Create 2D array from 1D array using loop (JavaScript)

Question

I have an array: [1, 2, 3, 4, 5, 6, 7, 8, 9]

I want to create a 2D array with three 1D arrays. Each NUM in the function variables is the length of each 1D array. The result should be [[1, 2, 3], [4, 5, 6], [7, 8, 9]]

But all I get is ,,3,,,6,,,9. What am I doing wrong?

function infiniteLoop(arr, num) {
 var answer = [];
 var count = 0;
 for (let i = 0; i < num.length; i++) {
 for (let j = 0; j < num[i]; j++, count++) {
 answer[i] = [];
 answer[i][j] = arr[count];
 }
 }
 return answer;
}
document.write(infiniteLoop([1, 2, 3, 4, 5, 6, 7, 8, 9], [3, 3, 3]));

• nnnnnn: Please don't edit suggestions into your question, because that invalidates answers to the previous version of your question. Regarding your edit, you have added the answer[i] = []; part in the wrong place. Have another look at my answer, or Thomas's (but don't use new Array()).
• Alexey Tseitlin: Sorry for the edit. Tried to help)

Answer 1

JavaScript doesn't have multidimensional arrays per se, what it has is arrays of arrays.

When you try to use answer[i][j] the answer[i] part of that is undefined because you haven't set it to anything yet - at that point answer is just an empty array. You need to set answer[i] = []; to set the first element of answer to be an empty array, and then you can use answer[i][j].

That will fit in your existing loop like this:

for (let i = 0; i < num.length; i++) {
 answer[i] = []; // <--- add this
 for (let j = 0; j < num[i]; j++, count++) {
 answer[i][j] = arr[count];
 }
}

Answer 2

Without testing it I believe you need to set answer[i] = [] before you loop your next array

Answer 3

answer[i] hasn't been created. You are trying to assign a value to something that doesn't exist. You need to create answer[i] like this:

answer[i] = new Array(num[i]);

So, the full code:

function infiniteLoop(arr, num) {
 var answer = [];
 var count = 0;
 for (var i = 0; i < num.length; i++) {
 answer[i] = new Array(num[i]);
 for (var j = 0; j < num[i]; j++, count++) {
 answer[i][j] = arr[count];
 }
 }
 return answer;
}
document.write(infiniteLoop([1, 2, 3, 4, 5, 6, 7, 8, 9], [3, 3, 3]));

Answer 4

The problem is that answer[i] is undefined. The solution would be to add the following statement to the beginning of the first for loop:

answer[i] = [];

This should initialize answer[i] as an array, thus allowing you to set individual indices for it.