scalar ops implementation missing for `usize - array`

[fzyzcjy]

Hi thanks for the lib! I see matrix - matrix, matrix - scalar both ok, but seems that there is not scalar - matrix, so I would appreciate it if we could have it.

[bluss]

It should be implemented according to https://docs.rs/ndarray/0.15.3/ndarray/struct.ArrayBase.html#arithmetic-operations, so this bug report needs instructions to reproduce - reopen with reproduction. Thanks :)

[fzyzcjy]

this errors:

    let a = 20usize;
    let b = array![10usize];
    let c = a - b;

with message

error[E0277]: cannot subtract `ArrayBase<OwnedRepr<usize>, Dim<[usize; 1]>>` from `usize`
  --> tests/macro.rs:12:15
   |
12 |     let c = a - b;
   |               ^ no implementation for `usize - ArrayBase<OwnedRepr<usize>, Dim<[usize; 1]>>`
   |
   = help: the trait `Sub<ArrayBase<OwnedRepr<usize>, Dim<[usize; 1]>>>` is not implemented for `usize`

while this compiles:

    let a = array![20usize];
    let b = array![10usize];
    let c = a - b;

this also compile:

    let a = array![20usize];
    let b = 10usize;
    let c = a - b;

of course this also compile:

    let a = 20usize;
    let b = 10usize;
    let c = a - b;

[fzyzcjy]

@bluss I have no permission to reopen it. So could you please reopen it? now it has reproducible sample

[bluss]

Ah yes, usize is missing: see here https://docs.rs/ndarray/0.15.3/src/ndarray/impl_ops.rs.html#331-341 It's not something we noticed before.

It's a bug/missing feature. We promise to implement these ops for at least all ScalarOperand types. Workaround until then: Use array-array ops or put the scalar on the rhs. Other types will work, for example f64 or u64.

[fzyzcjy]

Looking forward to the fix!