How can I initialize an array using a function?

Question

I want to create an array of 10 empty vectors in Rust, but [Vec::new(); 10] doesn't work as Vec doesn't implement Copy. How can I do this, and in more general terms how can I initialize a array by repeatedly calling a function?

Answer 1

For your specific case, you can just use Default:

let v: [Vec<String>; 10] = Default::default();

For the general case, you can create an iterator out of your function and then collect into the array using ArrayVec:

use arrayvec::ArrayVec; // 0.4.10
use std::iter;

fn make<R>(f: impl FnMut() -> R) -> [R; 10] {
    let a: ArrayVec<_> = iter::repeat_with(f).collect();
    a.into_inner()
        .unwrap_or_else(|_| panic!("Did not have enough elements"))
}

fn main() {
    let mut a = 0;
    let arr = make(|| {
        a += 3;
        a
    });

    println!("{:?}", arr);
    // [3, 6, 9, 12, 15, 18, 21, 24, 27, 30]
}

See also:

• How do I collect into an array?