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]));

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()). — nnnnnn
– nnnnnn, Commented May 27, 2016 at 23:32

nnnnnn · Accepted Answer · 2016-05-27 23:25:32Z

4

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];
    }
}

answered May 27, 2016 at 23:25

nnnnnn

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Comments

EJanuszewski · Accepted Answer · 2016-05-27 23:24:58Z

1

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

answered May 27, 2016 at 23:24

EJanuszewski

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Comments

user2684182 · Accepted Answer · 2016-05-27 23:27:34Z

1

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]));

answered May 27, 2016 at 23:27

user2684182

Comments

RamenChef · Accepted Answer · 2016-05-27 23:26:44Z

0

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.

answered May 27, 2016 at 23:26

RamenChef

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