How to get the result of a function in main when working with matrices?

Question

C++ / C#

Good day to all, I need to write a function for the sum of the elements of the main diagonal of the matrix. The program fully works with one exception, I can't move the code into a separate function. Perhaps an error in the parameters that I pass to the function, or in the function call itself

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

int sum (int n, int matrix[n][n]) {
    int sum = 0;
    for (int j = 0; j < n; ++j){
        for (int i = 0; i < n; ++i){
            if (j == i){
                sum += matrix[j][i];
            }
        }
    }
    return sum;
}

int main ()
{
    int n, i, j;
    int sum_of = 0;
    printf("n \n");
    scanf("%d",&n);
    int matrix[n][n];

    srand(time(0));
    for (i = 0; i < n; i++){
        for (j = 0; j < n; j++){
            matrix[i][j] = rand()%10;
        }
    }

    for (i = 0; i < n; i++){
        for (j = 0; j < n; j++){
            printf("%d ", matrix[i][j]);
        }
        printf("\n");
    }

    sum_of = sum(matrix[][]);

    printf("%d", sum_of);

    return 0;
}

Answer 1

sum_of = sum(matrix[][]);

should become

int sum (int n, int matrix[n][n]) {
    int sum = 0;
    for (int j = 0; j < n; ++j){
        for (int i = 0; i < n; ++i){
            if (j == i){
                sum += matrix[j][i];
            }
        }
    }
    return sum;
}

It certainly works, but it's better to write it like this:

int sum (int n, int matrix[n][n])
{
    int sum = 0;
    for (int i = 0; i < n; ++i) {
        sum += matrix[i][i];
    }
    return sum;
}

Answer 2

Something I doubt that your program works. You are trying to set an array of arbitrary length on the stack. On the stack, you can only create arrays of a fixed length, since memory is allocated for them at compile time. In your case, you need to use a heap (see the malloc() and free() functions) https://en.wikipedia.org/wiki/C_dynamic_memory_all...
This is if we are talking about C, if you write it in C ++ (please distinguish these languages) then the vector will help you

Answer 3

The program is fully functional

Doubtfully.
Like this:

printf("n \n");
    scanf("%d",&n);
    int matrix[n][n];

cannot be done. A static array is built at the time of compilation, so its dimension must be set - it's strange that your compiler didn't issue a comment - or was it simply ignored?
Memory allocation should be done like this:

int *matrix;

matrix = calloc( n * n, 1);

after which matrix is ​​a pointer
to int and the whole program, accordingly, needs to be redone for address arithmetic (I suspect that the task was to study address arithmetic :)
free(matrix);

Answer 4

Pay attention to how the function is declared: And how you call it: I hope you will understand further.