JavaScript - Create Array with data without looping

Question

I would like to know if it is possible to create an array and initialize it with the same object without having to loop on each element. I don't want to loop because i could have to insert many element . this is what i would like to be able to do:

var array=new Array(10000);

and I would like that each element of the array is the same object (other than undefined :) ) without having to do like this

for(i=0;i<array.length;i++)
    array[i]=object;

I hope that i want to do is clear to you

I came up with a solution but i uses the eval function so I am not sure if it is the best but it much efficient than a loop

Your advises are welcomed :)

here is how

var i="l,",l=new Object(),length=20000;
l.id=1;

while(i.length<length){
      i+=i;
}
i=i.substring(0,length-1);
i="["+i+"]";

var array=eval(i);

console.log(array);

thanks

Yes but I don't loop 10000 times, for 10000 elements I loop only 14 times which is much more efficient. — al7iss
– al7iss, Commented Jul 24, 2011 at 9:22

c-smile · Accepted Answer · 2011-07-23 15:10:13Z

3

There is no requirement for arrays in JS to allocate storage for elements when you do new Array(10000); You are getting undefined elements simply because there are no such elements - even no storage for them allocated. To create elements of the array you have to put them explicitly.

But to be honest I do not understand why do you need that. It is enough to put this:

var v = array[i] || object;

And v will always have either element or the object if element was not defined.

answered Jul 23, 2011 at 15:10

c-smile

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3 Comments

al7iss Over a year ago

thank you for your answer but what I need is that all the elements of the array be the same but not undefined. I need this because I will be modifying the array potion by portion. It does not seem to be doable, I will have to loop over the array witch will decrease performances unfortunately.

c-smile Over a year ago

Simply add pair of get/set functions to your array like myArr.get = function(i) { return this[i] || object; } and use them instead of [] accessor.

user137717 Over a year ago

if you really need all elements in an array of fixed length to be a certain Object and not undefined, I don't see how looping would reduce the performance of your program. The javascript environment would have to do it in the background in the same O(N) time that looping provides.

gaRex · Accepted Answer · 2011-07-23 15:17:12Z

1

No and yes @see array spec.

No, as in JS there is no such constructor like:

var a = new Array(10000, someObject);
// or
var a = Array.fill(0, 10000, someObject);

Yes, as you could do it manually :)

var a = new Array(someObject, someObject, someObject, ..., someObject); // 9996 elements ommited

And in you code it's better to do like this:

for (var i = 0, iMax = array.length; i < iMax; i++) {
    array[i]=object;
}

Add var inside for's counter initialization
Use another local variable instead of array.length, as it a little bit closer in scope

answered Jul 23, 2011 at 15:17

gaRex

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1 Comment

al7iss Over a year ago

Yes I thought about your solution but the creation of the array is done dynamically and its length is got from a server so it is not always the same.Thanks

Praveen Prasad · Accepted Answer · 2011-07-23 15:23:22Z

1

There is no such in built feature in js but you can create one for you; Yes iam looping inside function myArray.

function myArray(size,defaultObj)
{
    var _array=[];
    for (i=0;i<size;i++)
    {
        _array[i]=defaultObj
    }
    return _array;
}

var myArray= myArray(3,"hello");

alert(myArray[2]);

http://jsfiddle.net/ZySst/

answered Jul 23, 2011 at 15:23

Praveen Prasad

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Comments

Abdul Rahman · Accepted Answer · 2012-11-13 15:25:43Z

1

I think this is what you are looking for!

function createMatrix ()
       {
         var matrix = Array.prototype.slice.call(arguments);
         return matrix.length > 0 ? matrix : [0] ;
       }

create matrix of arbitrary dimension createMatrix () --> [0]
createMatrix (1, 2, 3) --> [1,2,3] - vector row

createMatrix ([1], [2], [3]) --> [[1],[2],[3]] - vector column

createMatrix ( [1, 2, 3], [4, 5, 6], [7, 8, 9] )

--> [[1,2,3],[4,5,6],[7,8,9]] - 3 × 3 matrix

edited Nov 13, 2012 at 15:25

Abdul Rahman

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answered Aug 26, 2011 at 11:00

MiljenkoH

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Comments

Duster · Accepted Answer · 2014-12-04 07:51:44Z

1

I think you can do something like below, I have tried filling 50 1's without using loop, but i think internally split and join both uses loop.

var a = new Array(50).join(1).split('').join(',');

answered Dec 4, 2014 at 7:51

Duster

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Comments

Gaurav · Accepted Answer · 2011-07-23 15:27:07Z

0

Probably not a practical solution, and won't work in IE < 9, but technically no (explicit) loop:

var a = [];
a.length = 10000;
a.forEach(function(e, i) {
    a[i] = object;
});

answered Jul 23, 2011 at 15:27

Gaurav

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2 Comments

al7iss Over a year ago

Yes but is forEach more efficient than looping ?

Gaurav Over a year ago

Hey, why the downvotes? The question was is it possible. The answer is (technically) yes.

FK82 · Accepted Answer · 2011-07-23 15:38:40Z

0

No, I don't think that is possible in vanilla JavaScript.

This code will create an Array containing 10 zeros:

var array = [ ] , filler = 1 ;
var i = 0 , length = 10 ;

while(i < length) array.push(filler) , i = i + 1;

alert("(!) array >> " + array) ;

(try it here)

answered Jul 23, 2011 at 15:38

FK82

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Comments

al7iss · Accepted Answer · 2011-07-24 09:52:18Z

0

This is the best solution I found, does not use eval and the loop is not too long so the browser does not crash:

var j=0,l=new Object();
l.id=1
var array=[l];

while(array.length<10000){
     array=array.concat(array);
}
array=array.slice(0,10000);
console.log(array.length);

Well of course the bigger the array's length is before the loop, the faster the loop is.

answered Jul 24, 2011 at 9:52

al7iss

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1 Comment

FK82 Over a year ago

Out of curiosity, have you actually tried some of the simpler patterns using Array#push or the bracket operator in terms of performance. This approach seems a tad bit convoluted tbh. Also, Array#concat creates a new Array with each call (developer.mozilla.org/en/JavaScript/Reference/Global_Objects/…) which seems like a wasteful thing to do.

Zane · Accepted Answer · 2013-10-16 13:55:13Z

0

I would say the answer to your question is: it can't be done.

Not in Java, and probably not in any other programming language.

If you think about the underlying machine, what you are asking for is: can a memory area of n values of a specific type be initialized to the same value without looping.

And I guess this is only possible without a loop if there is some special mechanism in the underlying machine, e.g. a way to nullify memory areas. But even then, in most cases the mechanism will in fact loop over all values.

answered Oct 16, 2013 at 13:55

Zane

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