Specifications

195
Description:
■ Creating a user-defined function
DEF FN creates a user-defined function.
• Definition of a user-defined function should appear preceding a calling statement
of the user-defined function in a source program.
• You cannot make double definition to a same function name.
• The
DEF FN statement should not be defined in the block-structured statements
(
DEF FN...END DEF, FOR...NEXT, FUNCTION...END FUNCTION, IF...THEN
...ELSE...END
IF, SELECT...CASE...END SELECT, SUB...END SUB and
WHILE...WEND), in the error-handling routine, event-handling routine, or in the
subroutines.
•
DEF FN functions cannot be recursive.
• The type of
functionname should match that of the function definition
expression.
• In defining a string function, you can specify the maximum
stringlength for a
return value. If its specification is omitted, the default value of 40 characters takes
effect.
•
dummyparameter, which corresponds to the variable having the same name in
the function definition
expression, is a local variable valid only in that
expression . Therefore, if a variable having the same name as dummypa-
rameter
is used outside DEF FN statement or used as a dummyparameter
of any other function in the same program, then it will be independently treated.
•
expression describes some operations for the user-defined function. It
should be within one program line including definition described left to the equal
sign (=).
•
expression can call other user-defined functions. You can nest DEF FN state-
ments to a maximum of 10 levels.
• If variables other than
dummyparameter(s) are specified in expression,
they will be treated as global variables whose current values are available.
•
stringlength should be enclosed with a pair of square brackets [ ].
■ Calling a user-defined function
FNfunctionname calls a user-defined function.
• The number of realparameters should be equal to that of dummyparame-
ter
s, and the types of the corresponding variables used in those parameters
should be identical.
• If you specify a global variable in
realparameter when calling a user-defined
function, the user-defined function cannot update the value of the global variable.
This is because all
realparameters are passed not by address but by value.
(So called "Call-by-value")