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is there a shorter, better way to generate 'n' length 2D array?

var a = (function(){ var i=9, arr=[]; while(i--) arr.push([]); return arr })(); a // [ [],[],[],[],[],[],[],[],[] ]

** old-school short-way**:

var a = (function(a){ while(a.push([]) < 9); return a})([]);

UPDATE - Using ES2015

Array(5).fill().map(a=>[]); // will create 5 Arrays in an Array // or Array.from({length:5}, a=>[])

Emptying 2D array (saves memory rather)

function make2dArray(len){ var a = []; while(a.push([]) < len); return a; } function empty2dArray(arr){ for( var i = arr.length; i--; ) arr[i].length = 0; } // lets make a 2D array of 3 items var a = make2dArray(3); // lets populate it a bit a[2].push('demo'); console.log(a); // [[],[],["demo"]] // clear the array empty2dArray(a); console.log(a); // [[],[],[]]
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7
  • 1
    Only if you put it in a function: var a = make2D(9); ;) Commented Jun 27, 2011 at 15:28
  • I am using it for a web-worker which needs a 2D empty array each time it's initialized Commented Jun 27, 2011 at 15:38
  • I wonder if there is some fancy EC5 for this Commented Jun 27, 2011 at 15:38
  • Btw. shorter is not necessarily better (just wanted to point out the differences in the title and the question body ;)). Commented Jun 27, 2011 at 15:52
  • And you could make it even shorter by passing the array as parameter instead of declaring it: var a = (function(a){ while(a.push([]) < 9); return a})([]); Commented Jun 27, 2011 at 15:59

10 Answers 10

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26

Another way:

for(var a = [];a.length < 10; a.push([])); // semicolon is mandatory here

Yet another way:

var a = []; while(a.push([]) < 10);

This works because .push() [docs] (specification) returns the new length of the array.

That said, this is the wrong way of "reducing code". Create a dedicated function with a meaningful name and use this one. Your code will be much more understandable:

function get2DArray(size) { size = size > 0 ? size : 0; var arr = []; while(size--) { arr.push([]); } return arr; } var a = get2DArray(9);

Code is read much more often than written.

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4 Comments

@vsync: True, somehow I thought I need to compare against 10 to get 9 elements. But anyway I like 10 more :P ;)
I believe the code should be small, but very well documenter in development, and on production it's compressed anyway, so you gain better compression rate this way.
I didn't know that 'a.push([])' returns a number. this is new for me that push returns a number, this is very interesting.
@vsync: Just added it to my answer. Because it is defined like that ;)
8

Just discovered another ES6 way with one line expression:

Array.from({length: N}, () => [])

Array.from(arrayLike[, mapFn[, thisArg]])

More detail about its implementation/polyfill ⇢ MDN Array.from()

Yet another neat solution with help of array spread syntax:

[...Array(N)].map(() => [])
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1 Comment

Note that this is not one function call bun N + 1 because you call once Array.from() and N times () => [] function for each iteration that happens inside Array.from.
3 
Array(cardinality).fill(0).map(function(item) {return [];});

where cardinality is the number of items you are looking at. In this case it would be 9. This was suggested by one of my colleagues actually. This is neat, I think :) This is valid from ECMA V6. Documentation: Array::fill

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3 Comments

If you are using ES6, then this should be: Array(5).fill(0).map(()=> [])
@vsync, True that. However, you can define the prototype function yourself and use it in ECMA 5 if you want. That's what, I did for the time-being
I use Babel in my build process, so it can transforms it to ES5 automatically
0 
var a = []; for(var i=0;i<10;i++) { a[i] = []; }
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4 Comments

Please post the output of console.log(a);. I'm curious to see whether it is the same as the OP posted. Ok, just kidding. -1 this just sets the length but it does not initialize anything.
This just sets the length, the elements are still 'undefined'
this won't create n entries with an empty array. in fact console.log return just a [].
I'm not sure why this was downvoted... it returns an empty 2D array. The output of JSON.stringify(a) is "[[],[],[],[],[],[],[],[],[],[]]".
0

shorter way can be : 

for(var i=9,a=[];i>=0;i--){ a.push([]) }
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1 Comment

this is exactly the SAME as mine..yes 'for' is less characters than 'while'..but still.
0 
var a = []; var max_length = 10; for(var i = 0; i < max_length; ++i){ a[i] = []; }
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6 Comments

you might as well do for(var i=9, a=[]; i--;){... - it's better
@vsync, why is starting at nine better?
@vsync -- i added a max_length param.
why not? it's less code then starting with zero then adding inside the loop. it gives the same result
@vsync, not when you are using a[i] = [] -- aalways better to start at 0
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0 
for(var i=9,a=[];i>=0;i--){ a.push([]) }
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1 Comment

hmmm..I would prefer a way which is more obvious that this is an assignment to some var..like var 2d = ....
0

In ES6: 

(m, n, initialElementValue) => Array(m).fill(Array(n).fill(initialElementValue))
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This would not create copies and would use the same array. So for example changing arr[0][1] would cause arr[1][1] and arr[2][1] to change. But it can be done that way
0 

var x = 3, y = 5, arr = Array(y).fill(); arr = arr.map(function(){return Array(x).fill(' ')}); console.log(arr);

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You answer does not provide any new information which wasn't already mentioned years ago by others here..
0

best way to generate 2D array in js by ES6 by Array.from

function twodimension(input) { let index = 0, sqrt = Math.sqrt(input.length); return Array.from({ length: sqrt }, (nothing, i) => Array.from({ length: sqrt }, (nothingTwo, j) => input[index++])) } console.log(twodimension('abcdefghijklmnopqrstupwxy')) console.log(twodimension([1,2,3,4,5,6,7,8,9]))

function input(length, fill) { let getNums = length * length; let fillNums = 1 if (fill == 'minus') { return Array.from({ length: length }, (nothing, i) => Array.from({ length: length }, (nothingTwo, j) => getNums--)) } else if (fill == 'plus') { return Array.from({ length: length }, (nothing, i) => Array.from({ length: length }, (nothingTwo, j) => fillNums++)) } // you can dping snake ladders also with Array.from if (fill === 'snakes') { return Array.from({ length: length }, (_, one) => Array.from({ length: length }, (_, two) => getNums--) ).map((el, i) => i % 2 == 1 && length % 2 == 0 ? el.reverse() : i % 2 == 0 && length % 2 == 1 ? el.reverse() : el ); } } console.log(input(8, 'minus')) console.log(input(10, 'plus')) console.log(input(5, 'snakes'))

you do anything with Array.from, it is easy to use and fast, this is the new method in ES6 syntax

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