Arrays in JS- Diagonal Difference

Problem Statement

Given a square matrix, calculate the absolute difference between the sums of its diagonals. Here is the link to problem HackerRank.

Example

1 2 3
4 5 6
9 8 9 

The left-to-right diagonal = 1+5+9=15. The right to left diagonal = 3+5+9=17. Their absolute difference is |15-17| = 2.

Solution

Let’s take an example:

11 2 4
4 5 6
10 8 -12

If we notice a pattern emerges. Our primary diagonal values are 11,5 and -12 that is if our input is a 2d integer array our primary diagonal values will be [[0,0],[1,1],[2,2]]. Similarly, for our secondary diagonal values are [[0,2],[1,1],[2,0]] that is 4,5 and 10.

Now, if we loop through array our primary values are where i = j. And secondary values can be defined as i+j = len-1; where length is length of the array.Let’s look at the code:

Code

function diagonalDifference(arr) {
    let primarySum = 0;
    let secondarySum = 0;
    const len = arr.length;

    for(let i = 0; i < len; i++){
        let dArr = arr[i];
        for(let j = 0; j < dArr.length; j++){
            if(i === j){
                primarySum += arr[i][j];
            }
            if(i+j === len-1){
                secondarySum += arr[i][j];
            }
        }
    }

    return Math.abs((primarySum - secondarySum));
}