Partitioning in JavaScript [duplicate]

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.

13

7 Answers

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;
}

3

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.

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:

2

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 );

1

One more solution, with no external library :

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

Demonstration :

5

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.

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...