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| Keyword | Abbreviation | Token (hex) | Version(s) | Classification | | FN | none | A5 | 1.0+ | Function and Preposition |
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 | |
Calculate the value of a user-defined function (evaluate expression with parameter replaced by argument).
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
READY.
PRINT FNY(3)
?UNDEF'D FUNCTION ERROR
READY.
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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.
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