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FN

Keyword Abbreviation Token (hex) Version(s) Classification
FN none A5 1.0+ Function and Preposition

  Syntax  
FN name ( argument )
 ~ or ~
DEF FN name ( parameter ) = expression
 
Parameters Type Legal Value(s) Default Value Note(s)
name Literal character(s)
first (required): letter
others (optional): letter or digit
 
Can not be (or include) a keyword,
but it may be the same as a reserved variable!
argument Numeric
depends on expression
parameter Literal character(s)
first (required): letter
others (optional): letter or digit
Can not be (or include) a keyword.
Can not be a reserved variable name.
expression BASIC text any mathematical expression
Should reference parameter.
May reference other variables, including reserved variables.
 
Returns Type Value(s) Note(s)
result Floating-point
depends on expression
 
  Purpose  
Calculate the value of a user-defined function (evaluate expression with parameter replaced by argument).

 
  Remarks  
The FN keyword acts like a preposition when defining a function; see DEF.
 
The primary use of FN is for calling a user-defining function.  The name used when calling the function must match that of a previously defined user function (see DEF).  If no matching name is found, an UNDEF'D FUNCTION ERROR is generated.  Per BASIC naming rules, only the first two characters are significant for matching, the first character must be a letter, the remaining characters (if any) may be either a letter or digit, and the name may not be (or have embedded inside of it) any BASIC keyword. 
 
The argument may be any numeric expression in general.  It may be a literal a number, a calculation, an integer or floating-point variable, or a function.  The argument may be (or include) a user-defined function, including the same function.  Each time the user-defined function is evaluated, the argument is used in place of any (all) occurance(s) in the expression used in the definition (i.e., it replaces the "dummy" parameter).  It is important to note that no "real" variable called parameter is created, and if one already exists it is not affected (or accessible) by the user function.
  
The legal values of argument and the returned result depend (in general) on the particular expression that defines the user function.  However, both the argument and the result must be numeric; custom string functions are not supported.  Otherwise a TYPE MISMATCH ERROR is generated.
 
Because a user function can only be defined in a program, calling such a function does not happen often in immediate/direct mode.  This is allowed, however, if the line defining the function is still in memory and has been executed by BASIC at least once.
 
Unlike the similar USR function, this custom user-function is defined in BASIC (see DEF), and you may have multiple custom functions by giving each a unique name.
 
Examples 1:
NEW

READY.
10 DEF FNY(X) = X*X
20 PRINT FNY(5)
30 PRINT X
RUN
 25
 0

READY.
PRINT FNY(3)
 9

READY.
NEW
PRINT FNY(3)

?UNDEF'D FUNCTION ERROR
READY.
 
Example 2:
NEW

READY.
10 DEF FNC(X) = 2*X+1
20 C = 1 : X = 10 : REM has no effect on function
30 PRINT FNC(3)
40 PRINT C : PRINT X
RUN
 7
 1
 10

READY.
NEW

READY.
10 DEF FNF(X) = X*X+Y : REM emulate function of 2 parameters
20 Y = 100 : PRINT FNF(5)
RUN
 125

READY.
 
  Compare With  
 
  Contrast With  
 
  See Also  

© H2Obsession, 2014
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