object array Group by an element?

Question

Please see this example: JsFiddle

Question: I have the following JSON Array

y= [ {"LngTrend":15,"DblValue":10,"DtmStamp":1358226000000},     
{"LngTrend":16,"DblValue":92,"DtmStamp":1358226000000},    
{"LngTrend":17,"DblValue":45,"DtmStamp":1358226000000},
{"LngTrend":18,"DblValue":87,"DtmStamp":1358226000000},


{"LngTrend":15,"DblValue":10,"DtmStamp":1358226060000},
{"LngTrend":16,"DblValue":87,"DtmStamp":1358226060000},
{"LngTrend":17,"DblValue":45,"DtmStamp":1358226060000},
{"LngTrend":18,"DblValue":92,"DtmStamp":1358226060000} ]

I was trying to group these object by DtmStamp end up having something like this :

 x =  [[1358226000000,10,92,45,87],[1358226060000,10,87,45,92], .......]

In other words:

x[0][0] = y[0].DtmStamp ;
x[0][1] = y[0].LngTrend ;
x[0][2] = y[1].LngTrend ;
x[0][3] = y[2].LngTrend ; 
x[0][4] = y[3].LngTrend ;

Unfortunately, it ends with something I don't want.

Here is what I have tried so far:

   var dataTrendArray = [];
           $.each(x, function (index, value) {
                var trendArray = [];
                if (index % 4 == 0) {
                    trendArray.push(x[index].DtmStamp);
                    for (var i = 0; i < 4; i++) {
                        index = eval(index + i);
                        trendArray.push(x[index].DblValue);
                    }
                }

               console.log(trendArray) ;
                dataTrendArray.push(trendArray);
            });

Can someone help me get on the right path?

Answer 1

You can leverage JavaScript objects as a key/value data structure similar to a map. The property name will serve as the key, while the property value will serve as the value. This will allow you to group.

var y = [
     {"LngTrend":15,"DblValue":10,"DtmStamp":1358226000000},     
     {"LngTrend":16,"DblValue":92,"DtmStamp":1358226000000},    
     {"LngTrend":17,"DblValue":45,"DtmStamp":1358226000000},
     {"LngTrend":18,"DblValue":87,"DtmStamp":1358226000000},
     {"LngTrend":15,"DblValue":10,"DtmStamp":1358226060000},
     {"LngTrend":16,"DblValue":87,"DtmStamp":1358226060000},
     {"LngTrend":17,"DblValue":45,"DtmStamp":1358226060000},
     {"LngTrend":18,"DblValue":92,"DtmStamp":1358226060000},
];

var x = {};

for (var i = 0; i < y.length; ++i) {
    var obj = y[i];

    //If a property for this DtmStamp does not exist yet, create
    if (x[obj.DtmStamp] === undefined)
        x[obj.DtmStamp] = [obj.DtmStamp]; //Assign a new array with the first element of DtmStamp.

    //x will always be the array corresponding to the current DtmStamp. Push a value the current value to it.
    x[obj.DtmStamp].push(obj.DblValue);
}

console.log(x); //x is now an object grouped by DtmStamp. You can easily turn it back into an array here.