create dynamic multidimmensional associative array in javascript from json

Question

I get a JSON object from PHP that holds multiple rows like this:

id | type   | amount| age
1  |  abc   |   1    |  1
2  |  def   |   2    |  1
3  |  def   |   4    |  2
4  |  def   |   13   |  3

I run it through JavaScript:

$.ajax({
    type: 'POST',
    url: "test.php",
    dataType: 'json',
    success: function (data) {
        var arr = [];
        for (var i = 0; i < data.length; i++) {
            arr[data[i].type][data[i].age] = data[i].ammount;
        }
    }
});

The idea is to get array that looks like this:

arr[def][1] = 2
arr[def][2] = 4
arr[def][3] = 13
arr[abc][1] = 1
arr[abc][2] = 0 // note that this one is just empty because it was not declared from json data.

Unfortunately I get an error:

Uncaught TypeError: Cannot set property '1' of undefined

1 is [data[i].age] and if I change its value error appears with other number.

what do you get and what do you end up with ? and why do you not use .push ? — Neta Meta
– Neta Meta, Commented Jan 12, 2014 at 0:18
i get json object in which i can access all the properties ie data[i].type so i can get from data[3].type = def. i end up with error as described. i cant use push because all data i get is in random order - using proper keys i can easily sort it — Kalreg
– Kalreg, Commented Jan 12, 2014 at 0:21
I think you should initialize the inner array first. I suggest you to examine this question: stackoverflow.com/questions/13725138/…. — kgd
– kgd, Commented Jan 12, 2014 at 0:22
well you cant create multiple D array without having the first array defined — Neta Meta
– Neta Meta, Commented Jan 12, 2014 at 0:22

Edgar Villegas Alvarado · Accepted Answer · 2014-01-12 00:26:09Z

5

You should do:

var arr = {};    
for (var i = 0; i < data.length; i++) {
   var type = data[i].type,
       age = data[i].age,
       amount = data[i].ammount;
   if(!arr[type]) arr[type] = {};  //Create arr[type] if it doesn't exist
   arr[type][age] = amount;
}

You could fill the empty cells with 0, but I'd better advise to do this for reading:

//returns the value or 0
function readArr(arr, type, age){
   if(!arr[type]) return 0;
   return arr[type][age] || 0;  
}

Then, instead of reading with arr["abc"][2], you'd read with readArr(arr, "abc", 2).

(If you prefer, of course you can avoid declaring a function but use its logic inline, you'd do something like (arr["abc"] ? arr["abc"][2] || 0 : 0) )

Cheers, from La Paz, Bolivia

edited Jan 12, 2014 at 0:26

answered Jan 12, 2014 at 0:21

Edgar Villegas Alvarado

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3 Comments

Kalreg Over a year ago

this one works perfectly fine. thank you so much, also for some extra info. greetings from Olsztyn, Poland, Central Europe :)

Edgar Villegas Alvarado Over a year ago

Welcome, Kalreg, I work with a couple friends from Poland :), nice to have one more friend from there!.

Beetroot-Beetroot Over a year ago

"You could fill the empty cells with 0" - what empty cells, there are none? The thing you call arr is a javascript plain object, which is absolutely NOT an associative array. Javascript does not have associative arrays.

Arnelle Balane · Accepted Answer · 2014-01-12 00:24:03Z

1

I think that's because arr['abc'] (or whatever the key is) is still undefined to be an array (or JSON object) when you set its abc property or key.

var arr = [];
for (var i = 0; i < data.length; i++) {
  if (!arr[data[i].type]) {
    arr[data[i].type] = [];
  }
  arr[data[i].type][data[i].age] = data[i].ammount;
}

Or if you want to use JSON objects

var arr = {};
for (var i = 0; i < data.length; i++) {
  if (!arr[data[i].type]) {
    arr[data[i].type] = {};
  }
  arr[data[i].type][data[i].age] = data[i].ammount;
}

answered Jan 12, 2014 at 0:24

Arnelle Balane

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1 Comment

Kalreg Over a year ago

your answer is also very good, unfortunatelly i can accept only one answer :(

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