How to create an immutable array of user input numbers?

Question

I am trying to create an immutable array of user input in rust but the problem is that I have to initialize the array first, and if I initialize the array I cannot insert user inputs in the array.

fn main() {
    //error: If I don't initialize the array then it throws and error.
    let arr: [i32;5] = [0;5];
    for i in 0..5 {
        // let i be the user input for now
        // if I do initialize the array I cannot reassign the array elements because of immutability
        arr[i] = i as i32;
    }
}

Error Message:

   Compiling playground v0.0.1 (/playground)
error[E0594]: cannot assign to `arr[_]`, as `arr` is not declared as mutable
 --> src/main.rs:7:9
  |
3 |     let arr: [i32;5] = [0;5];
  |         --- help: consider changing this to be mutable: `mut arr`
...
7 |         arr[i] = i as i32;
  |         ^^^^^^^^^^^^^^^^^ cannot assign

For more information about this error, try `rustc --explain E0594`.
error: could not compile `playground` due to previous error

What if you add mut where the error suggests? If you really want arr to be immutable you can write let arr = arr; later. Is that what you want? — rodrigo
– rodrigo, Commented Feb 18, 2022 at 20:31

rodrigo · Accepted Answer · 2022-02-18 20:43:27Z

4

In Rust you cannot use a variable without initializing it first. But if at initialization time you do not have the final values, you have to declare it as mut:

let mut arr: [i32;5] = [0;5];

And then you initialize the values of arr freely.

If later you want to make the variable arr immutable you just write a rebind:

let arr = arr;

As a side note you can always rebind a value to make it mutable again:

let mut arr = arr;

There are other ways to write that, like a sub-context or a function, but are more or less equivalent:

let arr = {
    let mut arr: [i32; 5] = [0; 5];
    //add values to arr
    arr
};

answered Feb 18, 2022 at 20:43

rodrigo

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