Chapter 7: Creating SUBs and FUNCTIONs (Part II)
7.1 Introduction
As we've discussed in the previous chapter, creating SUBs isn't difficult. Creating FUNCTIONs in QBasic is similar to creating SUBs, the main difference is that a FUNCTION has a return value attached to it.
7.2 QBasic's built-in FUNCTIONs
QBasic offers many built-in functions, let's take the graphics function POINT for example. POINT is used in graphics mode to get the colour of the pixel at (X,Y). Just pretend we're doing a simple collision detection routine, and want to know the colour of the pixel at (100,50) 
       Colour = POINT(100,50)
See, POINT is a function because it returns a value. All functions must be on the RHS (right-hand side) of an assignment. Just like a SUB, a FUNCTION can accept any number of parameters you define.
7.3 Get to the POINT :)
Fine, let's get to the point, how do you create a FUNCTION? 
 FUNCTION ReturnMax%(Num1%, Num2%)
    IF Num1% >= Num2% THEN
       ReturnMax% = Num1%
    ELSE
       ReturnMax% = Num2%
    END IF
 END FUNCTION
Quite obviously, this function returns the max of two numbers. So how does it work? Since a function returns a value, we need to specify what data type (see Chapter 2). As you can see, ReturnMax% is an integer type, so it will return an ... integer value! Some people ignore the % sign, but let's avoid this bad programming style. 
ReturnMax% = Num1%
is assigning the return value to Num1%, but note that this DOES NOT terminate a FUNCTION, you can assign infinitely many return values. Some people don't understand this: 
 FUNCTION ReturnMax%(Num1%, Num2%)
    ReturnMax% = 1234  ' Keep going...
    IF Num1% >= Num2% THEN
       ReturnMax% = Num1%
    ELSE
       ReturnMax% = Num2%
    END IF
 END FUNCTION
When the function is first called, the return value is 1234, but we're not done yet... eventually the return value is one of Num1% or Num2% depending on which one is larger.
7.4 Final notes
There's a lot to cover when dealing with SUBs/FUNCTIONs, most of the information is documented in QBasic's help file. I haven't covered shared variables and changing values of the parameters. I'll cover variable sharing later, but changing the value of a parameter is quite simple, so try this yourself.
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