# Function std::iter::repeat_with

1.28.0 · source ·

    pub fn repeat_with<A, F>(repeater: F) -> RepeatWith<F> ⓘwhere
        F: FnMut() -> A,

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());

Run

Using 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