Coding Practices – The Modulus Operator..
August 11, 2008
During my travels of programming video games for a living, I take great pleasure in the fulfillment of learning, exploring & experimenting with algorithms & the intricacies of programming languages everyday. Being a gamplay programmer I tend to find that I make regular use of the modulus operator & it seems to be one of those little tools that most people seem to overlook but can be really useful for a variety of problems. So I thought it might be a good idea to write a little article about my experiences with lil’ old ‘%’ & hopefully it maybe able to encourage others share good practical uses for it too & hopefully teach me a thing or two.
Before we begin I’d just like to make a note that I’m going to be using C# for all my examples considering this blog are generally c#/XNA-centric (however i’m pretty sure the same functions for the modulus operator can be applied in C/C++ & most other languages)..
So What Is The Ol’ Mod..?
The Modulus operator (sometimes termed ‘Modulo‘ in computing fields) defines an operation which returns the remainder of a division of one number by another. So for example if we take the any dividend ‘a’, & a divisor ‘n’ then we can define a % n (a modulo n) as the remainder.
So if we have 7 % 3 then we look at it as an integer division of only the proportion of the numbers which can evaluate to a whole number. In this case we see that 7 / 3 doesn’t equal a whole number (equals 2.333) however 6/3 does (leaving no remainder) so to find the modulus of this expression we can treat 7 / 3 as 6/3 + remainder which tells us that the remainder is 1. Here’s a few more examples just to help you get your head around the idea:-
8 % 4 = 0
12 % 16 = 12
So a few rules to bear in mind which will help you use the operator in practice:-
- if a / n equals a whole number then a % n always equals 0
- if a is less than n then a % n always equals a
- if a is greater than n then a % n always equals a value between 1 and n-1
As we can see these three simple rules help put things into perspective & should now hopefully give us a really sound idea of some of the ways we can use the operator in practice.
Let’s Get To The Main Course Already..!
OK so now we’re ready to begin looking at some of the ways we can use the modulus operator & it’s unique properties for something useful. Here’s what I have so far:-
Wrap-Around & Looping Number Iteration
Sometimes in our travels we may need to iterate through an array in such a way that we can use an incremental index value that can be reset once it goes beyond the bounds of the array. Modulus is really useful for doing these tasks as, looking at rules 1, 2 & 3, we can see that it’s zero-based in nature & perfectly suited to this kind of ‘wrap-around’ processing. So where we would have once had something like this
// increment index & return the item in the array
if (i >= maxNumValues) i = 0;
We can now substitute it for something like this
// increment index & return the item in the array
return foo[ (i++) % maxNumValues ];
Not a massive change but at least we’ve saved ourselves a few extra lines of code & removed the branch statement.
2D Coordinate Indexing
On several occasions i’ve found that I needed to index data in a 2D grid format. Whether it be indexing tiles on a spritesheet or onscreen items in a grid there’s generally a tendencies to define the data in a ‘flattened’ array format & make the grid-like behaviour implicit in it’s use. So say we have a sprite sheet containing a grid of tiled sprites of size d x d pixels and arranged in a grid of 4 x 4 tiles. if we treat the tiles as a 1D array of data then we can index each value from left-to-right, going from top-to-bottom starting at 0 and ending at 15 (4 x 4 -1 since it’s zero-based). in this case if we wanted to get the coordinates of a tile as say index = 9 we could do the following
- count up the indices from 0 to 9 & in each step:-
- add 1 to x & if x is greater than the number of tiles in a row then remove the number of tiles in a row from x & add one to y
- multiply x by the tileWidth & y by the tileHeight & return the two values
This will work if you use a while loop and branch statements but it will end up taking up several lines of code & could end up being overly complex. However if we use the modulus operator we can do it much quicker/cleaner
x = index % numTilesRow;
y = ( index – x ) / numTilesRow;
return new TileDims( x * tileW, y * tileH );
I added the struct TileDims just for illustration purposes but beyond that it’s a pretty neat & cheap solution. Also we can reverse this & retrieve the index from the coordinate values in a simple fashion
// return the index for x & y
return x + ( numTilesRow * y );
Detemining Odd or Even Numbers
We can use % to determine whether a number is odd or even by examining exactly what it is we mean by ‘odd’ & by ‘even’ first. by definition an odd number is any number that cannot divide into 2 (i.e. odd / 2 != wholeNum). With this in mind we can do a simple check for this
// odd number check
if ( num % 2 != 0)
// odd number..
// even number..
Simple to use but it can save alot of time trying to determine whether this is true using a brute force, iterative approach.
How Many 10s, 100s etc In A Decimal Value
Sometimes we may need to figure out how many 10s are in a number for example, which can be especially useful of you need to separate the digits of a number for rendering on multiple quads. Again the implementation is pretty simple
// find the number of 10s & 100s in num
tens = ( num – (num % 10) ) / 10;
hundrds = ( num – (num % 100) ) / 100;
So that’s it from me!
Please feel free to drop any comments you have & also I’d love to hear about any other neat little % tricks you’ve discovered during your own coding travels..!
– The Hog