How to map an array reference in Rust

Question

I have this array

let buffer: &[u8] = &[0; 40000];

But when I want to map it like this:

*buffer.map( |x| 0xff);

I have the following error:

error[E0599]: no method named `map` found for type `&[u8]` in the current scope   
     --> src/bin/save_png.rs:12:13
    | 
 12 |     *buffer.map( |x| 0xff); //.map(|x| 0xff);
    |             ^^^
    |
    = note: the method `map` exists but the following trait bounds were not satisfied:
            `&mut &[u8] : std::iter::Iterator`
            `&mut [u8] : std::iter::Iterator`

I tried several ways to do the elements mutable but I can't get the right syntax. Anyone have experience? I'm trying to work with a png image buffer.

Answer 1

The type &[T] doesn't have a map method. If you look at the error message, it is telling you that a method called map exists but it won't work for &mut &[u8] or &mut [u8] because those types do not implement Iterator. Arrays and other collections usually have a method, or set of methods, for creating an iterator. For a slice or an array, you have the choice of either iter() (iterating over references) or into_iter() (iterating over moved values and consuming the source collection).

Usually, you'll also want to collect the values into some other collection:

let res: Vec<u8> = buffer
    .iter()
    .map(|x| 0xff)
    .collect();