Here is the program to find the sum of the primary diagonal elements (From left top to right bottom) of the entered integer matrix. The program asks for the order (M, N)  [where M should equal N] of the matrix. Soon after entering the order, the cursor takes to the proper position in the screen to input the matrices. User need to hit ‘Enter’ button after each entry. Matrix is stored as A.

Logic :  The diagonal elements, are the elements having equal column and row value throughout. By keeping this in mind, we can trace the matrix till the end using the help of a for loop.  The flag ‘sum’ is updated in each of the iterations.
Finally, we’ll get the sum of the diagonal elements after the end of the while loop.

You are always welcome with your suggestion and doubts into our discussion forum.

#include<stdio.h>
void main()
{
int A[5][5],i,j,m,n,sum = 0;
clrscr();
printf(“nnt ENTER A ORDER OF THE MATRIX M,N…: “);
scanf(“%d,%d”,&m,&n);
printf(“nnt ENTER THE ELEMENTS OF THE MATRIX..:nn”);
if(m == n)
{
for(i=1;i<=m;i++)
{
for(j=1;j<=n;j++)
{
gotoxy(20+j*4,12+i*2);
scanf(“%d”,&A[i][j]);
}
printf(“n”);
}
for(i=1;i<=m;i++)
sum = sum + A[i][i];
printf(“nt THE SUM OF PRIMARY DIAGONAL OF A MATRIX IS…: %d”, sum);
}
else
{
printf(“nt THE ORDER OF THE MATRIX IS NOT CORRECT”);
printf(“nnt HELP : ‘M’ SHOULD BE EQUAL TO ‘N’”);
}
getch();
}
Download exe and source code here.