Function from_fn

pub fn from_fn<T, const N: usize, F>(f: F) -> [T; N]
where
    F: FnMut(usize) -> T,

Creates an array where each element is produced by calling f with that element’s index while walking forward through the array.

This is essentially the same as writing

[f(0), f(1), f(2), …, f(N - 2), f(N - 1)]

and is similar to (0..i).map(f), just for arrays not iterators.

If N == 0, this produces an empty array without ever calling f.

Example

// type inference is helping us here, the way `from_fn` knows how many
// elements to produce is the length of array down there: only arrays of
// equal lengths can be compared, so the const generic parameter `N` is
// inferred to be 5, thus creating array of 5 elements.

let array = core::array::from_fn(|i| i);
// indexes are:    0  1  2  3  4
assert_eq!(array, [0, 1, 2, 3, 4]);

let array2: [usize; 8] = core::array::from_fn(|i| i * 2);
// indexes are:     0  1  2  3  4  5   6   7
assert_eq!(array2, [0, 2, 4, 6, 8, 10, 12, 14]);

let bool_arr = core::array::from_fn::<_, 5, _>(|i| i % 2 == 0);
// indexes are:       0     1      2     3      4
assert_eq!(bool_arr, [true, false, true, false, true]);

You can also capture things, for example to create an array full of clones where you can’t just use [item; N] because it’s not Copy:

let my_string = String::from("Hello");
let clones: [String; 42] = std::array::from_fn(|_| my_string.clone());
assert!(clones.iter().all(|x| *x == my_string));

The array is generated in ascending index order, starting from the front and going towards the back, so you can use closures with mutable state:

let mut state = 1;
let a = std::array::from_fn(|_| { let x = state; state *= 2; x });
assert_eq!(a, [1, 2, 4, 8, 16, 32]);