How do I initialize an array of numbers with a variable?-CodePudding

I am from Java, I like using arrays in this manner.

public int do_something(int width, int height){
     int[][] nums = new int[height][width];
     return 1;
}

In the code above this isn't a problem and Java will create a 2D array of int values and fill them with zeros.

I want to achieve this with Rust as well. This is what I tried.

fn do_something(n: usize, m: usize) -> i32 {
    let memo: [[i32; m]; n] = [[0; m]; n];
    1
}

The program wouldn't compile, it tells me that I cannot initialize arrays with non-constants. My problem is that I don't know the values of n and m beforehand.

I tried using a vector instead of arrays like this:

fn do_something(n: usize, m: usize) -> i32 {
    let mut nums: Vec<Vec<i32>> = Vec::new();

    for _i in 0..n{
        let mut each: Vec<i32> = Vec::new();
        for _j in 0..m{
            each.push(0);
        }

        nums.push(each);
    }

    println!("{:?}", nums);
    1
}

Is there a simpler approach to initializing arrays with variables and filling them with zeros? Or should I suffice myself with the above approach?

CodePudding user response：

Is there a simpler approach to initializing arrays with variables and filling them with zeros?

You can just convert the array literals (which require a compile-time length) to vec pseudo-literals, which don't:

fn do_something(n: usize, m: usize) -> i32 {
    let memo = vec![vec![0i32; m]; n];
    1
}

CodePudding user response：

If you need the values to have a runtime-defined, but fixed size (i.e. if they won't be resized after creation), you can strip one layer of indirection by using Box<[i32]> instead of Vec<Vec<i32>>:

fn do_something(n: usize, m: usize) -> i32 {
    let memo: Box<[i32]> = vec![0; m * n].into_boxed_slice();
    // and then access the values with e.g. `memo[x * m   y]`
    1
}

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