Sum of Distincts in Javascript Object using Underscore

Question

Only just introduced to _Underscore so this may be totally nonsense but I'm wondering if the library can indeed do what I want.

I have an object containing 5 items:

Name, v2007,v2008,v2009,v2010

I simply want to go through the entire object and get the SUM of each distinct value in Name (much similar to SQL's GroupBy and SUM).

I have the following:

var distincts = [];
$.each(tmp, function (key, value) {
    $.each(value, function (fieldName, fieldValue) {
        if (fieldName == 'Name') {
            if ($.inArray(fieldValue, distincts) == -1) {
                distincts.push(fieldValue);
            }
        }
    });
});

$.each(distincts, function (a, b) {
    var t = _.where(tmp, {Name: b});
    var sum = _.reduce(t.v2007, function (memo, num) {
        return memo + num;
    }, 0)
    console.log(sum);
});

Which is returning 0 all the time, in fact I'm not sure how to retrieve a SUM of fields v2011, v2013 other than hard-coding them in which is simply not practical!

Can someone help me a little here, is there a simple way of getting the _Underscore library to SUM all fields (that, say, start with 'v') based on a Group of the Name?

Could you show us an example tmp object (in JSON format)? Btw, you shouldn't mix Underscore's and jQuery's each functions — Bergi
– Bergi, Commented Oct 26, 2012 at 13:12

Bergi · Accepted Answer · 2012-10-26 13:20:39Z

3

I think this does what you want:

var groups = _.groupBy(function(name) { return name.charAt(0) });
var sums = _.map(groups, function(group) {
    return _.reduce(group, function (a, b) { return a+b; }, 0);
});

answered Oct 26, 2012 at 13:20

Bergi

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2 Comments

Jess Over a year ago

No offense, but shouldn't there be a simpler way to do this with underscore?

Bergi Over a year ago

@Jess: I don't think so, Underscore is not a query language but a toolset of some functional helpers. I you find a simpler way, please provide another answer :-)

Jess · Accepted Answer · 2014-01-06 19:09:58Z

1

Here is a way I came up with that will take an array of objects and return a single object where the "keys" of the object are the grouped value and the "values" of the object are the aggregated value:

function groupBy(input, xCol, yCol) {

    var output = {};

    // This function can be easily changed to do max, min, avg, sum.
    var sum = function(row, idx) {
        if (!(row[xCol] in output)) {
            output[row[xCol]] = row[yCol];
        }
        else {
            output[row[xCol]] += row[yCol];         
        }
    };
    _.each(input, sum);

    return output;
}

And a test:

var input = [ {
    pn : "M1",
    date : "2014-01-01T00:00:00Z",
    qty : 1
}, {
    pn : "M1",
    date : "2014-01-01T00:00:00Z",
    qty : .5
}, {
    pn : "M2",
    date : "2014-01-04T00:00:00Z",
    qty : 2
} ];

it('should group by abc value, showing max.', function() {
    expect(groupBy(input, "pn", "qty")).toEqual({
        "M1" : 1.5,
        "M2" : 2
    });
});

This answer only nominally uses underscore for the foreach loop.

edited Jan 6, 2014 at 19:09

answered Jan 6, 2014 at 18:46

Jess

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1 Comment

Jess Over a year ago

What do you think @Bergi?

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