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up vote 1 down vote favorite

can anyone suggest a snippet or a short method to solve this:

array = [a,b,c,d,e,f]

currentIndex = 2;

getOffset(array,currentIndex,2); // 2+2 = 4 -> return 'e'

getOffset(array,currentIndex,-2); // -> return 'a'

getOffset(array,currentIndex,-3); // -> return 'f'

getOffset(array,currentIndex,-4); // -> return 'e'

getOffset(array,currentIndex, 5); // -> return 'b'

So if the the targetted index is bigger than array.length or < 0 -> simulate a circle loop inside the array and continue to step inside the indexes.

Can anyone help me? I tried, but got a buggy script :(

TY!

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3 Answers

up vote 4 down vote accepted

Try this:

function getOffset(arr,index, offset){   
    return arr[(arr.length+index+(offset%arr.length))%arr.length];
}
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1  
This is the correct answer for the reason described here: javascript.about.com/od/problemsolving/a/modulobug.htm The double modulo/dividend is necessary. Nice answer Amber. –  Cecchi Jun 22 '12 at 9:37
 
Yep. But getOffset(arr,2,-9) would return undefined –  KooiInc Jun 22 '12 at 9:56
 
No, it does not: jsfiddle.net/bTWwn –  Amberlamps Jun 22 '12 at 10:01
 
Did you use array = [a, b, c, d, e, f] to test it? Then it obviously returns undefined because there are no values defined for the variable a to f –  Amberlamps Jun 22 '12 at 10:04
 
@Amberlamps: I stand corrected, and no, I tested with ["a", "b", "c", "d", "e", "f"], but switched offset/index. –  KooiInc Jun 22 '12 at 10:10
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up vote 2 down vote

This should do the trick, I suppose:

function getOffset(arr,n,offset) {
   offset = offset || 0;
   var raw = (offset+n)%arr.length;
   return raw < 0 ? arr[arr.length-Math.abs(raw)] : arr[raw];
}

var arr = ["a", "b", "c", "d", "e", "f"];
getOffset(arr,-3,2); //=> 'f'
getOffset(arr,-3);   //=> 'd'
//but also ;~)
getOffset(arr,-56,2);  //=> 'a'
getOffset(arr,1024,2); //=> 'a'
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Where does i come from? –  Amberlamps Jun 22 '12 at 9:37
 
Earlier code. Removed it, and edited my answer. –  KooiInc Jun 22 '12 at 9:53
 
thank you for the answer ! –  zsitro Jun 22 '12 at 10:28
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up vote 1 down vote

Use the modulus operator:

function getOffset(arr, index, step) {
  return arr[(((index + step) % arr.length) + arr.length) % arr.length];
}
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1  
Why the downvote? –  Amberlamps Jun 22 '12 at 9:21
 
Seriously, why downvote? –  bvukelic Jun 22 '12 at 9:26
 
When |offset| is > index it will result in a negative index! –  Amberlamps Jun 22 '12 at 9:29
 
@Amberlamps, bvukelic: What when index is 1 and step is -5? Modulus, IMO, isn't defined for negative numbers. I did the downvote. –  Thrustmaster Jun 22 '12 at 9:30
 
% in JavaScript actually returns the dividend. If we want to only allow positive index's we could get the abs but I'm not sure that's the intended functionality, or necessary. –  Cecchi Jun 22 '12 at 9:32
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