Commodore VIC-20 - Page 358

344 The VIC 20 User Guide
DEFFN
The DEF function (DEF FN) allows special purpose functions to be
defined and used within BASIC programs.
Format:
DEF FNnvar(arg)=expression
Floating point variable nvar identifies the function, which is sub-
sequently referenced using the name
FNnvar(data). (If nvar has more than
five letters, a syntax error is reported. A syntax error is also reported if nvar
is a string or integer variable.)
The function
is specified by expression, which can be any arithmetic
expression containing any combination
of numeric constants, variables, or
operators. The dummy variable name arg can (and usually does) appear in
expression.
arg is the only variable in expression that can be specified when
FNnvar(data) is referenced. Any other variables in expression must be
defined before
FNnvar(data) is referenced for the first time. FNnvar(data)
evaluates expression using data as the value for argo
The entire DEF FN statement must appear on a single 80-character
line; however, a previously defined function can be included in
expression,
so user-defined functions of any complexity can be developed.
The function name
var can be reused and therefore redefined by
another
DEF FN statement appearing later in the same program.
The
DEF FN definition statement is illegal in immediate mode. How-
ever, a user-defined function that has been defined by a
DEF FN statement
in the current stored program can be referenced in
an immediate mode
statement.
Example:
10 DEF FNC(R)-«*Rf2
?FNC(1)
33 IF FNC(X»60 GOTO 150
Defines afunction that calculates the circumference of
a circle. It takes a single argument R. the radius of
the circle. and returns a single numeric value. the
circumference
of the circle
Prints
3.141159265 (the value of 71")
Uses the value calculated by the user-definedfunction
FNC as a branch condition. The current contents of
variable X are used when calculating the user-
defined function