Merge array of objects to array of arrays each containing 3 objects

Question

I have the following JavaScript array of many many objects:

[
    {...},
    {...},
    {...},
    .
    .
    .
    .
    .
    {...},
    {...}
]

I would like to transform the array to rows of 3 objects, something like this:

[
    [ {...}, {...}, {...} ],
    [ {...}, {...}, {...} ],
    [ {...}, {...}, {...} ],
    .
    .
    .
    [ {...}, {...}, {...} ],
    [ {...}, {...}, {...} ]
]

What is the best way to do it?

Do you know the ways to do that and want to know which is the best or do you not know any way to do that? — Renato Lochetti
– Renato Lochetti, Commented Oct 23, 2012 at 15:16
What criteria will you use to decide which array each object should go into? — jalynn2
– jalynn2, Commented Oct 23, 2012 at 15:16
@Renato Lochetti: I always can do it with for statements and loops. I am looking for the efficiect and lesdd code as possible. Maybe there is any hidden function of an array that do such thing that I don't know. — Naor
– Naor, Commented Oct 23, 2012 at 15:17
@jalynn2: I don't have criteria, each 3 objects should be in one row. I need to transform array to grid. — Naor
– Naor, Commented Oct 23, 2012 at 15:18
In terms of asymptotic time you always need to iterate for the entire vector. Just a loop and get three at a time. — Renato Lochetti
– Renato Lochetti, Commented Oct 23, 2012 at 15:19

jAndy · Accepted Answer · 2012-10-23 15:29:48Z

3

Yet another snippet (adapted from @Florent)

var result = [];

do {
   result.push( data.splice( 0, 3 ) )
} while( data.length );

edited Oct 23, 2012 at 15:29

answered Oct 23, 2012 at 15:24

jAndy

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3 Comments

Naor Over a year ago

What if data.length % 3 <> 0 ?

jAndy Over a year ago

@Naor: well, this code will greedily grab 3 items from the target array and write those into results. If there are less than 3, it'll take whatever is left.

jAndy Over a year ago

it was flawed anways, fixed that with .push instead of .concat

Florent · Accepted Answer · 2012-10-23 15:17:34Z

2

Use a for loop:

var i = 0
  , n = data.length
  , result = [];

for (; i < n; i += 3) {
  result.push([data[i], data[i + 1], data[i + 2]]);
}

answered Oct 23, 2012 at 15:17

Florent

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5 Comments

jAndy Over a year ago

preceeding... commas.. argll.. it.. hurts... my.. eyes.. argggh.. +1 anyway :p

jAndy Over a year ago

fixed it. By the way, the code can get improved by invoking Array.prototype.splice instead using offsets i+n.

Naor Over a year ago

The last row might contains undefined where data length is not devided bt 3.

Florent Over a year ago

According to the OP's question I supposed that data.length % 3 == 0

Florent Over a year ago

The suggested resulting array suggests that :P

Guffa · Accepted Answer · 2012-10-23 15:18:06Z

2

You can do it with a loop. This takes the array items and creates the array result:

var result = [];
var line;
for (var i = 0; i < items.length; i++) {
  if (i % 3 == 0) {
    line = [];
    result.push(line);
  }
  line.push(items[i]);
}

(This way of looping works even if the source array is not exactly divisible by three.)

answered Oct 23, 2012 at 15:18

Guffa

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2 Comments

Naor Over a year ago

You never push the last row if it doesn't divided by 3.

Guffa Over a year ago

@Naor: Yes, it does. The row is pushed to the array right after it is created, not after it has been filled with items.

Eugene · Accepted Answer · 2012-10-23 15:51:19Z

0

Just another option :)

var result = arr.reduce(
    function(prev, curr, index) { 
        if(index > 1) { 
            if(prev[prev.length-1].length == 3) 
                prev.push([]); 
            prev[prev.length-1].push(curr); 
            return prev; 
        }
        return [[prev, curr]]; 
    }
);

answered Oct 23, 2012 at 15:51

Eugene

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