JavaScript: Get unique values and their counts from an array of objects?

Question

Using jQuery, how can I iterate over an object, and get the unique values of a key with a count of each value?

For example, for this array:

var electrons = [
    { name: 'Electron1', distance: 1 }, 
    { name: 'Electron2', distance: 1 }, 
    { name: 'Electron3', distance: 2 }, 
    { name: 'Electron4', distance: 2 }, 
    { name: 'Electron5', distance: 2 }, 
    { name: 'Electron6', distance: 2 }, 
    { name: 'Electron7', distance: 2 }, 
    { name: 'Electron8', distance: 2 }, 
    { name: 'Electron9', distance: 2 }, 
    { name: 'Electron10', distance: 2 }, 
    { name: 'Electron11', distance: 3 }, 
];

I'd like to get back the following:

var distance_counts = {1: 2, 2: 8, 3: 1};

I've got this, which works but is a bit clumsy:

var radius_counts = {};
for (var i = 0; i < electrons.length; i++) { 
    if (electrons[i].distance in radius_counts) { 
         radius_counts[electrons[i].distance] += 1;
    } else { 
         radius_counts[electrons[i].distance] = 1;
    } 
}

Try this stackoverflow.com/questions/1960473/unique-values-in-an-array — Artem Vyshniakov
– Artem Vyshniakov, Commented Sep 7, 2012 at 13:10
@Artem - That's a completely different question about getting an array of unique values. — Richard
– Richard, Commented Sep 7, 2012 at 13:25
Would it be ok to use underscore.js? You could use _.pluck - although this will probably save you only one or two lines of code (and add the library....), a little like: jsfiddle.net/mYxVs — m90
– m90, Commented Sep 7, 2012 at 13:52

Avinash R · Accepted Answer · 2012-09-07 15:19:58Z

8

you could use map for this purpose as:

var distances = {};
$.map(electrons,function(e,i) {
   distances[e.distance] = (distances[e.distance] || 0) + 1;
});

or

var distances = {};
$.each(electrons,function(i,e) {
   distances[this.distance] = (distances[this.distance] || 0) + 1;
});

Also may I point out to you that although this code good to look and compact, this is not generally faster. Better make your code more faster and more easy to look at as:

var distances = {},e;
for (var i = 0,l=electrons.length; i < l; i++) { 
    e = electrons[i];
    distances[e.distance] = (distances[e.distance] || 0) + 1;
}

edited Sep 7, 2012 at 15:19

answered Sep 7, 2012 at 14:12

Avinash R

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3 Comments

gen_Eric Over a year ago

You don't need $.map here, I'd use $.each instead.

Avinash R Over a year ago

@Rocket, i used $.map coz, that was the first thing that came to my mind at that time, although in this particular context both would serve the purpose of removing the clumsiness. Also i've put the code for the same.

Richard Over a year ago

Thanks! I hadn't seen that use of || before - very useful.

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