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 again
14 /// may or may not ever return `Some` again. `next()` and `next_back()` are
15 /// interchangeable for this purpose.
17 /// [`Iterator`]: trait.Iterator.html
24 /// let numbers = vec![1, 2, 3, 4, 5, 6];
26 /// let mut iter = numbers.iter();
28 /// assert_eq!(Some(&1), iter.next());
29 /// assert_eq!(Some(&6), iter.next_back());
30 /// assert_eq!(Some(&5), iter.next_back());
31 /// assert_eq!(Some(&2), iter.next());
32 /// assert_eq!(Some(&3), iter.next());
33 /// assert_eq!(Some(&4), iter.next());
34 /// assert_eq!(None, iter.next());
35 /// assert_eq!(None, iter.next_back());
37 #[stable(feature = "rust1", since = "1.0.0")]
38 pub trait DoubleEndedIterator: Iterator {
39 /// Removes and returns an element from the end of the iterator.
41 /// Returns `None` when there are no more elements.
43 /// The [trait-level] docs contain more details.
45 /// [trait-level]: trait.DoubleEndedIterator.html
52 /// let numbers = vec![1, 2, 3, 4, 5, 6];
54 /// let mut iter = numbers.iter();
56 /// assert_eq!(Some(&1), iter.next());
57 /// assert_eq!(Some(&6), iter.next_back());
58 /// assert_eq!(Some(&5), iter.next_back());
59 /// assert_eq!(Some(&2), iter.next());
60 /// assert_eq!(Some(&3), iter.next());
61 /// assert_eq!(Some(&4), iter.next());
62 /// assert_eq!(None, iter.next());
63 /// assert_eq!(None, iter.next_back());
68 /// The elements yielded by `DoubleEndedIterator`'s methods may differ from
69 /// the ones yielded by `Iterator`'s methods:
72 /// let vec = vec![(1, 'a'), (1, 'b'), (1, 'c'), (2, 'a'), (2, 'b')];
73 /// let uniq_by_fst_comp = || {
74 /// let mut seen = std::collections::HashSet::new();
75 /// vec.iter().copied().filter(move |x| seen.insert(x.0))
78 /// assert_eq!(uniq_by_fst_comp().last(), Some((2, 'a')));
79 /// assert_eq!(uniq_by_fst_comp().next_back(), Some((2, 'b')));
82 /// uniq_by_fst_comp().fold(vec![], |mut v, x| {v.push(x); v}),
83 /// vec![(1, 'a'), (2, 'a')]
86 /// uniq_by_fst_comp().rfold(vec![], |mut v, x| {v.push(x); v}),
87 /// vec![(2, 'b'), (1, 'c')]
91 #[stable(feature = "rust1", since = "1.0.0")]
92 fn next_back(&mut self) -> Option<Self::Item>;
94 /// Returns the `n`th element from the end of the iterator.
96 /// This is essentially the reversed version of [`nth`]. Although like most indexing
97 /// operations, the count starts from zero, so `nth_back(0)` returns the first value from
98 /// the end, `nth_back(1)` the second, and so on.
100 /// Note that all elements between the end and the returned element will be
101 /// consumed, including the returned element. This also means that calling
102 /// `nth_back(0)` multiple times on the same iterator will return different
105 /// `nth_back()` will return [`None`] if `n` is greater than or equal to the length of the
108 /// [`nth`]: crate::iter::Iterator::nth
115 /// let a = [1, 2, 3];
116 /// assert_eq!(a.iter().nth_back(2), Some(&1));
119 /// Calling `nth_back()` multiple times doesn't rewind the iterator:
122 /// let a = [1, 2, 3];
124 /// let mut iter = a.iter();
126 /// assert_eq!(iter.nth_back(1), Some(&2));
127 /// assert_eq!(iter.nth_back(1), None);
130 /// Returning `None` if there are less than `n + 1` elements:
133 /// let a = [1, 2, 3];
134 /// assert_eq!(a.iter().nth_back(10), None);
137 #[stable(feature = "iter_nth_back", since = "1.37.0")]
138 fn nth_back(&mut self, mut n: usize) -> Option<Self::Item> {
139 for x in self.rev() {
148 /// This is the reverse version of [`try_fold()`]: it takes elements
149 /// starting from the back of the iterator.
151 /// [`try_fold()`]: Iterator::try_fold
158 /// let a = ["1", "2", "3"];
159 /// let sum = a.iter()
160 /// .map(|&s| s.parse::<i32>())
161 /// .try_rfold(0, |acc, x| x.and_then(|y| Ok(acc + y)));
162 /// assert_eq!(sum, Ok(6));
165 /// Short-circuiting:
168 /// let a = ["1", "rust", "3"];
169 /// let mut it = a.iter();
172 /// .map(|&s| s.parse::<i32>())
173 /// .try_rfold(0, |acc, x| x.and_then(|y| Ok(acc + y)));
174 /// assert!(sum.is_err());
176 /// // Because it short-circuited, the remaining elements are still
177 /// // available through the iterator.
178 /// assert_eq!(it.next_back(), Some(&"1"));
181 #[stable(feature = "iterator_try_fold", since = "1.27.0")]
182 fn try_rfold<B, F, R>(&mut self, init: B, mut f: F) -> R
185 F: FnMut(B, Self::Item) -> R,
188 let mut accum = init;
189 while let Some(x) = self.next_back() {
190 accum = f(accum, x)?;
195 /// An iterator method that reduces the iterator's elements to a single,
196 /// final value, starting from the back.
198 /// This is the reverse version of [`fold()`]: it takes elements starting from
199 /// the back of the iterator.
201 /// `rfold()` takes two arguments: an initial value, and a closure with two
202 /// arguments: an 'accumulator', and an element. The closure returns the value that
203 /// the accumulator should have for the next iteration.
205 /// The initial value is the value the accumulator will have on the first
208 /// After applying this closure to every element of the iterator, `rfold()`
209 /// returns the accumulator.
211 /// This operation is sometimes called 'reduce' or 'inject'.
213 /// Folding is useful whenever you have a collection of something, and want
214 /// to produce a single value from it.
216 /// [`fold()`]: Iterator::fold
223 /// let a = [1, 2, 3];
225 /// // the sum of all of the elements of a
226 /// let sum = a.iter()
227 /// .rfold(0, |acc, &x| acc + x);
229 /// assert_eq!(sum, 6);
232 /// This example builds a string, starting with an initial value
233 /// and continuing with each element from the back until the front:
236 /// let numbers = [1, 2, 3, 4, 5];
238 /// let zero = "0".to_string();
240 /// let result = numbers.iter().rfold(zero, |acc, &x| {
241 /// format!("({} + {})", x, acc)
244 /// assert_eq!(result, "(1 + (2 + (3 + (4 + (5 + 0)))))");
247 #[stable(feature = "iter_rfold", since = "1.27.0")]
248 fn rfold<B, F>(mut self, init: B, mut f: F) -> B
251 F: FnMut(B, Self::Item) -> B,
253 let mut accum = init;
254 while let Some(x) = self.next_back() {
260 /// Searches for an element of an iterator from the back that satisfies a predicate.
262 /// `rfind()` takes a closure that returns `true` or `false`. It applies
263 /// this closure to each element of the iterator, starting at the end, and if any
264 /// of them return `true`, then `rfind()` returns [`Some(element)`]. If they all return
265 /// `false`, it returns [`None`].
267 /// `rfind()` is short-circuiting; in other words, it will stop processing
268 /// as soon as the closure returns `true`.
270 /// Because `rfind()` takes a reference, and many iterators iterate over
271 /// references, this leads to a possibly confusing situation where the
272 /// argument is a double reference. You can see this effect in the
273 /// examples below, with `&&x`.
275 /// [`Some(element)`]: Some
282 /// let a = [1, 2, 3];
284 /// assert_eq!(a.iter().rfind(|&&x| x == 2), Some(&2));
286 /// assert_eq!(a.iter().rfind(|&&x| x == 5), None);
289 /// Stopping at the first `true`:
292 /// let a = [1, 2, 3];
294 /// let mut iter = a.iter();
296 /// assert_eq!(iter.rfind(|&&x| x == 2), Some(&2));
298 /// // we can still use `iter`, as there are more elements.
299 /// assert_eq!(iter.next_back(), Some(&1));
302 #[stable(feature = "iter_rfind", since = "1.27.0")]
303 fn rfind<P>(&mut self, predicate: P) -> Option<Self::Item>
306 P: FnMut(&Self::Item) -> bool,
310 mut predicate: impl FnMut(&T) -> bool,
311 ) -> impl FnMut((), T) -> ControlFlow<(), T> {
313 if predicate(&x) { ControlFlow::Break(x) } else { ControlFlow::CONTINUE }
317 self.try_rfold((), check(predicate)).break_value()
321 #[stable(feature = "rust1", since = "1.0.0")]
322 impl<'a, I: DoubleEndedIterator + ?Sized> DoubleEndedIterator for &'a mut I {
323 fn next_back(&mut self) -> Option<I::Item> {
326 fn nth_back(&mut self, n: usize) -> Option<I::Item> {