Beware of the If-Semicolon Bugs!

This has happened to me enough times before for me to be embarrassed by it, and for that reason I'm writing this blog post about it. If you have an if statement that is always returning true (it feels something like, "GAAAHH, why does my if statement always return true!) then this could be your problem (and solution!). I noticed this mistake again today working in Actionscript 3 while fixing a bug in Head Exploder. Here's a simplified version of the as3 code:

var smallNumber:Number = 1;
var bigNumber:Number = 100;

if (smallNumber > bigNumber)
{
     trace("Uh oh!");
}

The code above is fine, and when run it will not print out the message "Uh Oh!" to the console. However, what if we put a semicolon after the closing parenthesis of the if comparison?

var smallNumber:Number = 1;
var bigNumber:Number = 100;

if (smallNumber > bigNumber);
{
     trace("Uh oh!");
}

Now this code is going to be trouble for the developers writing it! For some reason, the semicolon at the end of the statement forces it be true, and therefore everything in the if statement brackets is run. Yikes! This mistake is especially tricky to debug since it doesn't cause any errors, just unexpected behavior in the application, and nobody likes discovering unexpected behavior! I've tried this in Flash Builder and Intellij. In both IDE's this syntax error is not detected, and I'd be so bold as to say that this will slip by the compiler in any As3 code editor. Also note, this isn't really specific to As3. It happens in Java as well. For example, type this into your favorite Java IDE and watch what happens:

int smallNum = 1;
int bigNum = 100;
                
if (smallNum > bigNum);
{
     System.out.println("Uh Oh!");
}

Sadly, this code prints out "Uh oh!" as well. I'm almost too afraid to think about how many other languages are also vulnerable to this subtle mistake.

Some Help From an Else

It's interesting that (in both languages) if you try to follow up the if statement with an else then the IDE complains (as it should) with a error that prevents you from even running the program. Even an empty else {} will cause the author-time error to appear, but then deleting the else will make the code good-to-go again (even though you'll get the unwanted behavior).

What's Really Going On Here?

At first glance you might think that this is some unfixed issue with programming languages, but of course it's not. Remember, compilers allow for single line body if/for statements without braces! Let's take a look at the problem code again. The compiler thinks that the semicolon after the closing parenthesis is ending the block that occurs if the if conditional returns true. the opening and closing braces are no longer connected to the if statement since it has the single line bracket-less body. The compiler basically ignores these brackets since they are not connected to any if or for statement (or anything else) so the code inside always runs.  

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The If-Semicolon bug is not a bug in the language itself. It is a type of bug you may find in your applications that is caused by this programmer mistake of putting a semicolon after an if statement.