How to print main diagonal of matrix and how to fill matrix with random numbers

I have matrix a[i][j] for example i = j = d (for example 3) . How can I fill it with random numbers and then I need to show main diagonal and another diagonal over and below matrix. Almost all examples that I found were for c++ but I need to do it in C.

I thought about cycle but I don't know how to use it. According to my idea it should take point a[0][0] and i = 0, i++ , i < d and j=0, j++ ,j < d. This will be main diagonal. And another one is i = d , i--, j=0 , j++ . Therefore it will take both diagonals and then print it though printf

It should look like this

1  6  11 16  - main diagonal

13 10 7  4   - additional diagonal

1  2   3   4

5  6   7   8

9  10  11  12  - random matrix

13 14  15  16

1  6  11  16 - main diagonal

13 10 7   4  - additional diagonal

Solution

Here is a version that populates a square matrix with random numbers, and then stores the primary and secondary diagonals in two arrays. These two arrays are filled at the same time in a single loop. Then the program displays the contents of each of the three arrays.

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

int main(void)
{
    size_t d = 4;
    int a[d][d];
    int primary[d], secondary[d];
    size_t i, j;

    srand(time(NULL));

    /* Fill array with random numbers */
    for (i = 0; i < d; i++)
        for (j = 0; j < d; j++)
            a[i][j] = rand() % 100;

    /* Store diagonals */
    for (i = 0; i < d; i++) {
        primary[i] = a[i][i];
        secondary[i] = a[d - (i + 1)][i];
        }

    /* Display arrays */
    puts("2d Array:");
    for (i = 0; i < d; i++) {
        for (j = 0; j < d; j++) {
            printf("%-5d", a[i][j]);
        }
        putchar('\n');
    }
    putchar('\n');

    puts("Primary Diagonal:");
    for (i = 0; i < d; i++)
        printf("%-5d", primary[i]);
    printf("\n\n");

    puts("Secondary Diagonal:");
    for (i = 0; i < d; i++)
        printf("%-5d", secondary[i]);
    printf("\n\n");

    return 0;
}

Here is a sample run:

2d Array:
65   59   91   10   
17   25   19   44   
94   77   68   21   
91   76   18   19   

Primary Diagonal:
65   25   68   19   

Secondary Diagonal:
91   77   19   10