Javascript - sum 2d arrays

Question

I have 2 2d arrays

var arr1 = [

  [1, 'a'],

  [2, 'b']

]



var arr2 = [

  [3, 'a'],

  [5, 'c']

]

I would like to sum these 2 arrays to get this result

var output = [

  [4, 'a'],

  [2, 'b'],

  [5, 'c']

]

I tried writing 2

.map

functions but along with the desired results this will return a lot of duplicates:

function sumArrays (arr1, arr2) {

  var output = [];

  arr2.map(function(i) {

    arr1.map(function(n) {

      if (i[1] === n[1]) {

        output.push([i[0] + n[0], i[1]])

      } else {

        output.push(i)

      }

    })

  })

  return output;

}

Is there an easier way to do this, or should I now be removing everything but the highest value for a specific string?

Thanks for the help.

Answer 1

Please use not Array#map, when you do not need a new array, which this method returns.

You could use a hash table for the inventory and check against and update arr2 with Array#forEach.

Proposal which uses arr1 for update

var arr1 = [[1, 'a'], [2, 'b']],
    arr2 = [[3, 'a'], [5, 'c']],
    inventory = Object.create(null);

arr1.forEach(function (a) {
    this[a[1]] = a;
}, inventory);

arr2.forEach(function (a) {
    if (!this[a[1]]) {
        this[a[1]] = [0, a[1]];
        arr1.push(this[a[1]]);
    }
    this[a[1]][0] += a[0];
}, inventory);

console.log(arr1);

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Proposal with new array for result.

var arr1 = [[1, 'a'], [2, 'b']],
    arr2 = [[3, 'a'], [5, 'c']],
    inventory = Object.create(null),
    result = arr1.map(function (a) {
        return this[a[1]] = [a[0], a[1]];
    }, inventory);

arr2.forEach(function (a) {
    if (!this[a[1]]) {
        this[a[1]] = [0, a[1]];
        result.push(this[a[1]]);
    }
    this[a[1]][0] += a[0];
}, inventory);

console.log(result);

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Javascript - sum 2d arrays

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