Tell me more ×
Stack Overflow is a question and answer site for professional and enthusiast programmers. It's 100% free, no registration required.
up vote 2 down vote favorite

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

share|improve this question
1  
Your solution has a while loop. – Gaurav Jul 23 '11 at 22:53
Yes but I don't loop 10000 times, for 10000 elements I loop only 14 times which is much more efficient. – al7iss Jul 24 '11 at 9:22

7 Answers

up vote 2 down vote

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.

share|improve this answer
thank you for your answer. – al7iss Jul 23 '11 at 15:15
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. – al7iss Jul 23 '11 at 15:24
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. – c-smile Jul 23 '11 at 15:42
up vote 1 down vote

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;
}
  1. Add var inside for's counter initialization
  2. Use another local variable instead of array.length, as it a little bit closer in scope
share|improve this answer
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 – al7iss Jul 23 '11 at 15:29
up vote 1 down vote

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/

share|improve this answer
Just a little bit elegant solution is here: jsfiddle.net/Ng3g2 – gaRex Jul 23 '11 at 16:59
up vote 1 down vote

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

share|improve this answer
up vote 0 down vote

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)

share|improve this answer
up vote 0 down vote

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.

share|improve this answer
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. – FK82 Jul 24 '11 at 13:00
up vote -1 down vote

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;
});
share|improve this answer
Yes but is forEach more efficient than looping ? – al7iss Jul 23 '11 at 15:31
Like I said, not practical. – Gaurav Jul 23 '11 at 15:32
Hey, why the downvotes? The question was is it possible. The answer is (technically) yes. – Gaurav Jul 23 '11 at 15:49

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.