C program to find the sum of diagonal elements of a square matrix

This C program is to find the sum of diagonal elements of a square matrix.For example, for a 2 x 2 matrix, the sum of diagonal elements of the matrix {1,2,3,4} will be equal to 5.

1     2

3    4

 Sum = 1+4 = 5

Logic

Here the procedure is almost same as the sum of elements of a matrix, only one condition needs to be added which is, we add only those elements of the matrix for which row number and column number is same, like 1^st row and 1^st column, 2^nd row and 2^nd column and so on(i==j).

Dry Run of the Program

Take input mat[][] and store elements in mat{1,2}{3,4}

Take input ‘row’ and no of rows(row) as 2

Take input ‘col’ and no of columns(col) as 2

Initialize sum=0

1st iteration for(i=0;i<row;i++) i.e. for(i=0;0<2;i++) Outer loop

1st iteration for(j=0;j<col;j++) i.e. for(j=0;0<2;j++) Inner loop

if(i==j) i.e. if(0==0)   true

sum=sum+mat[i][j];  i.e.  sum=0+mat[0][0] i.e. sum=0+1  i.e. sum=1

2nd iteration for(j=1;j<col;j++) i.e. for(j=1;1<2;j++) Inner loop

if(i==j) i.e. if(0==1)   false

2nd iteration for(i=1;i<row;i++) i.e. for(i=1;1<2;i++) Outer loop

1st iteration for(j=0;j<col;j++) i.e. for(j=0;0<2;j++) Inner loop

if(i==j) i.e. if(1==0)   false

2nd iteration for(j=1;j<col;j++) i.e. for(j=1;1<2;j++) Inner loop

if(i==j) i.e. if(1==1)   true

sum=sum+mat[i][j];  i.e.  sum=1+mat[1][1] i.e. sum=1+4  i.e. sum=5

Now we break out of inner loop and then outer loop.

Hence the sum of diagonal elements of a square matrix[1,2][3,4] would be sum=5

Program

#include<stdio.h>

void main()
{
    int mat[12][12];
    int i,j,row,col,sum=0;
    printf("Enter the number of rows and columns for 1st matrix\n");
    scanf("%d%d",&row,&col);
    printf("Enter the elements of the matrix\n");
    for(i=0;i<row;i++)
    {
        for(j=0;j<col;j++)
        {
            scanf("%d",&mat[i][j]);
        }
    }

    printf("The matrix\n");
    for(i=0;i<row;i++)
    {
        for(j=0;j<col;j++)
        {
            printf("%d\t",mat[i][j]);
        }
        printf("\n");
    }
    //To add diagonal elements
    for(i=0;i<row;i++)
    {
        for(j=0;j<col;j++)
        {
            if(i==j)
            {
                sum=sum+mat[i][j];
            }
        }
    }

    printf("The sum of diagonal elements of a square matrix = %d\n",sum);
}

Output