1 use crate::ops::{ControlFlow, Try};
3 /// An iterator able to yield elements from both ends.
5 /// Something that implements `DoubleEndedIterator` has one extra capability
6 /// over something that implements [`Iterator`]: the ability to also take
7 /// `Item`s from the back, as well as the front.
9 /// It is important to note that both back and forth work on the same range,
10 /// and do not cross: iteration is over when they meet in the middle.
12 /// In a similar fashion to the [`Iterator`] protocol, once a
13 /// `DoubleEndedIterator` returns [`None`] from a [`next_back()`], calling it
14 /// again may or may not ever return [`Some`] again. [`next()`] and
15 /// [`next_back()`] are interchangeable for this purpose.
17 /// [`next_back()`]: DoubleEndedIterator::next_back
18 /// [`next()`]: Iterator::next
25 /// let numbers = vec![1, 2, 3, 4, 5, 6];
27 /// let mut iter = numbers.iter();
29 /// assert_eq!(Some(&1), iter.next());
30 /// assert_eq!(Some(&6), iter.next_back());
31 /// assert_eq!(Some(&5), iter.next_back());
32 /// assert_eq!(Some(&2), iter.next());
33 /// assert_eq!(Some(&3), iter.next());
34 /// assert_eq!(Some(&4), iter.next());
35 /// assert_eq!(None, iter.next());
36 /// assert_eq!(None, iter.next_back());
38 #[stable(feature = "rust1", since = "1.0.0")]
39 #[cfg_attr(not(test), rustc_diagnostic_item = "DoubleEndedIterator")]
40 pub trait DoubleEndedIterator: Iterator {
41 /// Removes and returns an element from the end of the iterator.
43 /// Returns `None` when there are no more elements.
45 /// The [trait-level] docs contain more details.
47 /// [trait-level]: DoubleEndedIterator
54 /// let numbers = vec![1, 2, 3, 4, 5, 6];
56 /// let mut iter = numbers.iter();
58 /// assert_eq!(Some(&1), iter.next());
59 /// assert_eq!(Some(&6), iter.next_back());
60 /// assert_eq!(Some(&5), iter.next_back());
61 /// assert_eq!(Some(&2), iter.next());
62 /// assert_eq!(Some(&3), iter.next());
63 /// assert_eq!(Some(&4), iter.next());
64 /// assert_eq!(None, iter.next());
65 /// assert_eq!(None, iter.next_back());
70 /// The elements yielded by `DoubleEndedIterator`'s methods may differ from
71 /// the ones yielded by [`Iterator`]'s methods:
74 /// let vec = vec![(1, 'a'), (1, 'b'), (1, 'c'), (2, 'a'), (2, 'b')];
75 /// let uniq_by_fst_comp = || {
76 /// let mut seen = std::collections::HashSet::new();
77 /// vec.iter().copied().filter(move |x| seen.insert(x.0))
80 /// assert_eq!(uniq_by_fst_comp().last(), Some((2, 'a')));
81 /// assert_eq!(uniq_by_fst_comp().next_back(), Some((2, 'b')));
84 /// uniq_by_fst_comp().fold(vec![], |mut v, x| {v.push(x); v}),
85 /// vec![(1, 'a'), (2, 'a')]
88 /// uniq_by_fst_comp().rfold(vec![], |mut v, x| {v.push(x); v}),
89 /// vec![(2, 'b'), (1, 'c')]
92 #[stable(feature = "rust1", since = "1.0.0")]
93 fn next_back(&mut self) -> Option<Self::Item>;
95 /// Advances the iterator from the back by `n` elements.
97 /// `advance_back_by` is the reverse version of [`advance_by`]. This method will
98 /// eagerly skip `n` elements starting from the back by calling [`next_back`] up
99 /// to `n` times until [`None`] is encountered.
101 /// `advance_back_by(n)` will return [`Ok(())`] if the iterator successfully advances by
102 /// `n` elements, or [`Err(k)`] if [`None`] is encountered, where `k` is the number of
103 /// elements the iterator is advanced by before running out of elements (i.e. the length
104 /// of the iterator). Note that `k` is always less than `n`.
106 /// Calling `advance_back_by(0)` can do meaningful work, for example [`Flatten`] can advance its
107 /// outer iterator until it finds an inner iterator that is not empty, which then often
108 /// allows it to return a more accurate `size_hint()` than in its initial state.
110 /// [`advance_by`]: Iterator::advance_by
111 /// [`Flatten`]: crate::iter::Flatten
112 /// [`next_back`]: DoubleEndedIterator::next_back
119 /// #![feature(iter_advance_by)]
121 /// let a = [3, 4, 5, 6];
122 /// let mut iter = a.iter();
124 /// assert_eq!(iter.advance_back_by(2), Ok(()));
125 /// assert_eq!(iter.next_back(), Some(&4));
126 /// assert_eq!(iter.advance_back_by(0), Ok(()));
127 /// assert_eq!(iter.advance_back_by(100), Err(1)); // only `&3` was skipped
133 #[unstable(feature = "iter_advance_by", reason = "recently added", issue = "77404")]
134 fn advance_back_by(&mut self, n: usize) -> Result<(), usize> {
136 self.next_back().ok_or(i)?;
141 /// Returns the `n`th element from the end of the iterator.
143 /// This is essentially the reversed version of [`Iterator::nth()`].
144 /// Although like most indexing operations, the count starts from zero, so
145 /// `nth_back(0)` returns the first value from the end, `nth_back(1)` the
146 /// second, and so on.
148 /// Note that all elements between the end and the returned element will be
149 /// consumed, including the returned element. This also means that calling
150 /// `nth_back(0)` multiple times on the same iterator will return different
153 /// `nth_back()` will return [`None`] if `n` is greater than or equal to the
154 /// length of the iterator.
161 /// let a = [1, 2, 3];
162 /// assert_eq!(a.iter().nth_back(2), Some(&1));
165 /// Calling `nth_back()` multiple times doesn't rewind the iterator:
168 /// let a = [1, 2, 3];
170 /// let mut iter = a.iter();
172 /// assert_eq!(iter.nth_back(1), Some(&2));
173 /// assert_eq!(iter.nth_back(1), None);
176 /// Returning `None` if there are less than `n + 1` elements:
179 /// let a = [1, 2, 3];
180 /// assert_eq!(a.iter().nth_back(10), None);
183 #[stable(feature = "iter_nth_back", since = "1.37.0")]
184 fn nth_back(&mut self, n: usize) -> Option<Self::Item> {
185 self.advance_back_by(n).ok()?;
189 /// This is the reverse version of [`Iterator::try_fold()`]: it takes
190 /// elements starting from the back of the iterator.
197 /// let a = ["1", "2", "3"];
198 /// let sum = a.iter()
199 /// .map(|&s| s.parse::<i32>())
200 /// .try_rfold(0, |acc, x| x.and_then(|y| Ok(acc + y)));
201 /// assert_eq!(sum, Ok(6));
204 /// Short-circuiting:
207 /// let a = ["1", "rust", "3"];
208 /// let mut it = a.iter();
211 /// .map(|&s| s.parse::<i32>())
212 /// .try_rfold(0, |acc, x| x.and_then(|y| Ok(acc + y)));
213 /// assert!(sum.is_err());
215 /// // Because it short-circuited, the remaining elements are still
216 /// // available through the iterator.
217 /// assert_eq!(it.next_back(), Some(&"1"));
220 #[stable(feature = "iterator_try_fold", since = "1.27.0")]
221 fn try_rfold<B, F, R>(&mut self, init: B, mut f: F) -> R
224 F: FnMut(B, Self::Item) -> R,
227 let mut accum = init;
228 while let Some(x) = self.next_back() {
229 accum = f(accum, x)?;
234 /// An iterator method that reduces the iterator's elements to a single,
235 /// final value, starting from the back.
237 /// This is the reverse version of [`Iterator::fold()`]: it takes elements
238 /// starting from the back of the iterator.
240 /// `rfold()` takes two arguments: an initial value, and a closure with two
241 /// arguments: an 'accumulator', and an element. The closure returns the value that
242 /// the accumulator should have for the next iteration.
244 /// The initial value is the value the accumulator will have on the first
247 /// After applying this closure to every element of the iterator, `rfold()`
248 /// returns the accumulator.
250 /// This operation is sometimes called 'reduce' or 'inject'.
252 /// Folding is useful whenever you have a collection of something, and want
253 /// to produce a single value from it.
255 /// Note: `rfold()` combines elements in a *right-associative* fashion. For associative
256 /// operators like `+`, the order the elements are combined in is not important, but for non-associative
257 /// operators like `-` the order will affect the final result.
258 /// For a *left-associative* version of `rfold()`, see [`Iterator::fold()`].
265 /// let a = [1, 2, 3];
267 /// // the sum of all of the elements of a
268 /// let sum = a.iter()
269 /// .rfold(0, |acc, &x| acc + x);
271 /// assert_eq!(sum, 6);
274 /// This example demonstrates the right-associative nature of `rfold()`:
275 /// it builds a string, starting with an initial value
276 /// and continuing with each element from the back until the front:
279 /// let numbers = [1, 2, 3, 4, 5];
281 /// let zero = "0".to_string();
283 /// let result = numbers.iter().rfold(zero, |acc, &x| {
284 /// format!("({x} + {acc})")
287 /// assert_eq!(result, "(1 + (2 + (3 + (4 + (5 + 0)))))");
289 #[doc(alias = "foldr")]
291 #[stable(feature = "iter_rfold", since = "1.27.0")]
292 fn rfold<B, F>(mut self, init: B, mut f: F) -> B
295 F: FnMut(B, Self::Item) -> B,
297 let mut accum = init;
298 while let Some(x) = self.next_back() {
304 /// Searches for an element of an iterator from the back that satisfies a predicate.
306 /// `rfind()` takes a closure that returns `true` or `false`. It applies
307 /// this closure to each element of the iterator, starting at the end, and if any
308 /// of them return `true`, then `rfind()` returns [`Some(element)`]. If they all return
309 /// `false`, it returns [`None`].
311 /// `rfind()` is short-circuiting; in other words, it will stop processing
312 /// as soon as the closure returns `true`.
314 /// Because `rfind()` takes a reference, and many iterators iterate over
315 /// references, this leads to a possibly confusing situation where the
316 /// argument is a double reference. You can see this effect in the
317 /// examples below, with `&&x`.
319 /// [`Some(element)`]: Some
326 /// let a = [1, 2, 3];
328 /// assert_eq!(a.iter().rfind(|&&x| x == 2), Some(&2));
330 /// assert_eq!(a.iter().rfind(|&&x| x == 5), None);
333 /// Stopping at the first `true`:
336 /// let a = [1, 2, 3];
338 /// let mut iter = a.iter();
340 /// assert_eq!(iter.rfind(|&&x| x == 2), Some(&2));
342 /// // we can still use `iter`, as there are more elements.
343 /// assert_eq!(iter.next_back(), Some(&1));
346 #[stable(feature = "iter_rfind", since = "1.27.0")]
347 fn rfind<P>(&mut self, predicate: P) -> Option<Self::Item>
350 P: FnMut(&Self::Item) -> bool,
353 fn check<T>(mut predicate: impl FnMut(&T) -> bool) -> impl FnMut((), T) -> ControlFlow<T> {
355 if predicate(&x) { ControlFlow::Break(x) } else { ControlFlow::CONTINUE }
359 self.try_rfold((), check(predicate)).break_value()
363 #[stable(feature = "rust1", since = "1.0.0")]
364 impl<'a, I: DoubleEndedIterator + ?Sized> DoubleEndedIterator for &'a mut I {
365 fn next_back(&mut self) -> Option<I::Item> {
368 fn advance_back_by(&mut self, n: usize) -> Result<(), usize> {
369 (**self).advance_back_by(n)
371 fn nth_back(&mut self, n: usize) -> Option<I::Item> {