In most cases, it's either a typo, or you're trying to use a function which is not supported under Rapid-Q. This occurs most often when you don't have $TYPECHECK ON. I can't stress this enough, unless you've used Rapid-Q for awhile, I strongly encourage everybody to turn this directive on. Rapid-Q has a very poor checking mechanism, and relies heavily on the programmer to write proper code. For Example:
PRINT SPC(10)
SPC is not supported in Rapid-Q (in QBasic, SPC just skips spaces). If typechecking is off, the above statement is perfectly valid. SPC is a variable, initialized to 0, and (10) has no effect (ie. the number 10 is pushed on the stack, but no operation is done), so the above output will be 0.
Passing by reference
Only QObjects, Arrays, and Variants are passed by reference, all other variables are passed by value. To pass a variable by reference, you can either explicitly specify this by adding BYREF to your parameter list, or prepend an @ symbol to any variables you want passed by reference when calling the function. Do not use the @ symbol for Arrays, Variants, or QObjects since they are passed by reference by default. Read more about this in Chapter 3 or Appendix C: Procedures. Typechecking is done only for Arrays, Variants, and QObjects, since you can't pass an array of integers to a SUB that expects an array of doubles. You should get an error message if you try this, however, for all other variables which are passed by value, NO checking is done. Which means you could easily pass an integer to a SUB expecting a string. So becareful that you know the type of parameters for your SUB/FUNCTION, since Rapid-Q won't complain. Also make sure if you have DECLARE SUBs, that your SUB will have matching parameters, including matching parameter names.
Using Undocumented features
Rapid-Q has some undocumented features, most of which are in a beta-like stage. You can embed SUBs or FUNCTIONs but they can only be one level deep, and there are some consequences with using them. If you don't know what an embedded SUB looks like, take this for example:
SUB MySub
DIM I AS INTEGER
SUB MyChildSub (X AS INTEGER)
DIM N AS DOUBLE
PRINT "In Child Sub"
END SUB
CALL MyChildSub(100)
END SUB
There are some special issues involved, such as all variables are local to the SUBs that follow. Meaning, the variable I is local to SUB MyChildSub. Also, since Rapid-Q doesn't actually clean things up, you can actually call MyChildSub in your main program as well. This is a problem I'm working to correct when I find the time. One other feature which might interest people is postfix ++ and --
DIM I AS INTEGER
I++ '-- same as I = I + 1
I-- '-- same as I = I - 1
I -= 10 '-- same as DEC(I, 10)
I += 15 '-- same as INC(I, 15)
Arrays are a special case, anything but Arrays can be incremented/decremented. Rapid-Q doesn't do any optimizing, so the above code and the given translation runs exactly the same.
Other errors
If you don't have a clue as to what is causing your programs to crash, your only hope is to put several PRINT statements in your program and incrementally check each block of code so that it works properly. If you're using Unix or Linux, try using gdb, sometimes it will give you some useful information, but most of the time it's not really that helpful in this kind of setting.