Cannot read property 'push' of undefined multidimensional array

Question

I'm trying to solve this problem: Write a function that splits an array (first argument) into groups the length of size (second argument) and returns them as a multidimensional array. For example:

chunk(['a', 'b', 'c', 'd'], 2)

should return

[['a'. 'b'], ['c', 'd']]

My code is as follows:

function chunk(arr, size) {
  var newArr = [[]];
  for(i = 0; i < arr.length; i++) {
    for(j = 0; j < size; j++) {
      newArr[i].push(arr[i + j]);
    }
  }
  return newArr;
}

It gives an error: Cannot read property 'push' of undefined. Why is this happening and how can I fix this?

Michael Geary · Accepted Answer · 2015-07-20 08:24:32Z

2

You could do this with nested loops, but why not go for a simpler approach and use array.slice()?

function chunk( input, size ) {
    var output = [];
    for( i = 0;  i < input.length;  i += size ) {
        output.push( input.slice( i, i + size ) );
    }
    return output;
}

answered Jul 20, 2015 at 8:24

Michael Geary

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Glorfindel · Accepted Answer · 2015-07-20 08:14:58Z

2

After

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

you must initialize the one-dimensional array:

newArr[i] = [];

This will solve the error, but not produce the result you want. I think you need something like this:

for (i = 0; i < ceil(arr.length / size); i++) {
    newArr[i] = [];
    for (j = 0; j < size; j++) {
        if (i * size + j >= arr.length)
            break;
        newArr[i].push(arr[i * size + j]);
    }
}

answered Jul 20, 2015 at 8:14

Glorfindel

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2 Comments

Andrew Over a year ago

can someone explain why there's a break on line 5?

Glorfindel Over a year ago

Suppose size = 3, but the original arr has 7 members. The last 'chunk' will contain only 1 element - after that, there will be no more members to fill the remaining 2 spots in the last chunk. This is where the break comes in.

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