created_at2021-06-03 20:22:41.483774
updated_at2022-07-24 16:34:43.753758
descriptionInitialize arrays itemwise



Initialize arrays itemwise

Click here for the documentation.

Normally, when using fixed size arrays, you can only initialize them with a const value.


// Literals work.
let arr = [0; 5];
// Const values work too.
const STRING: String = String::new();
let arr = [STRING; 5];
// Function calls don't work.
let arr = [computation(); 5];

There are a few different ways of initializing an array itemwise, including:

  • Using an array of Options, initializing them all to None and then initializing each one to Some(computation()).
  • Using a Vec and incrementally pushing items to it.
  • Using an array of MaybeUninits, gradually initializing them and then transmuting the array. This requires usage of unsafe code.

This crate uses the third method but hides it behind a safe interface, so that no unsafe code is needed on the User end. It provides three functions to initialize arrays itemwise:

  • init_array to initialize a stack-based fixed-size array.
  • init_boxed_array to initialize a heap-allocated fixed-size array.
  • init_boxed_slice to initialize a heap-allocated dynamically-sized slice.

There is also a try_init_array function which allows to initialize an array fallibly and return the error early.

If you have the nightly feature enabled, you will have access to additional versions of the init_boxed_... functions compliant with the new Allocator API.

If you turn off the alloc feature, which is enabled by default, you can use this crate in a #[no_std] context without an allocator. The crate is fully #[no_std] compatible.

In addition to the 3 functions mentioned above, there are also two extension traits provided, ArrayExt and SliceExt, which provide the same functionality as the free functions.

All of these functions share the property that, if the initialization of any item panics (i.e. if the stack unwinds), all the already initialized items are dropped, minimizing the risk of a memory leak.


use init_array::*;

let arr = init_array(|i| i * i);
assert_eq!(arr, [0, 1, 4, 9, 16]);

let arr = init_boxed_array(|i| i * i);
assert_eq!(arr, Box::new([0, 1, 4, 9, 16]));

let arr = init_boxed_slice(5, |i| i * i);
assert_eq!(&*arr, &[0, 1, 4, 9, 16]);

let mut state = 0;
let arr = init_array(move |i| {
	state += i + 1;

assert_eq!(arr, [1, 3, 6, 10, 15, 21, 28, 36, 45, 55]);
Commit count: 17

cargo fmt