Javascript - zip and sum variable number of arrays in single expression

Question

The answer to Javascript equivalent of Python's zip function shows a neat way of taking any number of arrays, and "zipping" them into a merged array in a single statement.

zip= rows=>rows[0].map((_,c)=>rows.map(row=>row[c]))

This question Javascript - Sum two arrays in single iteration shows a similar idea, but summing the entries as they're zipped up.

I'm wondering how to combine the two techniques - so given an unknown number of arrays:

[
  [1,2,3],
  [1,2,3],
  [1,2,3]
]

... the result is [3, 6, 9], ie the sum of the first elements, the second and so forth. For the sake of simplicity, assume each array contains the same number of items.

Richard Hamilton · Answer 1 · 2016-06-19T22:44:51.557

First transpose the arrays, so the columns become rows and viceaversa. After that simply call the reduce function.

The first version of the code is mostly typescript for brevity. The second version is JavaScript.

Note that ES5 map and reduce functions are only available on IE9 and higher.

var numbers = [[1,2,3],[1,2,3],[1,2,3]]

// TypeScript

var colSums = numbers[0].map(function(col, i) { 
  return numbers.map((row) => row[i]).reduce((a,b) => a + b );
});

// JavaScript

/*var colSums = numbers[0].map(function(col, i) {
  return numbers.map(function(row) {
    return row[i];
  }).reduce(function(a,b) {
    return a + b;
  });
});*/
document.getElementById("col-sums").innerHTML = colSums;

<p id="col-sums"></p>

Looks like we had the same idea - yours is ES5 compatible (map and reduce available since IE9) — Party Ark, Jun 19 '16 at 22:43

score 0 · Answer 2 · edited May 23 '17 at 12:32

0

I think I have it - this answer was helpful - we can extend the original zipping method thus:

zip = rows => rows[0].map((_,c) => rows.map(row=>row[c]).reduce((a,b) => a+b,0))

edited May 23 '17 at 12:32

Community

1
1

answered Jun 19 '16 at 22:40

Party Ark

1,061
9
20

omarjmh · Answer 3 · 2016-06-19T22:53:05.273

0

No need for map, map, reduce, two maps will get you there:

Quick Demo

var m = [
  [1,2,3],
  [1,2,3],
  [1,2,3]
];

var total = m.map(function(arr, i) {
  var count = 0;
  return arr.map(function(el, idx) {
    return count += m[idx][i];
  })
}).pop()

edited Jun 19 '16 at 22:53

answered Jun 19 '16 at 22:43

omarjmh

13,632
6
34
42

sorry, that's just going to add the first two, results in [2,4,6]! – Party Ark Jun 19 '16 at 22:45

score 0 · Accepted Answer · answered Jun 19 '16 at 22:53

Similar to other answers, it just doesn't have to be so complicated:

numbers = [[1,2,3],[1,2,3],[1,2,3]];

var result = numbers[0].map(function(_, i) { 
  return numbers.reduce(function(prev, row) {
    return row[i] + prev;
  }, 0);
});

console.log(result);

Javascript - zip and sum variable number of arrays in single expression

4 Answers4