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.

Peter Hall · Accepted Answer · 2020-07-21 09:10:26Z

14

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();

edited Jul 21, 2020 at 9:10

answered Nov 30, 2018 at 17:41

Peter Hall

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6 Comments

ixx Over a year ago

Can you get an array back, instead of converting to Vec?

Kevin M Granger Over a year ago

@Ixx you can't get an array back because .collect() doesn't know how long the input is, and arrays are of fixed length in rust. You can see all the things you can collect into by seeing what implements FromIterator: doc.rust-lang.org/std/iter/trait.FromIterator.html#implementors

Tim MB Over a year ago

It would be nice to implement map directly on array that can return the same type.

Peter Hall Over a year ago

@TimMB That is how it is done in languages like Javascript because these languages aim to prioritise simplicity over performance. If map was a method of slice or Vec then it would need to allocate a new Vec each time it is called. The API design in Rust means that you only allocate when you specifically choose to do so (typically by collecting), and you can build complex iterator chains that do no new allocating at all.

Peter Hall Over a year ago

And FWIW, if you really want that kind of API, you can create it yourself with an extension trait which you can implement for [T].

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