error: cannot assign to immutable indexed content `i[..]`

Question

In the following rust code I am trying to change the contents of an array:

    let mut example_state = [[0;8]; 2];
    for mut i in example_state.iter() {
        let mut k = 0;
        for j in i.iter(){
            i[k] = 9u8;
            k +=1
        }
    }

However I get the error message:

src/main.rs:18:13: 18:23 error: cannot assign to immutable indexed content `i[..]`
src/main.rs:18             i[k] = 9u8;

which I'm confused by because I am defining i to be mut and example_state is also mutable.

I also don't know if this is the best way to change the contents of an array - do I need the counter k or can I simply use the iterator j in some way?

UPDATE: So I found that this block of code works:

let mut example_state = [[n;8]; 2];
for i in example_state.iter_mut() {
    for j in i.iter_mut(){
        *j = 9u8;
    }
}

but I would appreciate some explanation of what the difference is between them, iter_mut doesn't throw up much on Google.

Answer 1

Let's look at the signatures of the two methods, iter and iter_mut:

fn iter(&self) -> Iter<T>;
fn iter_mut(&mut self) -> IterMut<T>;

And the structs they return, Iter and IterMut, specifically the implementation of Iterator:

// Iter
type Item = &'a T
// IterMut
type Item = &'a mut T 

These are associated types, but basically in this case, they specify what the return type of calling Iterator::next. When you used iter, even though it was on a mutable variable, you were asking for an iterator to immutable references to a type T (&T). That's why you weren't able to mutate them!

When you switched to iter_mut, the return type of Iterator::next is &mut T, a mutable reference to a type T. You are allowed to set these values!

As an aside, your question used arrays, not slices, but there aren't documentation links for arrays (that I could find quickly), and slices are close enough to arrays so I used them for this explanation.