List of arrays: find item by array key, create or update as appropriate

Question

I have lists of arrays like this:

var a = [
   {  month: 'January', count: 3 },
   {  month: 'February', count: 5 }, 
   {  month: 'March', count: 4 }
];
var b = [
   {  month: 'January', count: 4 },
   {  month: 'February', count: 5 }, 
   {  month: 'March', count: 1 }
];

And I want to create a "total" list, using Underscore, that looks like this:

var totals = [
   {  month: 'January', count: 7 },
   {  month: 'February', count: 10 }, 
   {  month: 'March', count: 5 }
];

I don't know in advance what the values of "month" will be, so I need to allow for this.

This is as far as I've got, but it's not very elegant. Is there a nicer way?

var totals = [];
_.each([a, b], function(d) { 
   _.each(items, function(e) { 
    var matchingItem = _.filter(totals, function(item, i) {
      return (item.month === e.month);
    });
    if (matchingItem) {
       var i = totals.indexOf(matchingItem);
       totals[i].count += e.count;
    } else { 
       var newItem = e;
       totals.push(e);
    }

   });
};

glmxndr · Accepted Answer · 2014-01-16 16:46:44Z

2

Using Underscore functions, you may do it like this, using a temporary object:

var total = [];
var tmp = {};

_.each(_.flatten([a,b]), function (obj) {
    tmp[obj.month] = tmp[obj.month] ? tmp[obj.month] + obj.count : obj.count;
});
_.each(tmp, function (c,m) {
    total.push({month: m, count: c});
});

See code there : http://jsfiddle.net/8cxQB/

answered Jan 16, 2014 at 16:46

glmxndr

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Comments

Gruff Bunny · Accepted Answer · 2014-01-16 18:39:23Z

2

This solution concatenates all the arrays, groups them by month and then spits out the totals:

var months = a.concat(b);

var totals = _.map( _.groupBy(months, 'month'), function(values, month){
    return {
        month: month,
        count: _.reduce(values, function(memo,value){
            return memo + value.count
        }, 0)
    }
});

If there are more data arrays then the months array could be built like so:

var months = a.concat(b, c, d); // etc

answered Jan 16, 2014 at 18:39

Gruff Bunny

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wachme · Accepted Answer · 2014-01-16 16:42:28Z

1

var totals = {};

a.concat(b).forEach(function(el) {
  if(totals[el.month])
    totals[el.month].count = totals[el.month].count + el.count;
  else
    totals[el.month] = el;
})

console.log(totals);

answered Jan 16, 2014 at 16:42

wachme

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1 Comment

thefourtheye Over a year ago

I get { undefined: NaN }

Alexandru Puiu · Accepted Answer · 2014-01-16 19:33:50Z

1

If you're willing to add another plugin, linq.js would be your best bet http://linqjs.codeplex.com/

EDIT:

var a = [
   {  month: 'January', count: 3 },
   {  month: 'February', count: 5 }, 
   {  month: 'March', count: 4 }
];
var b = [
   {  month: 'January', count: 4 },
   {  month: 'February', count: 5 }, 
   {  month: 'March', count: 1 }
];

var queryResult = Enumerable.From(a).Union(b)
.GroupBy("$.month", "", function (key, g) { var result = { month: key, count: g.Sum("$.count") }; return result;})
.ToArray();

console.log(queryResult);

edited Jan 16, 2014 at 19:33

answered Jan 16, 2014 at 16:47

Alexandru Puiu

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2 Comments

Bergi Over a year ago

Linq instead of Underscore might be a solution, but then at least provide the code to solve the OP's problem instead of a random sample.

Alexandru Puiu Over a year ago

Sorry about that, thought the first example was straightforward enough. I posted a real answer using linq.js now. Hope this helps

parchambault · Accepted Answer · 2014-01-16 16:37:59Z

0

Why not have done an object like this

var counts =  {
   'January': 3,
   'February': 5 , 
   'March': 4
}

and then you can acces it with counts.January directly and modify it or within a loop with counts[month_name] = ...

hope that help

answered Jan 16, 2014 at 16:37

parchambault

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1 Comment

flossfan Over a year ago

That would undoubtedly be better, but unfortunately that's not the nature of the data I'm dealing with - it's returned from an API and I don't control the structure...

Victor Ivens · Accepted Answer · 2021-07-06 19:41:41Z

0

Although not very elegant, this solution is one-lineish and doesn't use third party libs.

Array.from([...a, ...b].reduce((p, c) => p.set(c.month, {month: c.month, count: (p.get(c.month)?.count ?? 0) + c.count}), new Map())).map(x => x[1])

// readable version
Array.from([...a, ...b]
  .reduce((previous, current) => {
    const count = (previous.get(current.month) ?? 0) + current.count
    return previous.set(current.month, { month: current.month, count })
  }, new Map()))
  .map(x => x[1])

answered Jul 6, 2021 at 19:41

Victor Ivens

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