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\":13582260         


        

Answer 1

Your code pushes empty trendArrays even if they are not filled (index % 4 != 0). Instead, use this:

var dataTrendArray = [];
for (var i = 0; i < x.length; i += 4) {
    var trendArray = [ x[i].DtmStamp ];
    for (var j = i, l = Math.max(i + 4, x.length); j < l; j++) {
        trendArray.push(x[j].DblValue);
    }
    // console.log(trendArray) ;
     dataTrendArray.push(trendArray);
}

However, it might fit better if you just grouped the trendArrays into an object, using the DtmStamp as keys:

var dataTrend = {};
for (var i=0; i<x.length; i++) {
    var key = x[i].DtmStamp;
    if (key in dataTrend)
        dataTrend[key].push(x[i].DblValue);
    else
        dataTrend[key] = [ x[i].DblValue ];
}

Answer 2

 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 k in y){
  if(x[y[k]["DtmStamp"]] == undefined)
     x[y[k]["DtmStamp"]] = [];
     x[y[k]["DtmStamp"]].push(y[k]["DblValue"])
}
alert(JSON.stringify(x))
console.log(x);

See http://plnkr.co/edit/511sKSdzHGYuvpYqKCPD?p=preview

Answer 3

You can build a sparse array, indexed by DtmStamp.

var x = [];
$.each(y, function(i, obj) {
    var s = obj.DtmStamp;
    if(!x[s]) x[s] = [];
    x[s].push(obj.DblValue);
});

//x is now a sparse array, indexed by DtmStamp

This has the advantage over an object, that the array elements are in DtmStamp order.

//To loop through x
for(i in x) {
    ...
}

Answer 4

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.

Answer 5

You should use a hash. A hash will allow you to easily index all of the DblValue values by DtmStamp. Here is a full working example:

jsFiddle

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 = {};

var i = 0;
while(i++ < y.length) {
    var key = y[i].DtmStamp.toString();
    if (typeof(x[key]) == "undefined") x[key] = [];
    x[key].push(y[i].DblValue);
}

alert(JSON.stringify(x));

The key is to use a hash on the values you want to group by.

Result:

{
    "1358226060000": [
        92,
        45,
        87,
        10
    ],
    "1358226000000": [
        87,
        45,
        92,
        10
    ]
}

If you want to prevent duplicates, you can do so by adding if/then logic in conjunction with indexOf().