I can't get those variable declarations right…Why those parameters don't work? I just set the n value as arrays length

Question

I can't get those variable declarations right...Why those parameters don't work? I just set the n value as arrays length.

Why the compiler can understand the variable declaration?

void diag(int n, double a[n][n], double b[n],double x[n]){

    for(int i = 1; i <= n; i++){
        x[i] = b[i]/a[i][i];
    }
    return x;
}

Answer 1

You have to use dynamic arrays (for example std::vector) or pointer on array. In C++ You can't use arrays with not known size at compilation time ('n' is not know).

Answer 2

This is just not legal C++ or anything close to it. Arrays must have their dimension known at compile-time, and if you pass one to a function, all dimensions except for the innermost must be specified.

Furthermore, the name of a parameter is not in scope for other parameter declarations. (which is what your errors are about).

If you want to use "arrays" whose dimension is not known at compile-time, you can use std::vector which is a contiguous resizeable array.