I want to create a mutable array of a fixed size. The elements are initialized later in the program. How do I initialize the array?
I tried doing this:
let mut array: [String; 126] = [String::new(); 126];
and it gives me the error:
the trait bound 'std::string::String: std::marker::Copy' is not satisfied
the trait 'std::marker::Copy' is not implemented for 'std::string::String'
how do I initialize the array with new strings?
At the moment, initialization of arrays is still a bit quirky.
In particular, having Default would have been useful here, but it's only implemented for arrays up to 32:
let array: [String; 32] = Default::default();
Any number over that will fail to compile because, while Rust 1.47 now implements some traits over a generic size for array types, Default is yet not one of them. These 32 implementations of Default were "sort-of" added manually.
We can overcome that with alternative container types, such as Vec. The vec! macro will let you clone a string as many times as you wish to fill in the new vector:
let mut array: Vec<String> = vec![String::new(); 126];
But of course, depending on your use case, you might also consider going lazy and only collecting the final outcomes using the Iterator API.
There are ways that work for types without const ways to construct them, for example:
let mut a = [(); 126].map(|_| String::new());
The other answer with const EMPTY_STRING: String = String::new() is probably better for this particular case, as the documentation of the array::map method warns that this might be inefficient and consume a lot of stack space.
In nightly (as of Rust 1.57), there is also an even nicer function std::array::from_fn().
You can use (the result of) a const function, for example String::new(), to initialize an array:
const EMPTY_STRING: String = String::new();
let mut array: [String; 126] = [EMPTY_STRING; 126];
