Calling slice on a dynamic size array

[kali]

Hey folks,

I'm struggling to find a way to call slice() on a dynamic ranked array, and can find a way to make it work.

extern crate ndarray;
use ndarray::*;

fn main() {
    let array: ArrayD<f32> = Array::zeros(vec![8, 8, 8]);
    foo(&array.view());
}

fn foo<'a>(a: &'a ArrayViewD<'a, f32>) -> ArrayViewD<'a, f32> {
    let slicer: Vec<SliceOrIndex> = (0..a.ndim())
        .map(|_| SliceOrIndex::Slice {
            start: 2,
            end: Some(6),
            step: 1,
        }).collect();
    let slice = SliceInfo::new(slicer).unwrap();
    a.slice(slice)
}

The gist of the issue is the type of the AsRef<[SliceOrIndex]> in (slicer in the example).

   |
13 |     a.slice(slice)
   |             ^^^^^ expected reference, found struct `ndarray::SliceInfo`
   |
   = note: expected type `&ndarray::SliceInfo<[ndarray::SliceOrIndex], ndarray::Dim<ndarray::IxDynImpl>>`
              found type `ndarray::SliceInfo<std::vec::Vec<ndarray::SliceOrIndex>, _>`

I can't see how to make the typechecker happy here, and I can't instantiate SliceInfo<[ndarray::SliceOrIndex]> because [ndarray::SliceOrIndex] is not sized. Am I missing something obvious ?

[jturner314]

The solution is to call .as_ref() manually on slice to perform the conversion, like this:

a.slice(slice.as_ref())

This calls this AsRef implementation (SliceInfo docs):

impl<T, D> AsRef<SliceInfo<[SliceOrIndex], D>> for SliceInfo<T, D>
where
    T: AsRef<[SliceOrIndex]>,
    D: Dimension,

This is a somewhat ugly part of the API. I have a plan to fix this but haven't gotten around to it yet.

Fwiw, for this particular example, it's simpler to use a series of .slice_axis() or .slice_axis_inplace() calls, like this:

fn foo<'a>(a: &'a ArrayViewD<'a, f32>) -> ArrayViewD<'a, f32> {
    let mut view = a.clone();
    for axis in 0..view.ndim() {
        view.slice_axis_inplace(Axis(axis), Slice::from(2..6));
    }
    view
}

Edit: (Or, if you know the dimension of the array ahead-of-time, you can use the s![] macro.)

[kali]

@jturner314 Ha, thanks a lot. for the solution and, and for the slice_axis_inplace tip. Everything is dynamic in my case, s! is not an option.