Push value into array of arrays in JavaScript

Question

I have and array of arrays which looks like this:

var arr = [[1,2,3],[4,5,6],[7,8,9]];

After that I have a list of numbers and a loop

var list = [15,10,11,14,13,12]

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

    var val = list[i];

    if (val >= 10 && val < 13) {
     arr[arr.length].push(val);   
    }
    else if (val >= 13 && val < 16) {
     arr[arr.length+1].push(val);   
    }
}

So basically I like to have an output which will look like this:

arr = [[1,2,3],[4,5,6],[7,8,9],[10,11,12],[13,14,15]];

With this code I'm getting an error "Cannot read property 'push' of undefined"

Also important is I can't use arr[3].push or arr[4].push because my case is more complicated and always I need to push values to new array which will appear on the over of my array. No matter how many objects I have inside.

Try explaining the use case for this code. I guarantee there's a better way to solve it. — Oka
– Oka, Commented Jun 13, 2015 at 17:21
You have to only use arr.push(val); It will append value to array. — Sachin I
– Sachin I, Commented Jun 13, 2015 at 17:28

Mulan · Accepted Answer · 2015-06-13 17:38:38Z

4

This is happening because arr[arr.length] will always be undefined.

So you're basically doing

undefined.push(x);
// Cannot read property 'push' of undefined

"Also important is I can't use arr[3].push or arr[4].push because my case is more complicated and always I need to push values to new array which will appear on the over of my array. No matter how many objects I have inside."

This algorithm is a code smell tho and we could probably help you better if you post your actual code.

To see what I'm talking about, consider the following code

// your numbers in a random order
var xs = [7,10,2,15,4,9,14,1,8,12,5,11,3,6,13];

// sort them
xs.sort(function(a, b) { return a-b; });

// define a function that "chunks" a list into smaller parts
function chunk(xs, n) {
  function iter(ys, y, xs) {
    if (y.length === 0) return ys;
    return next(ys.concat([y]), xs);
  }
  function next(ys, xs) {
    return iter(ys, xs.slice(0,n), xs.slice(n));
  }
  return next([], xs);
}

// call our function on your sorted list
var ys = chunk(xs, 3);

console.log(JSON.stringify(ys));
//=> [[1,2,3],[4,5,6],[7,8,9],[10,11,12],[13,14,15]]

edited Jun 13, 2015 at 17:38

answered Jun 13, 2015 at 17:17

Mulan

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1 Comment

TaoPR Over a year ago

Naming variables in calculus taste. Awesome.

Moeri · Accepted Answer · 2015-06-13 17:16:54Z

2

arr[arr.length] can never return any meaningful, think about it: if you have an array of length 6, then you have indexes 0..5 to work with. arr[6] will always return undefined because there's nothing there.

You probably need something like this:

if (val >= 10 && val < 13) {
 arr[arr.length - 1].push(val);   
}
else if (val >= 13 && val < 16) {
 arr[arr.length].push([val]);   
}

answered Jun 13, 2015 at 17:16

Moeri

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Comments

Sachin I · Accepted Answer · 2015-06-13 17:32:30Z

0

If you are looking sort the array element then your code will not work. Refer below code to sort the element and it will also solve your undefined issue.

<script>
var arr = [[1,2,3],[4,5,6],[7,8,9]];
var list = [15,10,11,14,13,12];
var arr1=[];
var arr2=[];

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

    var val = list[i];

    if (val >= 10 && val < 13) {
     arr1.push(val);
    }
    else if (val >= 13 && val < 16) {
     arr2.push(val);
    }

}
arr1.sort(function(a, b){return a-b});
arr.push(arr1);
arr2.sort(function(a, b){return a-b});
arr.push(arr2);
console.log(arr);
</script>

answered Jun 13, 2015 at 17:32

Sachin I

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Comments

MT0 · Accepted Answer · 2015-06-13 17:32:46Z

0

You need something like this:

var arr = [[1,2,3],[4,5,6],[7,8,9]];
var list = [15,10,11,14,13,12];
    
for (var i=0; i<list.length; i++) {
    var val = list[i];
    var index = Math.floor((val-1)/3);
    if ( arr[index] === undefined )
        arr[index] = [];
    arr[index].push(val);
    arr[index].sort();
}
    
console.log( arr );

answered Jun 13, 2015 at 17:32

MT0

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