Skipping Diagonal Elements Within a Square Matrix

Wrote a program to display a square matrix (ex: 5x5) and I am trying to count the total of the elements excluding the elements that lie within a diagonal line (from top left to bottom right & from bottom left to top right)

EX:

In a 5x5 matrix, only the Xs would be counted

O X X X O
X O X O X
X X O X X
X O X O X
O X X X O

Thus:

1 1 1 1 1
1 1 1 1 1
1 1 1 1 1
1 1 1 1 1
1 1 1 1 1
Total: 16

OR

2 2 2 2 2
2 2 2 2 2
2 2 2 2 2
2 2 2 2 2
2 2 2 2 2
Total: 32

Here is my code

int calculateVal(int **mat, int size)
{
int count = 0;
for (int row = 0; row < size; row++)
{
for (int col = 0; col < size; col++)
{
if (isRightDiagonal(size, row, col) == true ||
isLeftDiagonal(size, row, col) == true)
{
col++;
}
count += mat[row][col];
}
}
return count;
}
bool isRightDiagonal(int size, int row, int col)
{
return (col = abs(row - size)) ? true : false; 
}
bool isLeftDiagonal(int size, int row, int col)
{
return (row = abs(col - size)) ? true : false; 
}

I tried using an if statement to check if the element is within a diagonal, and if so, skipping to the next element, but the output is wrong.

The test case I have been primarily using is a 5x5 of 1s like the example above; however, the total prints out as 10 rather than 16

The tests for the diagonals are incorrect:

• an element is part of the left diagonal if col == row
• an element is part of the right diagonal if size - 1 - col == row

Furthermore, if the element is part of either diagonal, it should just be ignored, incrementing col is incorrect as it may make mat[row][col] refer en element outside the matrix, causing undefined behavior.

Here is a modified version:

int calculateVal(int **mat, int size) {
int count = 0;
for (int row = 0; row < size; row++) {
for (int col = 0; col < size; col++) {
if (col != row && size - 1 - col != row) {
count += mat[row][col];
}
}
}
return count;
}

Using a sample matrix with 1 for the entries you don't want to count (if you sum them instead of count then the value is not 0 then you know something is off). You could use a similar crafted matrix to ensure that the the 16 entries you are interested in are counted:

#include <stdio.h>
#include <stdlib.h>
int main() {
int matrix[][5] = {
{1, 0, 0, 0, 1},
{0, 1, 0, 1, 0},
{0, 0, 1, 0, 0},
{0, 1, 0, 1, 0},
{1, 0, 0, 0, 1}
};
size_t sum = 0;
const size_t n = sizeof matrix / sizeof *matrix;
for(size_t r = 0; r < n; r++)
for(size_t c = 0; c < n; c++)
sum += (r != c && r + c + 1 != n) * matrix[r][c];
printf("sum = %zu\n", sum);
}

If you don't like the non-branching style here is another way to express it:

			if(r != c && r + c + 1 != n)
sum += matrix[r][c];