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I want to create an array in javascript to have 20 elements with a starting index of 15, assign every element with a value of 00000. What is the most efficient way, and can it be done without looping through the index?

TIA.

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You could just toy around with different ways to do different things (how will you create the array, how will you create the elements that are going to go into the array, how will you actually insert those elements, etc.) and then put the various techniques on jsperf.com – sdleihssirhc Sep 3 '11 at 7:19
micro optimizations are the root of all evil programmers.stackexchange.com/questions/80084/… – Lime Sep 3 '11 at 8:20
I managed to write and finished the code for a web tool to perform the tasks that I set out to do. But as I learned more, I want it to be better and faster for users, so now I am scrapping most of the code and rewrite to fit new algorithm. I don't think I will ever finish. – Jamex Sep 3 '11 at 9:07
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5 Answers

up vote 0 down vote accepted

It's better to use an object, if you want to use an offset:

var iOffset = 15;
var iCount = 20;
var oValues = {};
for (var i = iOffset ; i < iOffset + iCount; i++) {
    oValues[i] = 0;
}
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thanks, this seem to be the alternative to array. I guess I was just asking if there is some way to declare something like arrayx {15,0000,20}, but I guess not. – Jamex Sep 3 '11 at 9:00
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up vote 2 down vote

Array indexes in javascript are always in the interval [0, num_elements). If you do not want this, you need to use an object. Then you have arbitraty indexes but no fixed order.

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thanks, I guess I was just asking if there is some way to declare something like arrayx {15,0000,20}, but I guess not. – Jamex Sep 3 '11 at 9:02
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up vote 1 down vote

Without loop you can only use instant initialization:

var arr = [null, null, null, ... 15 times ...., '000000', '000000', '000000', '000000', '000000'];

But probably it's better to use loop :)

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Note that having a property with value null, or indeed undefined, is not quite the same as having no property under a given index. – bobince Sep 3 '11 at 8:00
yes, I know it. I've choose the null because it's shorter to write for example :) – Samich Sep 3 '11 at 8:01
Shorter would be: [,,,,,,,,,,,,,,,'000000', '000000',...] – Felix Kling Sep 3 '11 at 8:27
thanks, I guess I was just asking if there is some way to declare something like arrayx {15,0000,20}, but I guess not. – Jamex Sep 3 '11 at 9:01
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up vote 0 down vote

For the specific case where each item in the array is a string, you can do it without a loop with the new Array(length) constructor and a sneaky join:

new Array(15).concat(('000000'+new Array(20).join(',000000')).split(','))

Unlikely to be faster than the loop though... certainly less clear. I'd also question whether an Array with missing properties 0–14 is something it's generally sensible to have.

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thanks, this seem to be the alternative to array. I guess I was just asking if there is some way to declare something like arrayx {15,0000,20}, but I guess not. – Jamex Sep 3 '11 at 9:00
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up vote 0 down vote

You can't have an array starting with an index of 15. The index of an array always start with index 0, as is demonstrated with:

var a = [];
a[15] = '000';
alert(a.length); //=> 16

You could use a recursive function to create an array of 20 elements, something like:

function arr(len){
    return len ? arr(len-1).concat('0000') : [];
}
var myArr = arr(20);

Or create an array of small objects, containing your custom index:

function arr(len){
    return len ? arr(len-1).concat({index:len+14,value:'0000'}) : [];
}
var myArr = arr(20);
alert(myArr[0].index); //=> 15

Alternatively, using a loop, you can do something like:

var nwArr = function(len,start){
  var l = len+start, a = Array(l);
  while ((l-=1)>=start){ a[l] = '0000'; }
  return a;
}(20,15);
alert(nwArr[15]); //=> '0000'
alert(nwArr[0]);  //=> undefined
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thanks, I will keep these answers in mind, I think one of these is handy. – Jamex Sep 3 '11 at 9:01
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