stacking array without copying

[daniellga]

Hi! Having an array like [0, 1, 2, 3, 4, 5, 6], is it possible to create an array like the one below, without cloning the original (thus I want an ArrayView)?

[[0, 2, 4],
 [1, 3, 5],
 [2, 4, 6]]

Note that the result is just a combination (stack) of views from the first array, taking a step of 2 between each element.

[bluss]

Reading it, it looks like it has column stride of 1 and row stride of 2, which should be possible as an ArrayView and denied for ArrayViewMut - not sure if the ArrayView constructors that take strides allow it, or if they need to be changed to allow this (it's not safe for mutable views since elements overlap.)

[daniellga]

You mean like this?

fn main() {
    let arr = [0, 1, 2, 3, 4, 5, 6];
    let result = ArrayView::from_shape((3, 3).strides((2, 1)), &arr).unwrap();
}

I get the following error:

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: ShapeError/Unsupported: unsupported operation', src/main.rs:10:65
note: [run with `RUST_BACKTRACE=1` environment variable to display a backtrace](https://play.rust-lang.org/#)

[bluss]

It should be (3, 3).strides((1, 2)) but it's symmetric so it will fail the same way. Missing feature. It's been discussed in other issues somewhere.

[jturner314]

Fwiw, a temporary workaround is to use the unsafe from_shape_ptr method. (from_shape_ptr has fewer checks than from_shape.)

use ndarray::prelude::*;

fn main() {
    let a = [0, 1, 2, 3, 4, 5, 6];
    let v = unsafe { ArrayView::from_shape_ptr([3, 3].strides([1, 2]), a.as_ptr()) };
    println!("{:?}", v);
}

[daniellga]

Nice! Thanks! Is it possible to convert the same way if a was a ndarray (ArrayBase) instead of an array?

[daniellga]

Oh nvm... It works the same way it seems.