Function core::iter::repeat_with
1.28.0 · source · pub fn repeat_with<A, F: FnMut() -> A>(repeater: F) -> RepeatWith<F>ⓘ
Expand description
Creates a new iterator that repeats elements of type A
endlessly by
applying the provided closure, the repeater, F: FnMut() -> A
.
The repeat_with()
function calls the repeater over and over again.
Infinite iterators like repeat_with()
are often used with adapters like
Iterator::take()
, in order to make them finite.
If the element type of the iterator you need implements Clone
, and
it is OK to keep the source element in memory, you should instead use
the repeat()
function.
An iterator produced by repeat_with()
is not a DoubleEndedIterator
.
If you need repeat_with()
to return a DoubleEndedIterator
,
please open a GitHub issue explaining your use case.
Examples
Basic usage:
use std::iter;
// let's assume we have some value of a type that is not `Clone`
// or which we don't want to have in memory just yet because it is expensive:
#[derive(PartialEq, Debug)]
struct Expensive;
// a particular value forever:
let mut things = iter::repeat_with(|| Expensive);
assert_eq!(Some(Expensive), things.next());
assert_eq!(Some(Expensive), things.next());
assert_eq!(Some(Expensive), things.next());
assert_eq!(Some(Expensive), things.next());
assert_eq!(Some(Expensive), things.next());
RunUsing mutation and going finite:
use std::iter;
// From the zeroth to the third power of two:
let mut curr = 1;
let mut pow2 = iter::repeat_with(|| { let tmp = curr; curr *= 2; tmp })
.take(4);
assert_eq!(Some(1), pow2.next());
assert_eq!(Some(2), pow2.next());
assert_eq!(Some(4), pow2.next());
assert_eq!(Some(8), pow2.next());
// ... and now we're done
assert_eq!(None, pow2.next());
Run