Incorrect statement about assignment for matrix views in user guide about nalgebra

[sebcrozet]

With non-view matrices we can do this:

let mut a = matrix![1, 2; 3, 4];
let b = matrix![10, 11; 12, 13];
a = b;
assert_eq!(a, b);

With view matrices, we can’t do this:

let mut a = matrix![1, 2; 3, 4];
let b = matrix![10, 11; 12, 13];
let mut view_a = a.fixed_view_mut::<2, 2>();
view_a = b;

Which makes sense, since view_a and b don’t have the same type anyway.

But I’ve seen many beginners struggle with the fact that it doesn’t compile. They usually expect that this effectively fills view_a with the content of b.

Or even worst, they will often assume that doing view_a = view_b will actually change the content of the matrix targeted by view_a (just like a = b changes the value identified by a).

All this is obvious when you are very familiar with rust and have some experience with nalgebra. But many users of nalgebra come from straight from C++ or python before having a chance to fully grasp the subtlety (or rather, the lack of subtlety) of the semantic of rust’s =

So the message I would like to give in the doc is that view_a = b doesn’t compile, and even view_a = view_b won’t do what you might think it will when it compiles (i.e. it won’t replace the content of the matrix targeted by view_a).

[robbespo00]

I do not understand correctly. what is the meaning.

Do you mean that the assignment operator = does not work for views due to the fact that if you try:

use nalgebra::{matrix, vector};

fn main() {
    let mut a = matrix![1.0, 2.0; 3.0, 4.0];
    let mut b = a.fixed_view_mut::<2, 1>(0, 0);
    let c = vector![5.0, 6.0];
    b = c;
}

It fails because it is a vector?
But in general you are allowed to assign = a view to another view (if they have the same dimensions) like here:

use nalgebra::{matrix, vector};

fn main() {
    let mut a = matrix![1.0, 2.0; 3.0, 4.0];
    let mut b = a.fixed_view_mut::<2, 1>(0, 0);

    let mut d = matrix![5.0, 6.0; 7.0, 8.0];
    let mut c = d.fixed_view_mut::<2, 1>(0, 0);
    b = c;
}

[Andlon]

I'm... actually not sure what makes sense to say here. We also cannot use = to assign to a matrix, we also have to use .copy_from there, so there doesn't seem to be any particular limitations of views here. I'm not sure, @sebcrozet maybe you can elaborate on what you had in mind in the discussion forum post?

[robbespo00]

@Andlon and @sebcrozet thanks for your support.
I have opened a pull request on the repository of the documentation. I hope it solved the problem.