
1094 /// let mut iter = a.iter().take_while(|x| **x < 0);
1096 /// assert_eq!(iter.next(), Some(&-1));
1098 /// // We have more elements that are less than zero, but since we already
1099 /// // got a false, take_while() isn't used any more
1100 /// assert_eq!(iter.next(), None);
1103 /// Because `take_while()` needs to look at the value in order to see if it
1104 /// should be included or not, consuming iterators will see that it is
1108 /// let a = [1, 2, 3, 4];
1109 /// let mut iter = a.iter();
1111 /// let result: Vec<i32> = iter.by_ref()
1112 /// .take_while(|n| **n != 3)
1116 /// assert_eq!(result, &[1, 2]);
1118 /// let result: Vec<i32> = iter.cloned().collect();
1120 /// assert_eq!(result, &[4]);
1123 /// The `3` is no longer there, because it was consumed in order to see if
1124 /// the iteration should stop, but wasn't placed back into the iterator.
1126 #[stable(feature = "rust1", since = "1.0.0")]
1127 fn take_while<P>(self, predicate: P) -> TakeWhile<Self, P>
1130 P: FnMut(&Self::Item) -> bool,
1132 TakeWhile::new(self, predicate)
1135 /// Creates an iterator that both yields elements based on a predicate and maps.
1137 /// `map_while()` takes a closure as an argument. It will call this
1138 /// closure on each element of the iterator, and yield elements
1139 /// while it returns [`Some(_)`][`Some`].
1146 /// #![feature(iter_map_while)]
1147 /// let a = [-1i32, 4, 0, 1];
1149 /// let mut iter = a.iter().map_while(|x| 16i32.checked_div(*x));
1151 /// assert_eq!(iter.next(), Some(-16));
1152 /// assert_eq!(iter.next(), Some(4));
1153 /// assert_eq!(iter.next(), None);
1156 /// Here's the same example, but with [`take_while`] and [`map`]:
1158 /// [`take_while`]: Iterator::take_while
1159 /// [`map`]: Iterator::map
1162 /// let a = [-1i32, 4, 0, 1];
1164 /// let mut iter = a.iter()
1165 /// .map(|x| 16i32.checked_div(*x))
1166 /// .take_while(|x| x.is_some())
1167 /// .map(|x| x.unwrap());
1169 /// assert_eq!(iter.next(), Some(-16));
1170 /// assert_eq!(iter.next(), Some(4));
1171 /// assert_eq!(iter.next(), None);
1174 /// Stopping after an initial [`None`]:
1177 /// #![feature(iter_map_while)]
1178 /// use std::convert::TryFrom;
1180 /// let a = [0, 1, 2, -3, 4, 5, -6];
1182 /// let iter = a.iter().map_while(|x| u32::try_from(*x).ok());
1183 /// let vec = iter.collect::<Vec<_>>();
1185 /// // We have more elements which could fit in u32 (4, 5), but `map_while` returned `None` for `-3`
1186 /// // (as the `predicate` returned `None`) and `collect` stops at the first `None` encountered.
1187 /// assert_eq!(vec, vec![0, 1, 2]);
1190 /// Because `map_while()` needs to look at the value in order to see if it
1191 /// should be included or not, consuming iterators will see that it is
1195 /// #![feature(iter_map_while)]
1196 /// use std::convert::TryFrom;
1198 /// let a = [1, 2, -3, 4];
1199 /// let mut iter = a.iter();
1201 /// let result: Vec<u32> = iter.by_ref()
1202 /// .map_while(|n| u32::try_from(*n).ok())
1205 /// assert_eq!(result, &[1, 2]);
1207 /// let result: Vec<i32> = iter.cloned().collect();
1209 /// assert_eq!(result, &[4]);
1212 /// The `-3` is no longer there, because it was consumed in order to see if
1213 /// the iteration should stop, but wasn't placed back into the iterator.
1215 /// Note that unlike [`take_while`] this iterator is **not** fused.
1216 /// It is also not specified what this iterator returns after the first [`None`] is returned.
1217 /// If you need fused iterator, use [`fuse`].
1219 /// [`fuse`]: Iterator::fuse
1221 #[unstable(feature = "iter_map_while", reason = "recently added", issue = "68537")]
1222 fn map_while<B, P>(self, predicate: P) -> MapWhile<Self, P>
1225 P: FnMut(Self::Item) -> Option<B>,
1227 MapWhile::new(self, predicate)
1230 /// Creates an iterator that skips the first `n` elements.
1232 /// `skip(n)` skips elements until `n` elements are skipped or the end of the
1233 /// iterator is reached (whichever happens first). After that, all the remaining
1234 /// elements are yielded. In particular, if the original iterator is too short,
1235 /// then the returned iterator is empty.
1237 /// Rather than overriding this method directly, instead override the `nth` method.
1244 /// let a = [1, 2, 3];
1246 /// let mut iter = a.iter().skip(2);
1248 /// assert_eq!(iter.next(), Some(&3));
1249 /// assert_eq!(iter.next(), None);
1252 #[stable(feature = "rust1", since = "1.0.0")]
1253 fn skip(self, n: usize) -> Skip<Self>
1260 /// Creates an iterator that yields the first `n` elements, or fewer
1261 /// if the underlying iterator ends sooner.
1263 /// `take(n)` yields elements until `n` elements are yielded or the end of
1264 /// the iterator is reached (whichever happens first).
1265 /// The returned iterator is a prefix of length `n` if the original iterator
1266 /// contains at least `n` elements, otherwise it contains all of the
1267 /// (fewer than `n`) elements of the original iterator.
1274 /// let a = [1, 2, 3];
1276 /// let mut iter = a.iter().take(2);
1278 /// assert_eq!(iter.next(), Some(&1));
1279 /// assert_eq!(iter.next(), Some(&2));
1280 /// assert_eq!(iter.next(), None);
1283 /// `take()` is often used with an infinite iterator, to make it finite:
1286 /// let mut iter = (0..).take(3);
1288 /// assert_eq!(iter.next(), Some(0));
1289 /// assert_eq!(iter.next(), Some(1));
1290 /// assert_eq!(iter.next(), Some(2));
1291 /// assert_eq!(iter.next(), None);
1294 /// If less than `n` elements are available,
1295 /// `take` will limit itself to the size of the underlying iterator:
1298 /// let v = vec![1, 2];
1299 /// let mut iter = v.into_iter().take(5);
1300 /// assert_eq!(iter.next(), Some(1));
1301 /// assert_eq!(iter.next(), Some(2));
1302 /// assert_eq!(iter.next(), None);
1305 #[stable(feature = "rust1", since = "1.0.0")]
1306 fn take(self, n: usize) -> Take<Self>
1313 /// An iterator adaptor similar to [`fold`] that holds internal state and
1314 /// produces a new iterator.
1316 /// [`fold`]: Iterator::fold
1318 /// `scan()` takes two arguments: an initial value which seeds the internal
1319 /// state, and a closure with two arguments, the first being a mutable
1320 /// reference to the internal state and the second an iterator element.
1321 /// The closure can assign to the internal state to share state between
1324 /// On iteration, the closure will be applied to each element of the
1325 /// iterator and the return value from the closure, an [`Option`], is
1326 /// yielded by the iterator.
1333 /// let a = [1, 2, 3];
1335 /// let mut iter = a.iter().scan(1, |state, &x| {
1336 /// // each iteration, we'll multiply the state by the element
1337 /// *state = *state * x;
1339 /// // then, we'll yield the negation of the state
1343 /// assert_eq!(iter.next(), Some(-1));
1344 /// assert_eq!(iter.next(), Some(-2));
1345 /// assert_eq!(iter.next(), Some(-6));
1346 /// assert_eq!(iter.next(), None);
1349 #[stable(feature = "rust1", since = "1.0.0")]
1350 fn scan<St, B, F>(self, initial_state: St, f: F) -> Scan<Self, St, F>
1353 F: FnMut(&mut St, Self::Item) -> Option<B>,
1355 Scan::new(self, initial_state, f)
1358 /// Creates an iterator that works like map, but flattens nested structure.
1360 /// The [`map`] adapter is very useful, but only when the closure
1361 /// argument produces values. If it produces an iterator instead, there's
1362 /// an extra layer of indirection. `flat_map()` will remove this extra layer
1365 /// You can think of `flat_map(f)` as the semantic equivalent
1366 /// of [`map`]ping, and then [`flatten`]ing as in `map(f).flatten()`.
1368 /// Another way of thinking about `flat_map()`: [`map`]'s closure returns
1369 /// one item for each element, and `flat_map()`'s closure returns an
1370 /// iterator for each element.
1372 /// [`map`]: Iterator::map
1373 /// [`flatten`]: Iterator::flatten
1380 /// let words = ["alpha", "beta", "gamma"];
1382 /// // chars() returns an iterator
1383 /// let merged: String = words.iter()
1384 /// .flat_map(|s| s.chars())
1386 /// assert_eq!(merged, "alphabetagamma");
1389 #[stable(feature = "rust1", since = "1.0.0")]
1390 fn flat_map<U, F>(self, f: F) -> FlatMap<Self, U, F>
1394 F: FnMut(Self::Item) -> U,
1396 FlatMap::new(self, f)
1399 /// Creates an iterator that flattens nested structure.
1401 /// This is useful when you have an iterator of iterators or an iterator of
1402 /// things that can be turned into iterators and you want to remove one
1403 /// level of indirection.
1410 /// let data = vec![vec![1, 2, 3, 4], vec![5, 6]];
1411 /// let flattened = data.into_iter().flatten().collect::<Vec<u8>>();
1412 /// assert_eq!(flattened, &[1, 2, 3, 4, 5, 6]);
1415 /// Mapping and then flattening:
1418 /// let words = ["alpha", "beta", "gamma"];
1420 /// // chars() returns an iterator
1421 /// let merged: String = words.iter()
1422 /// .map(|s| s.chars())
1425 /// assert_eq!(merged, "alphabetagamma");
1428 /// You can also rewrite this in terms of [`flat_map()`], which is preferable
1429 /// in this case since it conveys intent more clearly:
1432 /// let words = ["alpha", "beta", "gamma"];
1434 /// // chars() returns an iterator
1435 /// let merged: String = words.iter()
1436 /// .flat_map(|s| s.chars())
1438 /// assert_eq!(merged, "alphabetagamma");
1441 /// Flattening only removes one level of nesting at a time:
1444 /// let d3 = [[[1, 2], [3, 4]], [[5, 6], [7, 8]]];
1446 /// let d2 = d3.iter().flatten().collect::<Vec<_>>();
1447 /// assert_eq!(d2, [&[1, 2], &[3, 4], &[5, 6], &[7, 8]]);
1449 /// let d1 = d3.iter().flatten().flatten().collect::<Vec<_>>();
1450 /// assert_eq!(d1, [&1, &2, &3, &4, &5, &6, &7, &8]);
1453 /// Here we see that `flatten()` does not perform a "deep" flatten.
1454 /// Instead, only one level of nesting is removed. That is, if you
1455 /// `flatten()` a three-dimensional array, the result will be
1456 /// two-dimensional and not one-dimensional. To get a one-dimensional
1457 /// structure, you have to `flatten()` again.
1459 /// [`flat_map()`]: Iterator::flat_map
1461 #[stable(feature = "iterator_flatten", since = "1.29.0")]
1462 fn flatten(self) -> Flatten<Self>
1465 Self::Item: IntoIterator,
1470 /// Creates an iterator which ends after the first [`None`].
1472 /// After an iterator returns [`None`], future calls may or may not yield
1473 /// [`Some(T)`] again. `fuse()` adapts an iterator, ensuring that after a
1474 /// [`None`] is given, it will always return [`None`] forever.
1476 /// [`Some(T)`]: Some
1483 /// // an iterator which alternates between Some and None
1484 /// struct Alternate {
1488 /// impl Iterator for Alternate {
1489 /// type Item = i32;
1491 /// fn next(&mut self) -> Option<i32> {
1492 /// let val = self.state;
1493 /// self.state = self.state + 1;
1495 /// // if it's even, Some(i32), else None
1496 /// if val % 2 == 0 {
1504 /// let mut iter = Alternate { state: 0 };
1506 /// // we can see our iterator going back and forth
1507 /// assert_eq!(iter.next(), Some(0));
1508 /// assert_eq!(iter.next(), None);
1509 /// assert_eq!(iter.next(), Some(2));
1510 /// assert_eq!(iter.next(), None);
1512 /// // however, once we fuse it...
1513 /// let mut iter = iter.fuse();
1515 /// assert_eq!(iter.next(), Some(4));
1516 /// assert_eq!(iter.next(), None);
1518 /// // it will always return `None` after the first time.
1519 /// assert_eq!(iter.next(), None);
1520 /// assert_eq!(iter.next(), None);
1521 /// assert_eq!(iter.next(), None);
1524 #[stable(feature = "rust1", since = "1.0.0")]
1525 fn fuse(self) -> Fuse<Self>
1532 /// Does something with each element of an iterator, passing the value on.
1534 /// When using iterators, you'll often chain several of them together.
1535 /// While working on such code, you might want to check out what's
1536 /// happening at various parts in the pipeline. To do that, insert
1537 /// a call to `inspect()`.
1539 /// It's more common for `inspect()` to be used as a debugging tool than to
1540 /// exist in your final code, but applications may find it useful in certain
1541 /// situations when errors need to be logged before being discarded.
1548 /// let a = [1, 4, 2, 3];
1550 /// // this iterator sequence is complex.
1551 /// let sum = a.iter()
1553 /// .filter(|x| x % 2 == 0)
1554 /// .fold(0, |sum, i| sum + i);
1556 /// println!("{}", sum);
1558 /// // let's add some inspect() calls to investigate what's happening
1559 /// let sum = a.iter()
1561 /// .inspect(|x| println!("about to filter: {}", x))
1562 /// .filter(|x| x % 2 == 0)
1563 /// .inspect(|x| println!("made it through filter: {}", x))
1564 /// .fold(0, |sum, i| sum + i);
1566 /// println!("{}", sum);
1569 /// This will print:
1573 /// about to filter: 1
1574 /// about to filter: 4
1575 /// made it through filter: 4
1576 /// about to filter: 2
1577 /// made it through filter: 2
1578 /// about to filter: 3
1582 /// Logging errors before discarding them:
1585 /// let lines = ["1", "2", "a"];
1587 /// let sum: i32 = lines
1589 /// .map(|line| line.parse::<i32>())
1590 /// .inspect(|num| {
1591 /// if let Err(ref e) = *num {
1592 /// println!("Parsing error: {}", e);
1595 /// .filter_map(Result::ok)
1598 /// println!("Sum: {}", sum);
1601 /// This will print:
1604 /// Parsing error: invalid digit found in string
1608 #[stable(feature = "rust1", since = "1.0.0")]
1609 fn inspect<F>(self, f: F) -> Inspect<Self, F>
1612 F: FnMut(&Self::Item),
1614 Inspect::new(self, f)
1617 /// Borrows an iterator, rather than consuming it.
1619 /// This is useful to allow applying iterator adaptors while still
1620 /// retaining ownership of the original iterator.
1627 /// let a = [1, 2, 3];
1629 /// let iter = a.iter();
1631 /// let sum: i32 = iter.take(5).fold(0, |acc, i| acc + i);
1633 /// assert_eq!(sum, 6);
1635 /// // if we try to use iter again, it won't work. The following line
1636 /// // gives "error: use of moved value: `iter`
1637 /// // assert_eq!(iter.next(), None);
1639 /// // let's try that again
1640 /// let a = [1, 2, 3];
1642 /// let mut iter = a.iter();
1644 /// // instead, we add in a .by_ref()
1645 /// let sum: i32 = iter.by_ref().take(2).fold(0, |acc, i| acc + i);
1647 /// assert_eq!(sum, 3);
1649 /// // now this is just fine:
1650 /// assert_eq!(iter.next(), Some(&3));
1651 /// assert_eq!(iter.next(), None);
1653 #[stable(feature = "rust1", since = "1.0.0")]
1654 fn by_ref(&mut self) -> &mut Self
1661 /// Transforms an iterator into a collection.
1663 /// `collect()` can take anything iterable, and turn it into a relevant
1664 /// collection. This is one of the more powerful methods in the standard
1665 /// library, used in a variety of contexts.
1667 /// The most basic pattern in which `collect()` is used is to turn one
1668 /// collection into another. You take a collection, call [`iter`] on it,
1669 /// do a bunch of transformations, and then `collect()` at the end.
1671 /// `collect()` can also create instances of types that are not typical
1672 /// collections. For example, a [`String`] can be built from [`char`]s,
1673 /// and an iterator of [`Result<T, E>`][`Result`] items can be collected
1674 /// into `Result<Collection<T>, E>`. See the examples below for more.
1676 /// Because `collect()` is so general, it can cause problems with type
1677 /// inference. As such, `collect()` is one of the few times you'll see
1678 /// the syntax affectionately known as the 'turbofish': `::<>`. This
1679 /// helps the inference algorithm understand specifically which collection
1680 /// you're trying to collect into.
1687 /// let a = [1, 2, 3];
1689 /// let doubled: Vec<i32> = a.iter()
1690 /// .map(|&x| x * 2)
1693 /// assert_eq!(vec![2, 4, 6], doubled);
1696 /// Note that we needed the `: Vec<i32>` on the left-hand side. This is because
1697 /// we could collect into, for example, a [`VecDeque<T>`] instead:
1699 /// [`VecDeque<T>`]: ../../std/collections/struct.VecDeque.html
1702 /// use std::collections::VecDeque;
1704 /// let a = [1, 2, 3];
1706 /// let doubled: VecDeque<i32> = a.iter().map(|&x| x * 2).collect();
1708 /// assert_eq!(2, doubled[0]);
1709 /// assert_eq!(4, doubled[1]);
1710 /// assert_eq!(6, doubled[2]);
1713 /// Using the 'turbofish' instead of annotating `doubled`:
1716 /// let a = [1, 2, 3];
1718 /// let doubled = a.iter().map(|x| x * 2).collect::<Vec<i32>>();
1720 /// assert_eq!(vec![2, 4, 6], doubled);
1723 /// Because `collect()` only cares about what you're collecting into, you can
1724 /// still use a partial type hint, `_`, with the turbofish:
1727 /// let a = [1, 2, 3];
1729 /// let doubled = a.iter().map(|x| x * 2).collect::<Vec<_>>();
1731 /// assert_eq!(vec![2, 4, 6], doubled);
1734 /// Using `collect()` to make a [`String`]:
1737 /// let chars = ['g', 'd', 'k', 'k', 'n'];
1739 /// let hello: String = chars.iter()
1740 /// .map(|&x| x as u8)
1741 /// .map(|x| (x + 1) as char)
1744 /// assert_eq!("hello", hello);
1747 /// If you have a list of [`Result<T, E>`][`Result`]s, you can use `collect()` to
1748 /// see if any of them failed:
1751 /// let results = [Ok(1), Err("nope"), Ok(3), Err("bad")];
1753 /// let result: Result<Vec<_>, &str> = results.iter().cloned().collect();
1755 /// // gives us the first error
1756 /// assert_eq!(Err("nope"), result);
1758 /// let results = [Ok(1), Ok(3)];
1760 /// let result: Result<Vec<_>, &str> = results.iter().cloned().collect();
1762 /// // gives us the list of answers
1763 /// assert_eq!(Ok(vec![1, 3]), result);
1766 /// [`iter`]: Iterator::next
1767 /// [`String`]: ../../std/string/struct.String.html
1768 /// [`char`]: type@char
1770 #[stable(feature = "rust1", since = "1.0.0")]
1771 #[must_use = "if you really need to exhaust the iterator, consider `.for_each(drop)` instead"]
1772 fn collect<B: FromIterator<Self::Item>>(self) -> B
1776 FromIterator::from_iter(self)
1779 /// Consumes an iterator, creating two collections from it.
1781 /// The predicate passed to `partition()` can return `true`, or `false`.
1782 /// `partition()` returns a pair, all of the elements for which it returned
1783 /// `true`, and all of the elements for which it returned `false`.
1785 /// See also [`is_partitioned()`] and [`partition_in_place()`].
1787 /// [`is_partitioned()`]: Iterator::is_partitioned
1788 /// [`partition_in_place()`]: Iterator::partition_in_place
1795 /// let a = [1, 2, 3];
1797 /// let (even, odd): (Vec<i32>, Vec<i32>) = a
1799 /// .partition(|&n| n % 2 == 0);
1801 /// assert_eq!(even, vec![2]);
1802 /// assert_eq!(odd, vec![1, 3]);
1804 #[stable(feature = "rust1", since = "1.0.0")]
1805 fn partition<B, F>(self, f: F) -> (B, B)
1808 B: Default + Extend<Self::Item>,
1809 F: FnMut(&Self::Item) -> bool,
1812 fn extend<'a, T, B: Extend<T>>(
1813 mut f: impl FnMut(&T) -> bool + 'a,
1816 ) -> impl FnMut((), T) + 'a {
1821 right.extend_one(x);
1826 let mut left: B = Default::default();
1827 let mut right: B = Default::default();
1829 self.fold((), extend(f, &mut left, &mut right));
1834 /// Reorders the elements of this iterator *in-place* according to the given predicate,
1835 /// such that all those that return `true` precede all those that return `false`.
1836 /// Returns the number of `true` elements found.
1838 /// The relative order of partitioned items is not maintained.
1840 /// See also [`is_partitioned()`] and [`partition()`].
1842 /// [`is_partitioned()`]: Iterator::is_partitioned
1843 /// [`partition()`]: Iterator::partition
1848 /// #![feature(iter_partition_in_place)]
1850 /// let mut a = [1, 2, 3, 4, 5, 6, 7];
1852 /// // Partition in-place between evens and odds
1853 /// let i = a.iter_mut().partition_in_place(|&n| n % 2 == 0);
1855 /// assert_eq!(i, 3);
1856 /// assert!(a[..i].iter().all(|&n| n % 2 == 0)); // evens
1857 /// assert!(a[i..].iter().all(|&n| n % 2 == 1)); // odds
1859 #[unstable(feature = "iter_partition_in_place", reason = "new API", issue = "62543")]
1860 fn partition_in_place<'a, T: 'a, P>(mut self, ref mut predicate: P) -> usize
1862 Self: Sized + DoubleEndedIterator<Item = &'a mut T>,
1863 P: FnMut(&T) -> bool,
1865 // FIXME: should we worry about the count overflowing? The only way to have more than
1866 // `usize::MAX` mutable references is with ZSTs, which aren't useful to partition...
1868 // These closure "factory" functions exist to avoid genericity in `Self`.
1872 predicate: &'a mut impl FnMut(&T) -> bool,
1873 true_count: &'a mut usize,
1874 ) -> impl FnMut(&&mut T) -> bool + 'a {
1876 let p = predicate(&**x);
1877 *true_count += p as usize;
1883 fn is_true<T>(predicate: &mut impl FnMut(&T) -> bool) -> impl FnMut(&&mut T) -> bool + '_ {
1884 move |x| predicate(&**x)
1887 // Repeatedly find the first `false` and swap it with the last `true`.
1888 let mut true_count = 0;
1889 while let Some(head) = self.find(is_false(predicate, &mut true_count)) {
1890 if let Some(tail) = self.rfind(is_true(predicate)) {
1891 crate::mem::swap(head, tail);
1900 /// Checks if the elements of this iterator are partitioned according to the given predicate,
1901 /// such that all those that return `true` precede all those that return `false`.
1903 /// See also [`partition()`] and [`partition_in_place()`].
1905 /// [`partition()`]: Iterator::partition
1906 /// [`partition_in_place()`]: Iterator::partition_in_place
1911 /// #![feature(iter_is_partitioned)]
1913 /// assert!("Iterator".chars().is_partitioned(char::is_uppercase));
1914 /// assert!(!"IntoIterator".chars().is_partitioned(char::is_uppercase));
1916 #[unstable(feature = "iter_is_partitioned", reason = "new API", issue = "62544")]
1917 fn is_partitioned<P>(mut self, mut predicate: P) -> bool
1920 P: FnMut(Self::Item) -> bool,
1922 // Either all items test `true`, or the first clause stops at `false`
1923 // and we check that there are no more `true` items after that.
1924 self.all(&mut predicate) || !self.any(predicate)
1927 /// An iterator method that applies a function as long as it returns
1928 /// successfully, producing a single, final value.
1930 /// `try_fold()` takes two arguments: an initial value, and a closure with
1931 /// two arguments: an 'accumulator', and an element. The closure either
1932 /// returns successfully, with the value that the accumulator should have
1933 /// for the next iteration, or it returns failure, with an error value that
1934 /// is propagated back to the caller immediately (short-circuiting).
1936 /// The initial value is the value the accumulator will have on the first
1937 /// call. If applying the closure succeeded against every element of the
1938 /// iterator, `try_fold()` returns the final accumulator as success.
1940 /// Folding is useful whenever you have a collection of something, and want
1941 /// to produce a single value from it.
1943 /// # Note to Implementors
1945 /// Several of the other (forward) methods have default implementations in
1946 /// terms of this one, so try to implement this explicitly if it can
1947 /// do something better than the default `for` loop implementation.
1949 /// In particular, try to have this call `try_fold()` on the internal parts
1950 /// from which this iterator is composed. If multiple calls are needed,
1951 /// the `?` operator may be convenient for chaining the accumulator value
1952 /// along, but beware any invariants that need to be upheld before those
1953 /// early returns. This is a `&mut self` method, so iteration needs to be
1954 /// resumable after hitting an error here.
1961 /// let a = [1, 2, 3];
1963 /// // the checked sum of all of the elements of the array
1964 /// let sum = a.iter().try_fold(0i8, |acc, &x| acc.checked_add(x));
1966 /// assert_eq!(sum, Some(6));
1969 /// Short-circuiting:
1972 /// let a = [10, 20, 30, 100, 40, 50];
1973 /// let mut it = a.iter();
1975 /// // This sum overflows when adding the 100 element
1976 /// let sum = it.try_fold(0i8, |acc, &x| acc.checked_add(x));
1977 /// assert_eq!(sum, None);
1979 /// // Because it short-circuited, the remaining elements are still
1980 /// // available through the iterator.
1981 /// assert_eq!(it.len(), 2);
1982 /// assert_eq!(it.next(), Some(&40));
1985 #[stable(feature = "iterator_try_fold", since = "1.27.0")]
1986 fn try_fold<B, F, R>(&mut self, init: B, mut f: F) -> R
1989 F: FnMut(B, Self::Item) -> R,
1992 let mut accum = init;
1993 while let Some(x) = self.next() {
1994 accum = f(accum, x)?;
1999 /// An iterator method that applies a fallible function to each item in the
2000 /// iterator, stopping at the first error and returning that error.
2002 /// This can also be thought of as the fallible form of [`for_each()`]
2003 /// or as the stateless version of [`try_fold()`].
2005 /// [`for_each()`]: Iterator::for_each
2006 /// [`try_fold()`]: Iterator::try_fold
2011 /// use std::fs::rename;
2012 /// use std::io::{stdout, Write};
2013 /// use std::path::Path;
2015 /// let data = ["no_tea.txt", "stale_bread.json", "torrential_rain.png"];
2017 /// let res = data.iter().try_for_each(|x| writeln!(stdout(), "{}", x));
2018 /// assert!(res.is_ok());
2020 /// let mut it = data.iter().cloned();
2021 /// let res = it.try_for_each(|x| rename(x, Path::new(x).with_extension("old")));
2022 /// assert!(res.is_err());
2023 /// // It short-circuited, so the remaining items are still in the iterator:
2024 /// assert_eq!(it.next(), Some("stale_bread.json"));
2027 #[stable(feature = "iterator_try_fold", since = "1.27.0")]
2028 fn try_for_each<F, R>(&mut self, f: F) -> R
2031 F: FnMut(Self::Item) -> R,
2035 fn call<T, R>(mut f: impl FnMut(T) -> R) -> impl FnMut((), T) -> R {
2039 self.try_fold((), call(f))
2042 /// Folds every element into an accumulator by applying an operation,
2043 /// returning the final result.
2045 /// `fold()` takes two arguments: an initial value, and a closure with two
2046 /// arguments: an 'accumulator', and an element. The closure returns the value that
2047 /// the accumulator should have for the next iteration.
2049 /// The initial value is the value the accumulator will have on the first
2052 /// After applying this closure to every element of the iterator, `fold()`
2053 /// returns the accumulator.
2055 /// This operation is sometimes called 'reduce' or 'inject'.
2057 /// Folding is useful whenever you have a collection of something, and want
2058 /// to produce a single value from it.
2060 /// Note: `fold()`, and similar methods that traverse the entire iterator,
2061 /// may not terminate for infinite iterators, even on traits for which a
2062 /// result is determinable in finite time.
2064 /// Note: [`reduce()`] can be used to use the first element as the initial
2065 /// value, if the accumulator type and item type is the same.
2067 /// # Note to Implementors
2069 /// Several of the other (forward) methods have default implementations in
2070 /// terms of this one, so try to implement this explicitly if it can
2071 /// do something better than the default `for` loop implementation.
2073 /// In particular, try to have this call `fold()` on the internal parts
2074 /// from which this iterator is composed.
2081 /// let a = [1, 2, 3];
2083 /// // the sum of all of the elements of the array
2084 /// let sum = a.iter().fold(0, |acc, x| acc + x);
2086 /// assert_eq!(sum, 6);
2089 /// Let's walk through each step of the iteration here:
2091 /// | element | acc | x | result |
2092 /// |---------|-----|---|--------|
2094 /// | 1 | 0 | 1 | 1 |
2095 /// | 2 | 1 | 2 | 3 |
2096 /// | 3 | 3 | 3 | 6 |
2098 /// And so, our final result, `6`.
2100 /// It's common for people who haven't used iterators a lot to
2101 /// use a `for` loop with a list of things to build up a result. Those
2102 /// can be turned into `fold()`s:
2104 /// [`for`]: ../../book/ch03-05-control-flow.html#looping-through-a-collection-with-for
2107 /// let numbers = [1, 2, 3, 4, 5];
2109 /// let mut result = 0;
2112 /// for i in &numbers {
2113 /// result = result + i;
2117 /// let result2 = numbers.iter().fold(0, |acc, &x| acc + x);
2119 /// // they're the same
2120 /// assert_eq!(result, result2);
2123 /// [`reduce()`]: Iterator::reduce
2124 #[doc(alias = "reduce")]
2125 #[doc(alias = "inject")]
2127 #[stable(feature = "rust1", since = "1.0.0")]
2128 fn fold<B, F>(mut self, init: B, mut f: F) -> B
2131 F: FnMut(B, Self::Item) -> B,
2133 let mut accum = init;
2134 while let Some(x) = self.next() {
2135 accum = f(accum, x);
2140 /// Reduces the elements to a single one, by repeatedly applying a reducing
2143 /// If the iterator is empty, returns [`None`]; otherwise, returns the
2144 /// result of the reduction.
2146 /// For iterators with at least one element, this is the same as [`fold()`]
2147 /// with the first element of the iterator as the initial value, folding
2148 /// every subsequent element into it.
2150 /// [`fold()`]: Iterator::fold
2154 /// Find the maximum value:
2157 /// fn find_max<I>(iter: I) -> Option<I::Item>
2158 /// where I: Iterator,
2161 /// iter.reduce(|a, b| {
2162 /// if a >= b { a } else { b }
2165 /// let a = [10, 20, 5, -23, 0];
2166 /// let b: [u32; 0] = [];
2168 /// assert_eq!(find_max(a.iter()), Some(&20));
2169 /// assert_eq!(find_max(b.iter()), None);
2172 #[stable(feature = "iterator_fold_self", since = "1.51.0")]
2173 fn reduce<F>(mut self, f: F) -> Option<Self::Item>
2176 F: FnMut(Self::Item, Self::Item) -> Self::Item,
2178 let first = self.next()?;
2179 Some(self.fold(first, f))
2182 /// Tests if every element of the iterator matches a predicate.
2184 /// `all()` takes a closure that returns `true` or `false`. It applies
2185 /// this closure to each element of the iterator, and if they all return
2186 /// `true`, then so does `all()`. If any of them return `false`, it
2187 /// returns `false`.
2189 /// `all()` is short-circuiting; in other words, it will stop processing
2190 /// as soon as it finds a `false`, given that no matter what else happens,
2191 /// the result will also be `false`.
2193 /// An empty iterator returns `true`.
2200 /// let a = [1, 2, 3];
2202 /// assert!(a.iter().all(|&x| x > 0));
2204 /// assert!(!a.iter().all(|&x| x > 2));
2207 /// Stopping at the first `false`:
2210 /// let a = [1, 2, 3];
2212 /// let mut iter = a.iter();
2214 /// assert!(!iter.all(|&x| x != 2));
2216 /// // we can still use `iter`, as there are more elements.
2217 /// assert_eq!(iter.next(), Some(&3));
2219 #[doc(alias = "every")]
2221 #[stable(feature = "rust1", since = "1.0.0")]
2222 fn all<F>(&mut self, f: F) -> bool
2225 F: FnMut(Self::Item) -> bool,
2228 fn check<T>(mut f: impl FnMut(T) -> bool) -> impl FnMut((), T) -> ControlFlow<()> {
2230 if f(x) { ControlFlow::CONTINUE } else { ControlFlow::BREAK }
2233 self.try_fold((), check(f)) == ControlFlow::CONTINUE
2236 /// Tests if any element of the iterator matches a predicate.
2238 /// `any()` takes a closure that returns `true` or `false`. It applies
2239 /// this closure to each element of the iterator, and if any of them return
2240 /// `true`, then so does `any()`. If they all return `false`, it
2241 /// returns `false`.
2243 /// `any()` is short-circuiting; in other words, it will stop processing
2244 /// as soon as it finds a `true`, given that no matter what else happens,
2245 /// the result will also be `true`.
2247 /// An empty iterator returns `false`.
2254 /// let a = [1, 2, 3];
2256 /// assert!(a.iter().any(|&x| x > 0));
2258 /// assert!(!a.iter().any(|&x| x > 5));
2261 /// Stopping at the first `true`:
2264 /// let a = [1, 2, 3];
2266 /// let mut iter = a.iter();
2268 /// assert!(iter.any(|&x| x != 2));
2270 /// // we can still use `iter`, as there are more elements.
2271 /// assert_eq!(iter.next(), Some(&2));
2274 #[stable(feature = "rust1", since = "1.0.0")]
2275 fn any<F>(&mut self, f: F) -> bool
2278 F: FnMut(Self::Item) -> bool,
2281 fn check<T>(mut f: impl FnMut(T) -> bool) -> impl FnMut((), T) -> ControlFlow<()> {
2283 if f(x) { ControlFlow::BREAK } else { ControlFlow::CONTINUE }
2287 self.try_fold((), check(f)) == ControlFlow::BREAK
2290 /// Searches for an element of an iterator that satisfies a predicate.
2292 /// `find()` takes a closure that returns `true` or `false`. It applies
2293 /// this closure to each element of the iterator, and if any of them return
2294 /// `true`, then `find()` returns [`Some(element)`]. If they all return
2295 /// `false`, it returns [`None`].
2297 /// `find()` is short-circuiting; in other words, it will stop processing
2298 /// as soon as the closure returns `true`.
2300 /// Because `find()` takes a reference, and many iterators iterate over
2301 /// references, this leads to a possibly confusing situation where the
2302 /// argument is a double reference. You can see this effect in the
2303 /// examples below, with `&&x`.
2305 /// [`Some(element)`]: Some
2312 /// let a = [1, 2, 3];
2314 /// assert_eq!(a.iter().find(|&&x| x == 2), Some(&2));
2316 /// assert_eq!(a.iter().find(|&&x| x == 5), None);
2319 /// Stopping at the first `true`:
2322 /// let a = [1, 2, 3];
2324 /// let mut iter = a.iter();
2326 /// assert_eq!(iter.find(|&&x| x == 2), Some(&2));
2328 /// // we can still use `iter`, as there are more elements.
2329 /// assert_eq!(iter.next(), Some(&3));
2332 /// Note that `iter.find(f)` is equivalent to `iter.filter(f).next()`.
2334 #[stable(feature = "rust1", since = "1.0.0")]
2335 fn find<P>(&mut self, predicate: P) -> Option<Self::Item>
2338 P: FnMut(&Self::Item) -> bool,
2341 fn check<T>(mut predicate: impl FnMut(&T) -> bool) -> impl FnMut((), T) -> ControlFlow<T> {
2343 if predicate(&x) { ControlFlow::Break(x) } else { ControlFlow::CONTINUE }
2347 self.try_fold((), check(predicate)).break_value()
2350 /// Applies function to the elements of iterator and returns
2351 /// the first non-none result.
2353 /// `iter.find_map(f)` is equivalent to `iter.filter_map(f).next()`.
2358 /// let a = ["lol", "NaN", "2", "5"];
2360 /// let first_number = a.iter().find_map(|s| s.parse().ok());
2362 /// assert_eq!(first_number, Some(2));
2365 #[stable(feature = "iterator_find_map", since = "1.30.0")]
2366 fn find_map<B, F>(&mut self, f: F) -> Option<B>
2369 F: FnMut(Self::Item) -> Option<B>,
2372 fn check<T, B>(mut f: impl FnMut(T) -> Option<B>) -> impl FnMut((), T) -> ControlFlow<B> {
2373 move |(), x| match f(x) {
2374 Some(x) => ControlFlow::Break(x),
2375 None => ControlFlow::CONTINUE,
2379 self.try_fold((), check(f)).break_value()
2382 /// Applies function to the elements of iterator and returns
2383 /// the first true result or the first error.
2388 /// #![feature(try_find)]
2390 /// let a = ["1", "2", "lol", "NaN", "5"];
2392 /// let is_my_num = |s: &str, search: i32| -> Result<bool, std::num::ParseIntError> {
2393 /// Ok(s.parse::<i32>()? == search)
2396 /// let result = a.iter().try_find(|&&s| is_my_num(s, 2));
2397 /// assert_eq!(result, Ok(Some(&"2")));
2399 /// let result = a.iter().try_find(|&&s| is_my_num(s, 5));
2400 /// assert!(result.is_err());
2403 #[unstable(feature = "try_find", reason = "new API", issue = "63178")]
2404 fn try_find<F, R>(&mut self, f: F) -> Result<Option<Self::Item>, R::Error>
2407 F: FnMut(&Self::Item) -> R,
2411 fn check<F, T, R>(mut f: F) -> impl FnMut((), T) -> ControlFlow<Result<T, R::Error>>
2416 move |(), x| match f(&x).into_result() {
2417 Ok(false) => ControlFlow::CONTINUE,
2418 Ok(true) => ControlFlow::Break(Ok(x)),
2419 Err(x) => ControlFlow::Break(Err(x)),
2423 self.try_fold((), check(f)).break_value().transpose()
2426 /// Searches for an element in an iterator, returning its index.
2428 /// `position()` takes a closure that returns `true` or `false`. It applies
2429 /// this closure to each element of the iterator, and if one of them
2430 /// returns `true`, then `position()` returns [`Some(index)`]. If all of
2431 /// them return `false`, it returns [`None`].
2433 /// `position()` is short-circuiting; in other words, it will stop
2434 /// processing as soon as it finds a `true`.
2436 /// # Overflow Behavior
2438 /// The method does no guarding against overflows, so if there are more
2439 /// than [`usize::MAX`] non-matching elements, it either produces the wrong
2440 /// result or panics. If debug assertions are enabled, a panic is
2445 /// This function might panic if the iterator has more than `usize::MAX`
2446 /// non-matching elements.
2448 /// [`Some(index)`]: Some
2455 /// let a = [1, 2, 3];
2457 /// assert_eq!(a.iter().position(|&x| x == 2), Some(1));
2459 /// assert_eq!(a.iter().position(|&x| x == 5), None);
2462 /// Stopping at the first `true`:
2465 /// let a = [1, 2, 3, 4];
2467 /// let mut iter = a.iter();
2469 /// assert_eq!(iter.position(|&x| x >= 2), Some(1));
2471 /// // we can still use `iter`, as there are more elements.
2472 /// assert_eq!(iter.next(), Some(&3));
2474 /// // The returned index depends on iterator state
2475 /// assert_eq!(iter.position(|&x| x == 4), Some(0));
2479 #[stable(feature = "rust1", since = "1.0.0")]
2480 fn position<P>(&mut self, predicate: P) -> Option<usize>
2483 P: FnMut(Self::Item) -> bool,
2487 mut predicate: impl FnMut(T) -> bool,
2488 ) -> impl FnMut(usize, T) -> ControlFlow<usize, usize> {
2489 #[rustc_inherit_overflow_checks]
2491 if predicate(x) { ControlFlow::Break(i) } else { ControlFlow::Continue(i + 1) }
2495 self.try_fold(0, check(predicate)).break_value()
2498 /// Searches for an element in an iterator from the right, returning its
2501 /// `rposition()` takes a closure that returns `true` or `false`. It applies
2502 /// this closure to each element of the iterator, starting from the end,
2503 /// and if one of them returns `true`, then `rposition()` returns
2504 /// [`Some(index)`]. If all of them return `false`, it returns [`None`].
2506 /// `rposition()` is short-circuiting; in other words, it will stop
2507 /// processing as soon as it finds a `true`.
2509 /// [`Some(index)`]: Some
2516 /// let a = [1, 2, 3];
2518 /// assert_eq!(a.iter().rposition(|&x| x == 3), Some(2));
2520 /// assert_eq!(a.iter().rposition(|&x| x == 5), None);
2523 /// Stopping at the first `true`:
2526 /// let a = [1, 2, 3];
2528 /// let mut iter = a.iter();
2530 /// assert_eq!(iter.rposition(|&x| x == 2), Some(1));
2532 /// // we can still use `iter`, as there are more elements.
2533 /// assert_eq!(iter.next(), Some(&1));
2536 #[stable(feature = "rust1", since = "1.0.0")]
2537 fn rposition<P>(&mut self, predicate: P) -> Option<usize>
2539 P: FnMut(Self::Item) -> bool,
2540 Self: Sized + ExactSizeIterator + DoubleEndedIterator,
2542 // No need for an overflow check here, because `ExactSizeIterator`
2543 // implies that the number of elements fits into a `usize`.
2546 mut predicate: impl FnMut(T) -> bool,
2547 ) -> impl FnMut(usize, T) -> ControlFlow<usize, usize> {
2550 if predicate(x) { ControlFlow::Break(i) } else { ControlFlow::Continue(i) }
2555 self.try_rfold(n, check(predicate)).break_value()
2558 /// Returns the maximum element of an iterator.
2560 /// If several elements are equally maximum, the last element is
2561 /// returned. If the iterator is empty, [`None`] is returned.
2568 /// let a = [1, 2, 3];
2569 /// let b: Vec<u32> = Vec::new();
2571 /// assert_eq!(a.iter().max(), Some(&3));
2572 /// assert_eq!(b.iter().max(), None);
2575 #[stable(feature = "rust1", since = "1.0.0")]
2576 fn max(self) -> Option<Self::Item>
2581 self.max_by(Ord::cmp)
2584 /// Returns the minimum element of an iterator.
2586 /// If several elements are equally minimum, the first element is
2587 /// returned. If the iterator is empty, [`None`] is returned.
2594 /// let a = [1, 2, 3];
2595 /// let b: Vec<u32> = Vec::new();
2597 /// assert_eq!(a.iter().min(), Some(&1));
2598 /// assert_eq!(b.iter().min(), None);
2601 #[stable(feature = "rust1", since = "1.0.0")]
2602 fn min(self) -> Option<Self::Item>
2607 self.min_by(Ord::cmp)
2610 /// Returns the element that gives the maximum value from the
2611 /// specified function.
2613 /// If several elements are equally maximum, the last element is
2614 /// returned. If the iterator is empty, [`None`] is returned.
2619 /// let a = [-3_i32, 0, 1, 5, -10];
2620 /// assert_eq!(*a.iter().max_by_key(|x| x.abs()).unwrap(), -10);
2623 #[stable(feature = "iter_cmp_by_key", since = "1.6.0")]
2624 fn max_by_key<B: Ord, F>(self, f: F) -> Option<Self::Item>
2627 F: FnMut(&Self::Item) -> B,
2630 fn key<T, B>(mut f: impl FnMut(&T) -> B) -> impl FnMut(T) -> (B, T) {
2635 fn compare<T, B: Ord>((x_p, _): &(B, T), (y_p, _): &(B, T)) -> Ordering {
2639 let (_, x) = self.map(key(f)).max_by(compare)?;
2643 /// Returns the element that gives the maximum value with respect to the
2644 /// specified comparison function.
2646 /// If several elements are equally maximum, the last element is
2647 /// returned. If the iterator is empty, [`None`] is returned.
2652 /// let a = [-3_i32, 0, 1, 5, -10];
2653 /// assert_eq!(*a.iter().max_by(|x, y| x.cmp(y)).unwrap(), 5);
2656 #[stable(feature = "iter_max_by", since = "1.15.0")]
2657 fn max_by<F>(self, compare: F) -> Option<Self::Item>
2660 F: FnMut(&Self::Item, &Self::Item) -> Ordering,
2663 fn fold<T>(mut compare: impl FnMut(&T, &T) -> Ordering) -> impl FnMut(T, T) -> T {
2664 move |x, y| cmp::max_by(x, y, &mut compare)
2667 self.reduce(fold(compare))
2670 /// Returns the element that gives the minimum value from the
2671 /// specified function.
2673 /// If several elements are equally minimum, the first element is
2674 /// returned. If the iterator is empty, [`None`] is returned.
2679 /// let a = [-3_i32, 0, 1, 5, -10];
2680 /// assert_eq!(*a.iter().min_by_key(|x| x.abs()).unwrap(), 0);
2683 #[stable(feature = "iter_cmp_by_key", since = "1.6.0")]
2684 fn min_by_key<B: Ord, F>(self, f: F) -> Option<Self::Item>
2687 F: FnMut(&Self::Item) -> B,
2690 fn key<T, B>(mut f: impl FnMut(&T) -> B) -> impl FnMut(T) -> (B, T) {
2695 fn compare<T, B: Ord>((x_p, _): &(B, T), (y_p, _): &(B, T)) -> Ordering {
2699 let (_, x) = self.map(key(f)).min_by(compare)?;
2703 /// Returns the element that gives the minimum value with respect to the
2704 /// specified comparison function.
2706 /// If several elements are equally minimum, the first element is
2707 /// returned. If the iterator is empty, [`None`] is returned.
2712 /// let a = [-3_i32, 0, 1, 5, -10];
2713 /// assert_eq!(*a.iter().min_by(|x, y| x.cmp(y)).unwrap(), -10);
2716 #[stable(feature = "iter_min_by", since = "1.15.0")]
2717 fn min_by<F>(self, compare: F) -> Option<Self::Item>
2720 F: FnMut(&Self::Item, &Self::Item) -> Ordering,
2723 fn fold<T>(mut compare: impl FnMut(&T, &T) -> Ordering) -> impl FnMut(T, T) -> T {
2724 move |x, y| cmp::min_by(x, y, &mut compare)
2727 self.reduce(fold(compare))
2730 /// Reverses an iterator's direction.
2732 /// Usually, iterators iterate from left to right. After using `rev()`,
2733 /// an iterator will instead iterate from right to left.
2735 /// This is only possible if the iterator has an end, so `rev()` only
2736 /// works on [`DoubleEndedIterator`]s.
2741 /// let a = [1, 2, 3];
2743 /// let mut iter = a.iter().rev();
2745 /// assert_eq!(iter.next(), Some(&3));
2746 /// assert_eq!(iter.next(), Some(&2));
2747 /// assert_eq!(iter.next(), Some(&1));
2749 /// assert_eq!(iter.next(), None);
2752 #[doc(alias = "reverse")]
2753 #[stable(feature = "rust1", since = "1.0.0")]
2754 fn rev(self) -> Rev<Self>
2756 Self: Sized + DoubleEndedIterator,
2761 /// Converts an iterator of pairs into a pair of containers.
2763 /// `unzip()` consumes an entire iterator of pairs, producing two
2764 /// collections: one from the left elements of the pairs, and one
2765 /// from the right elements.
2767 /// This function is, in some sense, the opposite of [`zip`].
2769 /// [`zip`]: Iterator::zip
2776 /// let a = [(1, 2), (3, 4)];
2778 /// let (left, right): (Vec<_>, Vec<_>) = a.iter().cloned().unzip();
2780 /// assert_eq!(left, [1, 3]);
2781 /// assert_eq!(right, [2, 4]);
2783 #[stable(feature = "rust1", since = "1.0.0")]
2784 fn unzip<A, B, FromA, FromB>(self) -> (FromA, FromB)
2786 FromA: Default + Extend<A>,
2787 FromB: Default + Extend<B>,
2788 Self: Sized + Iterator<Item = (A, B)>,
2790 fn extend<'a, A, B>(
2791 ts: &'a mut impl Extend<A>,
2792 us: &'a mut impl Extend<B>,
2793 ) -> impl FnMut((), (A, B)) + 'a {
2800 let mut ts: FromA = Default::default();
2801 let mut us: FromB = Default::default();
2803 let (lower_bound, _) = self.size_hint();
2804 if lower_bound > 0 {
2805 ts.extend_reserve(lower_bound);
2806 us.extend_reserve(lower_bound);
2809 self.fold((), extend(&mut ts, &mut us));
2814 /// Creates an iterator which copies all of its elements.
2816 /// This is useful when you have an iterator over `&T`, but you need an
2817 /// iterator over `T`.
2824 /// let a = [1, 2, 3];
2826 /// let v_copied: Vec<_> = a.iter().copied().collect();
2828 /// // copied is the same as .map(|&x| x)
2829 /// let v_map: Vec<_> = a.iter().map(|&x| x).collect();
2831 /// assert_eq!(v_copied, vec![1, 2, 3]);
2832 /// assert_eq!(v_map, vec![1, 2, 3]);
2834 #[stable(feature = "iter_copied", since = "1.36.0")]
2835 fn copied<'a, T: 'a>(self) -> Copied<Self>
2837 Self: Sized + Iterator<Item = &'a T>,
2843 /// Creates an iterator which [`clone`]s all of its elements.
2845 /// This is useful when you have an iterator over `&T`, but you need an
2846 /// iterator over `T`.
2848 /// [`clone`]: Clone::clone
2855 /// let a = [1, 2, 3];
2857 /// let v_cloned: Vec<_> = a.iter().cloned().collect();
2859 /// // cloned is the same as .map(|&x| x), for integers
2860 /// let v_map: Vec<_> = a.iter().map(|&x| x).collect();
2862 /// assert_eq!(v_cloned, vec![1, 2, 3]);
2863 /// assert_eq!(v_map, vec![1, 2, 3]);
2865 #[stable(feature = "rust1", since = "1.0.0")]
2866 fn cloned<'a, T: 'a>(self) -> Cloned<Self>
2868 Self: Sized + Iterator<Item = &'a T>,
2874 /// Repeats an iterator endlessly.
2876 /// Instead of stopping at [`None`], the iterator will instead start again,
2877 /// from the beginning. After iterating again, it will start at the
2878 /// beginning again. And again. And again. Forever.
2885 /// let a = [1, 2, 3];
2887 /// let mut it = a.iter().cycle();
2889 /// assert_eq!(it.next(), Some(&1));
2890 /// assert_eq!(it.next(), Some(&2));
2891 /// assert_eq!(it.next(), Some(&3));
2892 /// assert_eq!(it.next(), Some(&1));
2893 /// assert_eq!(it.next(), Some(&2));
2894 /// assert_eq!(it.next(), Some(&3));
2895 /// assert_eq!(it.next(), Some(&1));
2897 #[stable(feature = "rust1", since = "1.0.0")]
2899 fn cycle(self) -> Cycle<Self>
2901 Self: Sized + Clone,
2906 /// Sums the elements of an iterator.
2908 /// Takes each element, adds them together, and returns the result.
2910 /// An empty iterator returns the zero value of the type.
2914 /// When calling `sum()` and a primitive integer type is being returned, this
2915 /// method will panic if the computation overflows and debug assertions are
2923 /// let a = [1, 2, 3];
2924 /// let sum: i32 = a.iter().sum();
2926 /// assert_eq!(sum, 6);
2928 #[stable(feature = "iter_arith", since = "1.11.0")]
2929 fn sum<S>(self) -> S
2937 /// Iterates over the entire iterator, multiplying all the elements
2939 /// An empty iterator returns the one value of the type.
2943 /// When calling `product()` and a primitive integer type is being returned,
2944 /// method will panic if the computation overflows and debug assertions are
2950 /// fn factorial(n: u32) -> u32 {
2951 /// (1..=n).product()
2953 /// assert_eq!(factorial(0), 1);
2954 /// assert_eq!(factorial(1), 1);
2955 /// assert_eq!(factorial(5), 120);
2957 #[stable(feature = "iter_arith", since = "1.11.0")]
2958 fn product<P>(self) -> P
2961 P: Product<Self::Item>,
2963 Product::product(self)
2966 /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
2972 /// use std::cmp::Ordering;
2974 /// assert_eq!([1].iter().cmp([1].iter()), Ordering::Equal);
2975 /// assert_eq!([1].iter().cmp([1, 2].iter()), Ordering::Less);
2976 /// assert_eq!([1, 2].iter().cmp([1].iter()), Ordering::Greater);
2978 #[stable(feature = "iter_order", since = "1.5.0")]
2979 fn cmp<I>(self, other: I) -> Ordering
2981 I: IntoIterator<Item = Self::Item>,
2985 self.cmp_by(other, |x, y| x.cmp(&y))
2988 /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
2989 /// of another with respect to the specified comparison function.
2996 /// #![feature(iter_order_by)]
2998 /// use std::cmp::Ordering;
3000 /// let xs = [1, 2, 3, 4];
3001 /// let ys = [1, 4, 9, 16];
3003 /// assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| x.cmp(&y)), Ordering::Less);
3004 /// assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| (x * x).cmp(&y)), Ordering::Equal);
3005 /// assert_eq!(xs.iter().cmp_by(&ys, |&x, &y| (2 * x).cmp(&y)), Ordering::Greater);
3007 #[unstable(feature = "iter_order_by", issue = "64295")]
3008 fn cmp_by<I, F>(mut self, other: I, mut cmp: F) -> Ordering
3012 F: FnMut(Self::Item, I::Item) -> Ordering,
3014 let mut other = other.into_iter();
3017 let x = match self.next() {
3019 if other.next().is_none() {
3020 return Ordering::Equal;
3022 return Ordering::Less;
3028 let y = match other.next() {
3029 None => return Ordering::Greater,
3034 Ordering::Equal => (),
3035 non_eq => return non_eq,
3040 /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
3046 /// use std::cmp::Ordering;
3048 /// assert_eq!([1.].iter().partial_cmp([1.].iter()), Some(Ordering::Equal));
3049 /// assert_eq!([1.].iter().partial_cmp([1., 2.].iter()), Some(Ordering::Less));
3050 /// assert_eq!([1., 2.].iter().partial_cmp([1.].iter()), Some(Ordering::Greater));
3052 /// assert_eq!([f64::NAN].iter().partial_cmp([1.].iter()), None);
3054 #[stable(feature = "iter_order", since = "1.5.0")]
3055 fn partial_cmp<I>(self, other: I) -> Option<Ordering>
3058 Self::Item: PartialOrd<I::Item>,
3061 self.partial_cmp_by(other, |x, y| x.partial_cmp(&y))
3064 /// [Lexicographically](Ord#lexicographical-comparison) compares the elements of this [`Iterator`] with those
3065 /// of another with respect to the specified comparison function.
3072 /// #![feature(iter_order_by)]
3074 /// use std::cmp::Ordering;
3076 /// let xs = [1.0, 2.0, 3.0, 4.0];
3077 /// let ys = [1.0, 4.0, 9.0, 16.0];
3080 /// xs.iter().partial_cmp_by(&ys, |&x, &y| x.partial_cmp(&y)),
3081 /// Some(Ordering::Less)
3084 /// xs.iter().partial_cmp_by(&ys, |&x, &y| (x * x).partial_cmp(&y)),
3085 /// Some(Ordering::Equal)
3088 /// xs.iter().partial_cmp_by(&ys, |&x, &y| (2.0 * x).partial_cmp(&y)),
3089 /// Some(Ordering::Greater)
3092 #[unstable(feature = "iter_order_by", issue = "64295")]
3093 fn partial_cmp_by<I, F>(mut self, other: I, mut partial_cmp: F) -> Option<Ordering>
3097 F: FnMut(Self::Item, I::Item) -> Option<Ordering>,
3099 let mut other = other.into_iter();
3102 let x = match self.next() {
3104 if other.next().is_none() {
3105 return Some(Ordering::Equal);
3107 return Some(Ordering::Less);
3113 let y = match other.next() {
3114 None => return Some(Ordering::Greater),
3118 match partial_cmp(x, y) {
3119 Some(Ordering::Equal) => (),
3120 non_eq => return non_eq,
3125 /// Determines if the elements of this [`Iterator`] are equal to those of
3131 /// assert_eq!([1].iter().eq([1].iter()), true);
3132 /// assert_eq!([1].iter().eq([1, 2].iter()), false);
3134 #[stable(feature = "iter_order", since = "1.5.0")]
3135 fn eq<I>(self, other: I) -> bool
3138 Self::Item: PartialEq<I::Item>,
3141 self.eq_by(other, |x, y| x == y)
3144 /// Determines if the elements of this [`Iterator`] are equal to those of
3145 /// another with respect to the specified equality function.
3152 /// #![feature(iter_order_by)]
3154 /// let xs = [1, 2, 3, 4];
3155 /// let ys = [1, 4, 9, 16];
3157 /// assert!(xs.iter().eq_by(&ys, |&x, &y| x * x == y));
3159 #[unstable(feature = "iter_order_by", issue = "64295")]
3160 fn eq_by<I, F>(mut self, other: I, mut eq: F) -> bool
3164 F: FnMut(Self::Item, I::Item) -> bool,
3166 let mut other = other.into_iter();
3169 let x = match self.next() {
3170 None => return other.next().is_none(),
3174 let y = match other.next() {
3175 None => return false,
3185 /// Determines if the elements of this [`Iterator`] are unequal to those of
3191 /// assert_eq!([1].iter().ne([1].iter()), false);
3192 /// assert_eq!([1].iter().ne([1, 2].iter()), true);
3194 #[stable(feature = "iter_order", since = "1.5.0")]
3195 fn ne<I>(self, other: I) -> bool
3198 Self::Item: PartialEq<I::Item>,
3204 /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
3205 /// less than those of another.
3210 /// assert_eq!([1].iter().lt([1].iter()), false);
3211 /// assert_eq!([1].iter().lt([1, 2].iter()), true);
3212 /// assert_eq!([1, 2].iter().lt([1].iter()), false);
3213 /// assert_eq!([1, 2].iter().lt([1, 2].iter()), false);
3215 #[stable(feature = "iter_order", since = "1.5.0")]
3216 fn lt<I>(self, other: I) -> bool
3219 Self::Item: PartialOrd<I::Item>,
3222 self.partial_cmp(other) == Some(Ordering::Less)
3225 /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
3226 /// less or equal to those of another.
3231 /// assert_eq!([1].iter().le([1].iter()), true);
3232 /// assert_eq!([1].iter().le([1, 2].iter()), true);
3233 /// assert_eq!([1, 2].iter().le([1].iter()), false);
3234 /// assert_eq!([1, 2].iter().le([1, 2].iter()), true);
3236 #[stable(feature = "iter_order", since = "1.5.0")]
3237 fn le<I>(self, other: I) -> bool
3240 Self::Item: PartialOrd<I::Item>,
3243 matches!(self.partial_cmp(other), Some(Ordering::Less | Ordering::Equal))
3246 /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
3247 /// greater than those of another.
3252 /// assert_eq!([1].iter().gt([1].iter()), false);
3253 /// assert_eq!([1].iter().gt([1, 2].iter()), false);
3254 /// assert_eq!([1, 2].iter().gt([1].iter()), true);
3255 /// assert_eq!([1, 2].iter().gt([1, 2].iter()), false);
3257 #[stable(feature = "iter_order", since = "1.5.0")]
3258 fn gt<I>(self, other: I) -> bool
3261 Self::Item: PartialOrd<I::Item>,
3264 self.partial_cmp(other) == Some(Ordering::Greater)
3267 /// Determines if the elements of this [`Iterator`] are [lexicographically](Ord#lexicographical-comparison)
3268 /// greater than or equal to those of another.
3273 /// assert_eq!([1].iter().ge([1].iter()), true);
3274 /// assert_eq!([1].iter().ge([1, 2].iter()), false);
3275 /// assert_eq!([1, 2].iter().ge([1].iter()), true);
3276 /// assert_eq!([1, 2].iter().ge([1, 2].iter()), true);
3278 #[stable(feature = "iter_order", since = "1.5.0")]
3279 fn ge<I>(self, other: I) -> bool
3282 Self::Item: PartialOrd<I::Item>,
3285 matches!(self.partial_cmp(other), Some(Ordering::Greater | Ordering::Equal))
3288 /// Checks if the elements of this iterator are sorted.
3290 /// That is, for each element `a` and its following element `b`, `a <= b` must hold. If the
3291 /// iterator yields exactly zero or one element, `true` is returned.
3293 /// Note that if `Self::Item` is only `PartialOrd`, but not `Ord`, the above definition
3294 /// implies that this function returns `false` if any two consecutive items are not
3300 /// #![feature(is_sorted)]
3302 /// assert!([1, 2, 2, 9].iter().is_sorted());
3303 /// assert!(![1, 3, 2, 4].iter().is_sorted());
3304 /// assert!([0].iter().is_sorted());
3305 /// assert!(std::iter::empty::<i32>().is_sorted());
3306 /// assert!(![0.0, 1.0, f32::NAN].iter().is_sorted());
3309 #[unstable(feature = "is_sorted", reason = "new API", issue = "53485")]
3310 fn is_sorted(self) -> bool
3313 Self::Item: PartialOrd,
3315 self.is_sorted_by(PartialOrd::partial_cmp)
3318 /// Checks if the elements of this iterator are sorted using the given comparator function.
3320 /// Instead of using `PartialOrd::partial_cmp`, this function uses the given `compare`
3321 /// function to determine the ordering of two elements. Apart from that, it's equivalent to
3322 /// [`is_sorted`]; see its documentation for more information.
3327 /// #![feature(is_sorted)]
3329 /// assert!([1, 2, 2, 9].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
3330 /// assert!(![1, 3, 2, 4].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
3331 /// assert!([0].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
3332 /// assert!(std::iter::empty::<i32>().is_sorted_by(|a, b| a.partial_cmp(b)));
3333 /// assert!(![0.0, 1.0, f32::NAN].iter().is_sorted_by(|a, b| a.partial_cmp(b)));
3336 /// [`is_sorted`]: Iterator::is_sorted
3337 #[unstable(feature = "is_sorted", reason = "new API", issue = "53485")]
3338 fn is_sorted_by<F>(mut self, compare: F) -> bool
3341 F: FnMut(&Self::Item, &Self::Item) -> Option<Ordering>,
3346 mut compare: impl FnMut(&T, &T) -> Option<Ordering> + 'a,
3347 ) -> impl FnMut(T) -> bool + 'a {
3349 if let Some(Ordering::Greater) | None = compare(&last, &curr) {
3357 let mut last = match self.next() {
3359 None => return true,
3362 self.all(check(&mut last, compare))
3365 /// Checks if the elements of this iterator are sorted using the given key extraction
3368 /// Instead of comparing the iterator's elements directly, this function compares the keys of
3369 /// the elements, as determined by `f`. Apart from that, it's equivalent to [`is_sorted`]; see
3370 /// its documentation for more information.
3372 /// [`is_sorted`]: Iterator::is_sorted
3377 /// #![feature(is_sorted)]
3379 /// assert!(["c", "bb", "aaa"].iter().is_sorted_by_key(|s| s.len()));
3380 /// assert!(![-2i32, -1, 0, 3].iter().is_sorted_by_key(|n| n.abs()));
3383 #[unstable(feature = "is_sorted", reason = "new API", issue = "53485")]
3384 fn is_sorted_by_key<F, K>(self, f: F) -> bool
3387 F: FnMut(Self::Item) -> K,
3390 self.map(f).is_sorted()
3393 /// See [TrustedRandomAccess]
3394 // The unusual name is to avoid name collisions in method resolution
3398 #[unstable(feature = "trusted_random_access", issue = "none")]
3399 unsafe fn __iterator_get_unchecked(&mut self, _idx: usize) -> Self::Item
3401 Self: TrustedRandomAccess,
3403 unreachable!("Always specialized");
3407 #[stable(feature = "rust1", since = "1.0.0")]
3408 impl<I: Iterator + ?Sized> Iterator for &mut I {
3409 type Item = I::Item;
3410 fn next(&mut self) -> Option<I::Item> {
3413 fn size_hint(&self) -> (usize, Option<usize>) {
3414 (**self).size_hint()
3416 fn advance_by(&mut self, n: usize) -> Result<(), usize> {
3417 (**self).advance_by(n)
3419 fn nth(&mut self, n: usize) -> Option<Self::Item> {