Sum values of attribute when the value of an other attribute is the same

[Burton449]

I have an array of objects like this one:

{'shapeId': 'shapeid1', 'latlong': '45.42342,-65.23424', 'number': 5}

I want the sum of the values of the attribute "number" when the value of the attribute "shapeId" is the same.

Is there a built in method for that? Thank you!

What I tried so far is this, using reduce() method, but the results are odd when the function is called many times.

    function selectMarkersInPoly() {
        allTotal = new Array();
        alert(allMarkers.length)
        for (var i=0; i < createdShapes.length; i++) {
        for (var j=0; j < allMarkers.length; j++){
                var latlong = allMarkers[j].getPosition();
                if(google.maps.geometry.poly.containsLocation(latlong, createdShapes[i]) == true) {
                    allMarkers[j].setOptions({
                        icon : "http://labs.google.com/ridefinder/images/mm_20_white.png",
                        shapeId:createdShapes[i].id
                    });
                }else{
                    allMarkers[j].setOptions({
                        icon : "http://labs.google.com/ridefinder/images/mm_20_red.png",
                        shapeId: 'shapeid0'
                    });
                }    
            }
        }            
    allTotal = allMarkers.reduce(function(res, obj) {
        if (!(obj.shapeId in res)){
        res.__array.push(res[obj.shapeId] = obj);
        }else {
        res[obj.shapeId].number += obj.number;
        }
        return res;
            }, {__array:[]}).__array
                .sort(function(a,b) { return b.shapeId - a.shapeId;
     });

        for (var j=0; j < allTotal.length; j++) {
        alert(allTotal[j].shapeId + ': ' + allTotal[j].nbr_electeur)
        }
    }
            
            
//There is 32600 number in total in allMarkers.number in 3505 objects
//first time i call the function, allTotal= [Object { number=32598,  shapeId='shapeid0'}, Object { number=2,  shapeId='shapeid1'}] where shapeid0 are not selected
//second time i call that function without changing anything, allTotal= [Object { number=65193,  shapeId='shapeid0'}, Object { number=2,  shapeId='shapeid1'}]
//third time: allTotal= [Object { number=97788,  shapeId='shapeid0'}, Object { number=2,  shapeId='shapeid1'}]

[Lesshardtoofind]

It looks like you are always appending to the variable aka +=. If you don't reset it back to zero when you call the function then you will just get an incrementally large result. Like if I did

<script>
  var x = 0;
  function AddTen()
  {
     x += 10;
  }
</script>

X would get ten larger every time I called the function. If I wanted X to only be 10 then I would have to put the x = 0 inside the function. (or just make the function x = 10, but I'm sure you see my point)

[Burton449]

Yes you are rigth. The problem is that inside the expression res.__array.push(res[obj.shapeId] = obj); I assign the object itself to the res, not a clone. With res[obj.shapeId].number += obj.number; then it manipulates the original object, which is why the results grow in subsequent calls.

Better:

var numberSums = allMarkers.reduce(function(res, obj) {
    var id = obj.shapeId;
    if (id in res)
        res[id] += obj.number;
    else
        res[id] = obj.number;
    return res;
}, {});
Object.keys(numberSums).sort(function(a,b){return a-b;}).forEach(function(id) {
    alert(id+": "+numberSums[id]);
});