使用d3或jQuery的json对象中有多少个不同的密钥对值

Question

I have an array of objects which looks somewhat like this

 [ {
      "created_date" : "2013-08-12T06:25:00",
      "descriptor" : "Noise, Barking Dog (NR5)"
    },
    {
      "created_date" : "2013-08-17T06:25:00",
      "descriptor" : "Noise, Barking Dog (NR5)"
    },
    {
      "created_date" : "2013-08-17T02:25:00",
      "descriptor" : "Noise, Barking Dog (NR5)"
    },
    {
      "created_date" : "2013-08-12T06:25:00",
      "descriptor" : "Loud Music"
    },
    {
      "created_date" : "2013-08-17T06:25:00",
      "descriptor" : "Loud Music"
    },
    {
      "created_date" : "2013-08-17T02:25:00",
      "descriptor" : "Construction Noise"
    },
    {
      "created_date" : "2013-08-17T02:25:00",
      "descriptor" : "Construction Noise"
    }]

The question is how do I count how many "descriptor" categories are there and their name How should I approach this using D3 or jquery. Basically the idea is to form different groups

Answer 1

You can use d3's d3.map() to map each descriptor to the number of its occurrences.

var map = d3.map();
data.forEach(function(d) {
    var descriptorCount = map.get(d.descriptor);
    map.set(d.descriptor, descriptorCount === undefined ? 1 : ++descriptorCount);
});
console.log(map); // {Noise, Barking Dog (NR5): 3, Loud Music: 2, Construction Noise: 2}

Fiddle

Answer 2

var UniqueDescriptors = "";
for (var i =0; i<array.length;i++) {
    var obj = array[i];
    if (obj.descriptor && UniqueDescriptors.indexOf(obj.descriptor) == -1) {
        UniqueDescriptors += obj.descriptor + '|';
    }
}
// this returns just the number
//return (UniqueDescriptors.split('|').length - 1);
// this returns an array of the different descriptors if you need the names. you also can count the number by accessing the length property
return UniqueDescriptors.split('|').pop();

of course the obvious flaw of this script is that it assumes there are no pipes in the descriptor values.

Answer 3

Just iterate over the array and collect each descriptor:

var descriptors = (function(data) {
    var seen = {};
    data.forEach(function(obj) {
        seen[obj.descriptor] = true;
    });
    return Object.keys(seen);
}(data));

To get the number of unique descriptors, just take the length of the array:

descriptors.length

---

If you need to support older browser that don't support Object.keys or Array#forEach :

var descriptors = (function(data) {
    var seen = {};
    var descriptors = [];
    for (var i = 0, l = data.length; i < l; i++) {
        var descriptor = data[i].descriptor;
        if (!seen[descriptor]) {
            descriptors.push(descriptor);
            seen[descriptor] = true;
        }
    }
    return descriptors;
}(data));