Why matrix[i+1][j] is not defined?

Question

Why matrix in [i+1] is not defined meanwhile it works properly for matrix [i]

 function matrixElementsSum(matrix) {
     let x = 0;
     let i = 0;
     let j=0;
     for (i=0; i<matrix.length; i++) {
         for (j=0; j<matrix[i].length; j++) {

         if (matrix[i][j] === 0) {
             matrix[i+1][j]=0;
            //Cannot set property '0' of undefined
         }
         x = matrix[i].reduce(function(a,b){return a+b},0);
         x +=x;
      }

     }
    return x;
}

Think about the case when `i` is `matrix.length-1` at that time there does not exists `i+1` — void, Feb 14 '18 at 11:15
In the very last iteration of the outer loop, when `i == matrix.length - 1`, then `i + 1` will be out of bounds. — Some programmer dude, Feb 14 '18 at 11:15
Possible duplicate of [Why is using "for...in" with array iteration a bad idea?](https://stackoverflow.com/questions/500504/why-is-using-for-in-with-array-iteration-a-bad-idea) — Ankit Patidar, Feb 14 '18 at 11:25

Victor M Perez · Answer 1 · 2018-02-15T10:12:36.497

0

The index - i - should go until matrix.length - 1 in your

    for (i=0; i<matrix.length; i++) {
       ///ETC

edited Feb 15 '18 at 10:12

answered Feb 14 '18 at 11:16

Victor M Perez

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score 0 · Answer 2 · answered Feb 14 '18 at 11:18

Why matrix in [i+1] is not defined meanwhile it works properly for matrix [i]

Simply because of your for-loop condition

for (i=0; i<matrix.length; i++) {

value of i can go upto matrix.length -1, and matrix[ matrix.length ] will always be undefined as array indexing starts from 0.

Why matrix[i+1][j] is not defined?

2 Answers2