Pattern for "column total" of an array of arrays

Question

I am looking for a pattern to get the totals for each n-th item of arrays. Conceptually this would be like getting column totals of a data table.

var dataRows = [
  [10, 11, 12, 13],
  [20, 21, 22, 23],
  [30, 31, 32, 33]
]
// The outcome should be:
// [ 60, 63, 66, 69]

I had some equivalent Python code on the server side, but decided it should be handled client side, thus in javascript. Here is the equivalent Python code:

[sum(i) for i in zip(*data_rows)]

How would you go about writing this? I am sure the code will be longer without zip, unpack and list comprehension, but can it be done elegantly? Also are there libraries that extend the core library of JS?

Answer 1

Just iterate over your arrays:

var dataRows = [
  [10, 11, 12, 13],
  [20, 21, 22, 23],
  [30, 31, 32, 33]
]

var result = [];

for(var i = 0; i < dataRows.length; i++){
    for(var j = 0; j < dataRows[i].length; j++){
        result[j] = (result[j] || 0) + dataRows[i][j];
    } // ^ Add the current value to the proper position of `result`.
}

alert(JSON.stringify(result));

As for the second part of your question:
There are libraries that add a lot of array manipulation functions to JS. Take a look at Underscore, for example.

Answer 2

Play with ES5 array methods:

var dataRows = [
  [10, 11, 12, 13],
  [20, 21, 22, 23],
  [30, 31, 32, 33]
];

dataRows.reduce(function(a, b) {
  return a.map(function(el, i) {
    return el + b[i];
  });
});
// [60, 63, 66, 69]

This is attempt to give a clue, so I didn't deal with error handling thing.