core::iter::repeat_with - Rust
Description: API documentation for the Rust `repeat_with` fn in crate `core`.
Keywords: rust, rustlang, rust-lang, repeat_with

# 1.28.0[−][src]Function core::iter::repeat_with

ⓘImportant traits for RepeatWith<F>

### Important traits for RepeatWith<F>

`impl<A, F: FnMut() -> A> Iterator for RepeatWith<F> type Item = A;`

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

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 and over and over and over and 🔁.

Infinite iterators like `repeat_with()` are often used with adapters like `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 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

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