CODEWITHSUNDEEP
javascriptunderscore.js
David Pardo
David Pardo

Reputation: 103

Mapping two (or more) arrays into one with underscore.js

I'd need to add element-wise several arrays. That is, I have several arrays of equal lenght, and I'd need just one with the same number of elements that are the sum of the inputs. Underscore has methods to fold all elements into one and to map every element using a function, but I can't find any way to combine two arrays piece wise.

If my original arrays were [1,2,3,4,5,6], [1,1,1,1,1,1] and [2,2,2,2,2,2] the result should be [4,5,6,7,8,9].

I know I can do it by iterating over the arrays, but wonder if it would be easier/faster using underscore.js functions. Can I do it? How?

Upvotes: 10

Views: 17274

Answers (5)

Penny Liu
Penny Liu

Reputation: 17438

Using Lodash 4:

var arrays = [
  [1, 2, 3, 4, 5, 6],
  [1, 1, 1, 1, 1, 1],
  [2, 2, 2, 2, 2, 2]
];

var result = _.map(_.unzip(arrays), _.sum);
console.log(result);
<script src="https://cdnjs.cloudflare.com/ajax/libs/lodash.js/4.17.11/lodash.js"></script>

Upvotes: 0

Daniel Margol
Daniel Margol

Reputation: 701

You could use lodash (https://lodash.com/) instead of underscore which has a pretty cool zipWith (https://lodash.com/docs#zipWith) operator that would work like the example below. (note _.add is also a lodash math function)

var a = [1,2,3,4,5,6]
  , b = [1,1,1,1,1,1]
  , c = [2,2,2,2,2,2];

var result = _.zipWith(a, b, c, _.add);

// result = [4, 5, 6, 7, 8, 9]

Upvotes: 4

benzonico
benzonico

Reputation: 10833

You can use zip in conjunction with map and reduce: Not tested but something like this might work

var result = _.map(_.zip(array1, array2, array3),function(zipped){return _.reduce(zipped,  function(memo, num){ return memo + num; }, 0)});

Upvotes: 0

Bergi
Bergi

Reputation: 664764

Easier yes, faster no. To emulate a zipWith, you can combine a zip with a sum-reduce:

var arrays = [[1,2,3,4,5,6], [1,1,1,1,1,1], [2,2,2,2,2,2]];

_.map(_.zip.apply(_, arrays), function(pieces) {
     return _.reduce(pieces, function(m, p) {return m+p;}, 0);
});

Upvotes: 11

elclanrs
elclanrs

Reputation: 94121

I don't think it would be easier with underscore. Here's two options:

var a = [1,2,3,4,5,6]
  , b = [1,1,1,1,1,1]
  , c = [2,2,2,2,2,2];

var result = a.map(function(_,i) {
  return a[i] + b[i] + c[i];
});

// OR

var result = [];
for (var i = 0; i < a.length; i++) {
  result.push(a[i] + b[i] + c[i]);
}

console.log(result); //=> [4,5,6,7,8,9]

Upvotes: 4

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