Support zero-length axis in .map_axis/_mut()

[jturner314]

The implementation of .map_axis/_mut() panics when the specified axis is zero-length. Ideally, we'd just apply mapping to empty views instead.

Example:

extern crate ndarray;

use ndarray::prelude::*;

fn main() {
    let a = Array2::<f32>::zeros((0, 2));
    a.map_axis(Axis(0), |_| {});
}

[andrei-papou]

Hi @jturner314 . I can work on this one if you don't mind.

[jturner314]

@andrei-papou Sure, please feel free.

[andrei-papou]

@jturner314 could you please point me to an example of creating an empty array? Thanks a lot.

[jturner314]

The simplest way to create a 1-dimensional, zero-length view in map_axis would be unsafe { ArrayView::new_(std::ptr::NonNull::<A>::dangling().as_ptr(), Ix1(0), Ix1(0)) }. See this commentary.

Edit: If that doesn't compile, you might need a cast: unsafe { ArrayView::new_(std::ptr::NonNull::<A>::dangling().as_ptr() as *const A, Ix1(0), Ix1(0)) }.

[andrei-papou]

@jturner314 thanks. please take a look at the PR #612

[jturner314]

Fwiw, I realized that a cleaner (safe) way to create a zero-length one-dimensional view is ArrayView1::from(&[]).

[jblondin]

PR was merged. Is anything still remaining here, or can this issue be closed?

[jturner314]

Fixed by #612. Thanks for following up.