Javascript - sum 2d arrays

arrays, javascript

Solution

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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Problem

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.

Original source