Partitioning in JavaScript

Question

Please consider an array such as :

arrayAll = [1,2,3,4,5,6,7,8,9]

Is there a package that enable to do partitioning to obtain :

arrayALLPartionned = [[1,2,3],[4,5,6],[7,8,9]]

I can see how to do this with a for loop but would appreciate a "pre-made" function if existing.

Answer 1

I think you will have to use a for loop, don't know of any inbuilt functions...

Try this function:

function splitarray(input, spacing)
{
    var output = [];

    for (var i = 0; i < input.length; i += spacing)
    {
        output[output.length] = input.slice(i, i + spacing);
    }

    return output;
}

Answer 2

Here's a recursive solution:

function partition(array, n) {
  return array.length ? [array.splice(0, n)].concat(partition(array, n)) : [];
}    

This takes advantage of the fact that Array#splice destructively remove the specified items, and returns them as the function value. Note that this will destroy the input array, leaving it empty.

Answer 3

If using Underscore.js, you can implement this with groupBy() and values()

function partition(items, size) {
    var result = _.groupBy(items, function(item, i) {
        return Math.floor(i/size);
    });
    return _.values(result);
}

(This is less ugly in CoffeeScript.)

jsFiddle: http://jsfiddle.net/MW3BS/

Answer 4

I've added this solution to @dystroy's jspref here and it appears to run twice as fast as the other solutions. Edit: in Safari & Chrome but not Firefox

Here is functional style solution to add to the mix of answers here.

It is a higher order function called toPartitions which returns a callback for underscore's reduce method or the native array reduce method.

Example usage:

[1,2,3,4,5,6,7,8,9].reduce( toPartitions( 3 ), [] );

The function:

function toPartitions ( size ) {
    var partition = [];
    return function ( acc, v ) {
        partition.push( v );
        if ( partition.length === size ) {
            acc.push( partition );
            partition = [];
        }
        return acc;
    };
}

Like Clojure's partition it will not include a tail partition when there are not enough elements.

In your example you could do:

arrayALLPartionned = arrayAll.reduce( toPartitions( 3 ), [] ) );

If you don't want to use this with reduce, but just have a function which takes an array and partition size you could do:

function partition ( arr, size ) {
    return arr.reduce( toPartitions( size ), [] );
}

Therefore the solution would just be:

arrayALLPartionned = partition( arrayAll, 3 );

Answer 5

One more solution, with no external library :

function partition(items, size) {
    var p = [];
    for (var i=Math.floor(items.length/size); i-->0; ) {
        p[i]=items.slice(i*size, (i+1)*size);
    }
    return p;
}

Demonstration : http://jsfiddle.net/dystroy/xtHXZ/

Answer 6

You can write your own prototype method to do this

Array.prototype.partition = function(length) {
  var result = [];
  for(var i = 0; i < this.length; i++) {
    if(i % length === 0) result.push([]);
    result[result.length - 1].push(this[i]);
  }
  return result;
};

If you prefer not to add to the native prototype, you can write a simple function:

var partition = function(arr, length) {
  var result = [];
  for(var i = 0; i < arr.length; i++) {
    if(i % length === 0) result.push([]);
    result[result.length - 1].push(arr[i]);
  }
  return result;
};

You can see it in action on this jsFiddle demo.

Answer 7

Prototype has an array.partition function as well as an eachSlice() function. Sounds like eachSlice() is what you're looking for. If you're using jquery, there's a plug in to be able to use prototype functions. Here's a link to it... http://www.learningjquery.com/2009/02/implementing-prototypes-array-methods-in-jquery