array-init

Documentation

Crates.io

(still kinda experimental, API may change, may be secretly unsafe)

The array-init crate allows you to initialize arrays with an initializer closure that will be called once for each element until the array is filled.

This way you do not need to default-fill an array before running initializers. Rust currently only lets you either specify all initializers at once, individually ([a(), b(), c(), ...]), or specify one initializer for a Copy type ([a(); N]), which will be called once with the result copied over.

Examples:

# #![allow(unused)]
# extern crate array_init;

// Initialize an array of length 50 containing
// successive squares

let arr: [u32; 50] = array_init::array_init(|i| (i*i) as u32);

// Initialize an array from an iterator
// producing an array of [1,2,3,4] repeated

let four = [1u32,2,3,4];
let mut iter = four.iter().cloned().cycle();
let arr: [u32; 50] = array_init::from_iter(iter).unwrap();

// Closures can also mutate state. We guarantee that they will be called
// in order from lower to higher indices.

let mut last = 1u64;
let mut secondlast = 0;
let fibonacci: [u64; 50] = array_init::array_init(|_| {
    let this = last + secondlast;
    secondlast = last;
    last = this;
    this
});

Currently, using from_iter and array_init will incur additional memcpys, which may be undesirable for a large array. This can be eliminated by using the nightly feature of this crate, which uses unions to provide panic-safety. Alternatively, if your array only contains Copy types, you can use array_init_copy and from_iter_copy.

Sadly, cannot guarantee right now that any of these solutions will completely eliminate a memcpy.