Useful hints: #1 – Odd/Even numbers (Modulo operator [%] and Bitwise And [&])

Eoin — Thu, 11 Mar 2010 19:21:50 +0000

I’m going to start posting some useful little bits of code up here as i think of them.
This is partly to help with my exceptionally poor memory, and partly to help out anyone who might be interested. I encourage anyone else who knows of some similar little helpful tips to send them to me and i’ll post them up here.

Ok, so here’s a nice simple one to start off. What’s an easy and simple method to determine if a number is odd or even?
The answer lies with the modulo operator (%) or the bitwise AND operator (&).

i%2

Whatever i is, i%2 will evaluate to either 0 or 1.
Or a faster method for determining odd or even numbers would be the Bitwise AND operator (represented by a single ampersand: &):

i&1

So what use is this?

Let’s say you’ve got a for loop:

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for(var i:int = 0; i < 50; i++){
if(i%2){ // Or i&1
//do something for odd numbers
}else{
//do something else for even numbers
}
}

So why would we use the modulo operator if a bitwise AND is faster?
To take this further, you could specify separate code for every nth number:

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for(var i:int = 0; i < 50; i++){
switch(i%4){
case 0:
trace ("run every 4 iterations starting from 0");
break;
case 1:
trace ("run every 4 iterations starting from 1");
break;
case 2:
trace ("run every 4 iterations starting from 2");
break;
case 3:
trace ("run every 4 iterations starting from 3");
break;
}
}

UPDATE
Some supporting code to show the speed difference.

Results in milliseconds:
Bitwise & : 2809
Modulo : 3456
Bitwise shift : 2801
Simple division : 2792

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package
{
import flash.display.Sprite;
import flash.utils.getTimer;

public class SpeedTest extends Sprite
{
public function SpeedTest()
{
var bAndResult:int = ( runTest( bAnd, 10000000 ) );
trace("Bitwise & :", bAndResult);
var modResult:int = ( runTest( mod, 10000000 ) );
trace("Modulo :", modResult);

var shiftResult:int = ( runTest( bShift, 10000000 ) );
trace("Bitwise shift :", shiftResult);
var divResult:int = ( runTest( div, 10000000 ) );
trace("Simple division :", divResult);
}

private function runTest( fnc:Function, count:uint ):int
{
var start:int = getTimer();
fnc( count );
return getTimer() - start;
}

private function bAnd( count:uint ):void
{
for( var i:uint = 0; i < count; ++i )
{
var bleh:int = i & 1;
}
}
private function mod( count:uint ):void
{
for( var i:uint = 0; i < count; ++i )
{
var bleh:int = i % 2;
}
}
private function bShift( count:uint ):void
{
for( var i:uint = 0; i < count; ++i )
{
var bleh:int = i >> 1;
}
}
private function div( count:uint ):void
{
for( var i:uint = 0; i < count; ++i )
{
var bleh:int = i / 2;
}
}
}
}

Work Calendar

Eoin — Wed, 27 Jan 2010 22:28:37 +0000

The work calendar is a personal project of mine. I wanted to create an application that would allow myself and others to easily track work hours from week to week, provide a simple, secure, easy-to-use printable interface and perform some calculations the the average user might want to know (totals, pay rate, tax deductions etc). As well as that, my friends kept losing the pieces of paper they used to write down their hours, so I thought this might help them out.

MySQL provides a powerful and efficient database which makes it easy to store and retrieve the data for each user.

Uses: XHTML(strict), CSS, PHP, MySQL, JavaScript, jQuery, AJAX

Work Calendar screenshot

This app was also created to help me brush up on some of my skills, so it’s also a bit of a demo to help prove I can do what I say I can do. If you’re a techy, you’ll understand what goes into making something like this

Work Calendar is free to use by anyone. If you like it, hate it or have any comments please let me know.

Eoin Griffin » PHP

Useful hints: #1 – Odd/Even numbers (Modulo operator [%] and Bitwise And [&])

Work Calendar