切片数组的切片 arrays

Question

I have an 2D array, and I want a function to change it. I don't know vectors yet, but since the array is going to have a fixed size, I don't need them. I wanted to give the function a slice of an array of slices of arrays, eg fn my_func (arr: &mut [&mut [char]]) {...} , but I don't know how to call it from my int main.

Here's an example I've been playing with.

fn main() {
    let mut map = [['-'; 3]; 3];

    for row in map {
        for col in row {
            print!("{} ", col);
        }

        println!("");
    }

    println!("Changing...");
    put_plus(???);

    for row in map {
        for col in row {
            print!("{} ", col);
        }

        println!("");
    }
}

fn put_plus(map: &mut [&mut [char]]) {
    map[map.len() - 1][map[map.len() - 1].len() - 2] = '+';
}

The output I expect is:

- - - 
- - - 
- - - 
Changing...
- - -
- - -
- + -

Is there an expression to turn [[char; 3]; 3] [[char; 3]; 3] [[char; 3]; 3] into [[char]] so that I could put this expression instead of ??? and everything works??

Answer 1

Here's a version of the plus function that works for all sizes:

fn put_plus<const COUNT: usize>(map: &mut [[char; COUNT]; COUNT]) {
    map[map.len() - 1][map[map.len() - 1].len() - 2] = '+';
}

As pointed out in the comments, you just need a mutable reference on the "outer" struct. In Rust, you don't control mutability at the "element" level. Instead, mutability of a container extends to all the elements within that container. You can't have, for example, a mutable vector containing immutable vectors or a mutable array containing immutable arrays.

So, just slapping a &mut to the "outside" of your array means everything in your array, including the inner arrays AND the values contained within those inner arrays, will be mutable.

EDIT: My version of put_plus will only work for square arrays. As commenter @cafce25 points out, the outer :COUNT can be removed. That will make it more general