Arrays with run-time-known-length and multi-dimensional arrays


Apologies first for putting a user question on the “internals” board, but it seems the “user” board doesn’t exist yet.

Question 1: is there some way of making arrays with a length known at run-time? Something like:

fn make_arr(len: usize) -> Box<[i32]>{
    Box::new([0u32; len])

Question 2: are there any libraries for multi-dimensional arrays? I have seen comments about such things but not any real code, yet.

What I’d prefer not to use is Vec<Vec<Vec<f64>>>, but at the moment it might be either that or a crude wrapper holding strides and un-sugared index function.


If you’re looking for things like mathematical vectors and matrices, I would look into nalgebra… I find it quite useful.


nalgebra apparently has dynamically-sized matrices which, for my purposes, can be used as a two-dimensional array. But I need three dimensions.


You can create Vec<T> and turn it into Box<[T]> (i.e. throw away capacity) with Vec::into_boxed_slice



You could create a Vec<T> and then turn it into a Box<[T]>. I don’t see another way to create a dynamic Box<[T]> with safe code.

pub fn make_arr(len: usize) -> Box<[i32]> {
    use std::iter::repeat;

The code above is pretty slow with the current rustc though. With unsafe code it is 9× faster:

pub fn make_arr_unsafe(len: usize) -> Box<[i32]> {
    use std::rt::heap::allocate;
    use std::mem::{min_align_of, transmute};
    use std::ptr::set_memory;
    use std::raw::Slice;
    use std::i32;

    let size = len * i32::BYTES;
    unsafe {
        let mem = allocate(size, min_align_of::<i32>());
        set_memory(mem, 0, size);
        let slice = Slice { data: mem as *const i32, len: len };


I’ve written rust-ndarray (docs) but mostly to learn how numpy’s ndarray works! It’s a real multidimensional array. The most useful part of it (for me) is multidimensional slices. I would be happy to hear feedback, and be wary, I’m sure it is not well optimized yet.


Looks quite cool blake! Question, why not fix the dimensions of the array in the struct, i.e. Array2, Array3, etc.?


Well the dimension is fixed by the type, it’s in the second type parameter. Array2 would be Array<A, (Ix, Ix)>. You’re free to type alias this in your own code, I think that’s the best anyway.