core/iter/sources/repeat.rs
1use crate::iter::{FusedIterator, TrustedLen};
2use crate::num::NonZero;
3
4/// Creates a new iterator that endlessly repeats a single element.
5///
6/// The `repeat()` function repeats a single value over and over again.
7///
8/// Infinite iterators like `repeat()` are often used with adapters like
9/// [`Iterator::take()`], in order to make them finite.
10///
11/// Use [`str::repeat()`] instead of this function if you just want to repeat
12/// a char/string `n`th times.
13///
14/// If the element type of the iterator you need does not implement `Clone`,
15/// or if you do not want to keep the repeated element in memory, you can
16/// instead use the [`repeat_with()`] function.
17///
18/// [`repeat_with()`]: crate::iter::repeat_with
19/// [`str::repeat()`]: ../../std/primitive.str.html#method.repeat
20///
21/// # Examples
22///
23/// Basic usage:
24///
25/// ```
26/// use std::iter;
27///
28/// // the number four 4ever:
29/// let mut fours = iter::repeat(4);
30///
31/// assert_eq!(Some(4), fours.next());
32/// assert_eq!(Some(4), fours.next());
33/// assert_eq!(Some(4), fours.next());
34/// assert_eq!(Some(4), fours.next());
35/// assert_eq!(Some(4), fours.next());
36///
37/// // yup, still four
38/// assert_eq!(Some(4), fours.next());
39/// ```
40///
41/// Going finite with [`Iterator::take()`]:
42///
43/// ```
44/// use std::iter;
45///
46/// // that last example was too many fours. Let's only have four fours.
47/// let mut four_fours = iter::repeat(4).take(4);
48///
49/// assert_eq!(Some(4), four_fours.next());
50/// assert_eq!(Some(4), four_fours.next());
51/// assert_eq!(Some(4), four_fours.next());
52/// assert_eq!(Some(4), four_fours.next());
53///
54/// // ... and now we're done
55/// assert_eq!(None, four_fours.next());
56/// ```
57#[inline]
58#[stable(feature = "rust1", since = "1.0.0")]
59#[rustc_diagnostic_item = "iter_repeat"]
60pub fn repeat<T: Clone>(elt: T) -> Repeat<T> {
61 Repeat { element: elt }
62}
63
64/// An iterator that repeats an element endlessly.
65///
66/// This `struct` is created by the [`repeat()`] function. See its documentation for more.
67#[derive(Clone, Debug)]
68#[stable(feature = "rust1", since = "1.0.0")]
69pub struct Repeat<A> {
70 element: A,
71}
72
73#[stable(feature = "rust1", since = "1.0.0")]
74impl<A: Clone> Iterator for Repeat<A> {
75 type Item = A;
76
77 #[inline]
78 fn next(&mut self) -> Option<A> {
79 Some(self.element.clone())
80 }
81
82 #[inline]
83 fn size_hint(&self) -> (usize, Option<usize>) {
84 (usize::MAX, None)
85 }
86
87 #[inline]
88 fn advance_by(&mut self, n: usize) -> Result<(), NonZero<usize>> {
89 // Advancing an infinite iterator of a single element is a no-op.
90 let _ = n;
91 Ok(())
92 }
93
94 #[inline]
95 fn nth(&mut self, n: usize) -> Option<A> {
96 let _ = n;
97 Some(self.element.clone())
98 }
99
100 fn last(self) -> Option<A> {
101 loop {}
102 }
103
104 fn count(self) -> usize {
105 loop {}
106 }
107}
108
109#[stable(feature = "rust1", since = "1.0.0")]
110impl<A: Clone> DoubleEndedIterator for Repeat<A> {
111 #[inline]
112 fn next_back(&mut self) -> Option<A> {
113 Some(self.element.clone())
114 }
115
116 #[inline]
117 fn advance_back_by(&mut self, n: usize) -> Result<(), NonZero<usize>> {
118 // Advancing an infinite iterator of a single element is a no-op.
119 let _ = n;
120 Ok(())
121 }
122
123 #[inline]
124 fn nth_back(&mut self, n: usize) -> Option<A> {
125 let _ = n;
126 Some(self.element.clone())
127 }
128}
129
130#[stable(feature = "fused", since = "1.26.0")]
131impl<A: Clone> FusedIterator for Repeat<A> {}
132
133#[unstable(feature = "trusted_len", issue = "37572")]
134unsafe impl<A: Clone> TrustedLen for Repeat<A> {}