Sum multidimensional array values and store them in another array

Question

I have a 2D array which is structured like so:

I would like to sum the values relative to their index ie. (-10000 + 100 + 100 + -100). Then store these in a separate array which would look something like:

[[500], [-10100], [9000], [18000], [18000]]

I would imagine I'd have to use map and reduce to achieve this, but I'm not having much luck.

Here's what I currently have:

for (var i=1; i<totals.length; i++) {
    for (var z=0; z<totals[i].length; z++) {
        console.log(totals[i][z] = totals[i-1][z] + totals[i][z]);
    }
}

However, that seems to output the following:

If someone could push me in the right direction, that would be awesome.

Answer 1

Use Array.prototype.map() and Array.prototype.reduce():

const arrays = [
  [500],
  [-10000, 100, 100, -100],
  [9000],
  [9000, 9000],
  [9000, 9000]
];

const result = arrays.map(xs => xs.reduce((sum, x) => sum + x, 0));

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

If you want each sum in an array, just enclose it in brackets:

const arrays = [
  [500],
  [-10000, 100, 100, -100],
  [9000],
  [9000, 9000],
  [9000, 9000]
];

const result = arrays.map(xs => [xs.reduce((sum, x) => sum + x, 0)]);

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

Answer 2

My recommendation is to follow jo_va approach: it is far more declarative and simpler.

Having said that, if you want to use for loop, here is your corrected version

const arrays = [
  [500],
  [-10000, 100, 100, -100],
  [9000],
  [9000, 9000],
  [9000, 9000]
];

let results = [];
for (var i = 0 ; i < arrays.length; i++) {
    let sum = 0;
    for (var z = 0; z < arrays[i].length; z++) {
        sum += arrays[i][z]
    }
    results.push(sum);
}
console.log(results);

As you can see the result is the same, but in comparison

• it is more imperative, rather than declarative
• it is more verbose, and larger, therefore, more error-prone

Your problem was that you were updating the array as well iterating it, causing the iteration to broke. You were not accesing the array properly either.

If you neeeded to return an array of results, it is better to create a new and push results there.

Again, consider using map + reduce; it is the best approach. I just wanted to show you the option you wanted to use