How to pivot a javascript array

Question

I have this javascript array:

[['a', 'x', 1],
 ['a', 'y', 2],
 ['b', 'x', 3],
 ['b', 'z', 4],
 ['c', 'y', 5],
 ['c', 'z', 6]]

How do I pivot it to something like below with the 2nd column ('x', 'y', 'z') from above going across.

[['a', 1, 2, null],
 ['b', 3, null, 4],
 ['c', null, 5, 6]]

EDIT: Sorry I was unclear. The answers so far seem to be referencing a static length/value for x, y, z. The array will be dynamic and can have anything in the 2nd column (ex. 't','u','v','w' instead of 'x','y','z'). I think I need to fill the array up first with nulls for all the possible combinations and then push in the values.

Thanks..

I apologise for the lack of mathematical acumen, but could you explain how "pivoting" works? I can't spot the pattern — user1726343
– user1726343, Commented Oct 24, 2012 at 22:16
@Asad: pivot is like a cross tab where you transpose a column into a row. en.wikipedia.org/wiki/Pivot_table — Txoov
– Txoov, Commented Oct 24, 2012 at 23:42

score 2 · Accepted Answer · 2012-10-24 22:50:26Z

Going by Fabricio's comment, here is how you can accomplish something similar:

var result = {};
for(var i=0;i< basearray.length;i++){
    if(!result[basearray[i][0]]){
        result[basearray[i][0]]={};
    }
    result[basearray[i][0]][basearray[i][1]]=basearray[i][2];
}

Note that this returns an object or hashmap, not strictly an array, but the data is more organised and it can easily be turned into an array if you so wish. Here is a demonstration (check your console).

By adding this code:

var count=0;
for(var key in result){
    result[count]=[];
    result[count][0]=key;
    result[count][1]=result[key].x||null;
    result[count][2]=result[key].y||null;
    result[count][3]=result[key].z||null;
    count++;
}

your result object now simulates both structures, your original array of arrays, and the suggested key value pairs. You can see the results here: http://jsfiddle.net/9Lakw/3/

Here is what result looks like:

{
   "a":{
      "x":1,
      "y":2
   },
   "b":{
      "x":3,
      "z":4
   },
   "c":{
      "y":5,
      "z":6
   },
   "0":[
      "a",
      1,
      2,
      null
   ],
   "1":[
      "b",
      3,
      null,
      4
   ],
   "2":[
      "c",
      null,
      5,
      6
   ]
}

Fabrício Matté · Accepted Answer · 2012-10-24 22:31:21Z

1

Here's how I'd do it, with arrays and null fillers as in the question. This assumes that coords for given points always come in succession.

var arr = [['a', 'x', 1],
 ['a', 'y', 2],
 ['b', 'x', 3],
 ['b', 'z', 4],
 ['c', 'y', 5],
 ['c', 'z', 6]];

var aux = {
        x: 1,
        y: 2,
        z: 3
    },
    output = [],
    lastItem,
    currItem;

for (var i = 0; i < arr.length; i++) {
    currItem = arr[i];
    if (currItem[0] !== lastItem) {
        lastItem = currItem[0];
        output.push([lastItem, null, null, null]);
    }
    output[output.length-1][aux[currItem[1]]] = currItem[2];
}
console.log(output); //[["a", 1, 2, null], ["b", 3, null, 4], ["c", null, 5, 6]]

Fiddle

answered Oct 24, 2012 at 22:31

Fabrício Matté

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1 Comment

Fabrício Matté Over a year ago

The test if (currItem[0] !== lastItem) would probably break, I could make it work with an Array.indexOf/filter search but if I remember correctly from my father's pivoting, those should come in order.

Eric · Accepted Answer · 2012-10-24 22:37:03Z

1

Are you sure that's the format you want? It seems more sensible to get:

{
    a: {x: 1, y: 2},
    b: {x: 3, z: 4},
    c: {y: 5, z: 6}
}

Which you can get from:

var results = {};
data.forEach(function(el) {
    var name = el[0];
    var prop = el[1];
    var value = el[2];
    results[name] = results[name] || {};
    results[name][prop] = value;
})

answered Oct 24, 2012 at 22:37

Eric

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