Summarize array of objects and calculate average value for each unique object name

Question

I have an array like so:

var array = [
     {
       name: "a",
       value: 1 
     },
     {
       name: "a",
       value: 2 
     },
     {
       name: "a",
       value: 3 
     },
     {
       name: "b",
       value: 0 
     },
     {
       name: "b",
       value: 1 
     }
 ];

And I need an array like this:

var newarray = [
     {
       name: "a",
       value: 2
     },
     {
       name: "b",
       value: 0.5
     }
 ]

Where the new array has each unique name as an object with the average value.

Is there an easy way to accomplish this?

Answer 1

You'll have to loop through the array, computing the sum and counts for each object. Here's a quick implementation:

function average(arr) {
    var sums = {}, counts = {}, results = [], name;
    for (var i = 0; i < arr.length; i++) {
        name = arr[i].name;
        if (!(name in sums)) {
            sums[name] = 0;
            counts[name] = 0;
        }
        sums[name] += arr[i].value;
        counts[name]++;
    }

    for(name in sums) {
        results.push({ name: name, value: sums[name] / counts[name] });
    }
    return results;
}

Demonstration

Note, this kind of thing can be made much easier if you use a library like Underscore.js:

var averages = _.chain(array)
                .groupBy('name')
                .map(function(g, k) {
                    return { 
                        name: k, 
                        value: _.chain(g)
                                .pluck('value')
                                .reduce(function(x, y) { return x + y })
                                .value() / g.length
                    };
                })
                .value();

Demonstration