How to sum elements at the same index in array of

Question

Let's say that I have an array of arrays, like so:

[
  [0, 1, 3],
  [2, 4, 6],
  [5, 5, 7],
  [10, 0, 3]
]

How do I generate a new array that sums all of the values at each position of the inner arrays in javascript? In this case, the result would be: [17, 10, 19]. I need to be able to have a solution that works regardless of the length of the inner arrays. I think that this is possible using some combination of map and for-of, or possibly reduce, but I can't quite wrap my head around it. I've searched but can't find any examples that quite match this one.

Answer 1

Assuming array is static as op showned.

a = [
      [0, 1, 3],
      [2, 4, 6],
      [5, 5, 7],
      [10, 0, 3]
    ]

b = []
     	
for(i = 0; i < a[0].length; i++){						
  count = 0
  for(j = 0; j < a.length; j++){				
     count += a[j][i]			
   }
   b.push(count)
}		
console.log(b)

Answer 2

You can use Array.prototype.reduce() in combination with Array.prototype.forEach().

var array = [
        [0, 1, 3],
        [2, 4, 6],
        [5, 5, 7],
        [10, 0, 3]
    ],
    result = array.reduce(function (r, a) {
        a.forEach(function (b, i) {
            r[i] = (r[i] || 0) + b;
        });
        return r;
    }, []);
document.write('<pre>' + JSON.stringify(result, 0, 4) + '</pre>');

Update, a shorter approach by taking a map for reducing the array.

var array = [[0, 1, 3], [2, 4, 6], [5, 5, 7], [10, 0, 3]],
    result = array.reduce((r, a) => a.map((b, i) => (r[i] || 0) + b), []);
    
console.log(result);

Answer 3

Using Lodash 4:

function sum_columns(data) {
  return _.map(_.unzip(data), _.sum);
}

var result = sum_columns([
  [1, 2],
  [4, 8, 16],
  [32]
]);

console.log(JSON.stringify(result));

<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.17.11/lodash.js"></script>

For older Lodash versions and some remarks

Lodash 4 has changed the way _.unzipWith works, now the iteratee gets all the values passed as spread arguments at once, so we cant use the reducer style _.add anymore. With Lodash 3 the following example works just fine:

function sum_columns(data) {
  return _.unzipWith(data, _.add);
}

var result = sum_columns([
  [1, 2],
  [4, 8, 16],
  [32],
]);

console.log(JSON.stringify(result));

<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/3.10.1/lodash.js"></script>

_.unzipWith will insert undefineds where the row is shorter than the others, and _.sum treats undefined values as 0. (as of Lodash 3)

If your input data can contain undefined and null items, and you want to treat those as 0, you can use this:

function sum_columns_safe(data) {
  return _.map(_.unzip(data), _.sum);
}

function sum_columns(data) {
  return _.unzipWith(data, _.add);
}

console.log(sum_columns_safe([[undefined]])); // [0]
console.log(sum_columns([[undefined]]));      // [undefined]

<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/3.10.1/lodash.js"></script>

This snipet works with Lodash 3, unfortunately I didn't find a nice way of treating undefined as 0 in Lodash 4, as now sum is changed so _.sum([undefined]) === undefined

Answer 4

const ar = [
  [0, 1, 3],
  [2, 4, 6],
  [5, 5, 7],
  [10, 0, 3]
]   

ar.map( item => item.reduce( (memo, value)=> memo+= value, 0 ) )
//result-> [4, 12, 17, 13]

Answer 5

One-liner in ES6, with map and reduce

var a = [ [0, 1, 3], [2, 4, 6], [5, 5, 7], [10, 0, 3] ];

var sum = a[0].map((_, i) => a.reduce((p, _, j) => p + a[j][i], 0));

document.write(sum);

Answer 6

Assuming that the nested arrays will always have the same lengths, concat and reduce can be used.

    function totalIt (arr) {
        var lng = arr[0].length;
       return [].concat.apply([],arr)  //flatten the array
                .reduce( function(arr, val, ind){ //loop over and create a new array
                    var i = ind%lng;  //get the column
                    arr[i] = (arr[i] || 0) + val; //update total for column
                    return arr;  //return the updated array
                }, []);  //the new array used by reduce
    }
    
    var arr = [
      [0, 1, 3],
      [2, 4, 6],
      [5, 5, 7],
      [10, 0, 3]
    ];
    console.log(totalIt(arr));  //[17, 10, 19]