User‐Defined‐Functions

Basic Syntax

Note: Name of user-defined function must have '$' as suffix

DEF FN functionName$(param1$, param2$, ...)
    ' Function body
    RETURN value
END DEF

Simple Function

DEF FN double$(x$)
    RETURN x$ * 2
END DEF

PRINT FN double$(21)    ' Output: 42
PRINT FN double$(5)     ' Output: 10

Multiple Parameters

DEF FN add$(a$, b$)
    RETURN a$ + b$
END DEF

DEF FN multiply$(a$, b$)
    RETURN a$ * b$
END DEF

PRINT FN add$(3, 4)         ' Output: 7
PRINT FN multiply$(3, 4)    ' Output: 12

Array data as parameter

DIM a$
a$("name") = "Foo"
a$("age") = 19
a$(1) = 1

DEF FN func$(a$, b$, c$)
    PRINT a$
    PRINT b$
    PRINT c$
    RETURN 0
END DEF

LET b$ = FN func$(a$("name"), a$("age"), a$(1))
' Output:
' Foo
' 19
' 1

String Functions

DEF FN greet$(name$)
    RETURN "Hello, " + name$ + "!"
END DEF

PRINT FN greet$("Alice")    ' Output: Hello, Alice!

Functions with Logic

DEF FN max$(a$, b$)
    IF a$ > b$ THEN
        RETURN a$
    ELSE
        RETURN b$
    END IF
END DEF

DEF FN min$(a$, b$)
    IF a$ < b$ THEN
        RETURN a$
    ELSE
        RETURN b$
    END IF
END DEF

PRINT FN max$(10, 25)    ' Output: 25
PRINT FN min$(10, 25)    ' Output: 10

Recursive Functions

Functions can call themselves:

DEF FN factorial$(n$)
    IF n$ <= 1 THEN
        RETURN 1
    END IF
    RETURN n$ * FN factorial$(n$ - 1)
END DEF

PRINT FN factorial$(5)    ' Output: 120
PRINT FN factorial$(10)   ' Output: 3628800

Fibonacci

DEF FN fib$(n$)
    IF n$ <= 1 THEN
        RETURN n$
    END IF
    RETURN FN fib$(n$ - 1) + FN fib$(n$ - 2)
END DEF

FOR i$ = 0 TO 10
    PRINT FN fib$(i$); " ";
NEXT
' Output: 0 1 1 2 3 5 8 13 21 34 55

Scope

User-defined functions have their own local scope that is completely isolated from the main program:

• Function parameters are local to the function
• Variables declared with LET inside a function are local to that function
• Functions cannot access or modify global variables from the main program
• Parameters are passed by value - changes to parameters don't affect the original variables

LET b$ = 100              ' Global variable
LET C# = "Foo"            ' Global constant
DEF FN test$(a$)          ' Parameter a$ is local
    LET b$ = 2            ' Local b$ (separate from global b$)
    PRINT C#              ' Global constants are available
    a$ = a$ * b$          ' Uses local a$ (100) and local b$ (2)
    RETURN a$             ' Returns 200
END DEF

In this example:

• The global b$ has value 100
• When calling FN test(b$), the value 100 is passed to parameter a$
• Inside the function, a new local variable b$ is created with value 2
• The local b$ is completely separate from the global b$
• The function calculates a$ * b$ = 100 × 2 = 200
• The global b$ remains unchanged at 100

Labels Inside Functions

Functions can have local labels for GOTO/GOSUB:

DEF FN countdown$(n$)
    [loop]
    IF n$ <= 0 THEN
        RETURN "Done!"
    END IF
    PRINT n$
    n$ = n$ - 1
    GOTO [loop]
END DEF

PRINT FN countdown$(5)

Output:

5
4
3
2
1
Done!

Function Isolation

GOTO and GOSUB inside a function cannot jump outside the function:

[outside_label]
PRINT "Outside"

DEF FN broken$(x$)
    GOTO [outside_label]    ' ERROR!
    RETURN x$
END DEF

This produces: GOTO cannot jump outside function: outside_label

This restriction ensures functions are self-contained and predictable.

Practical Examples

Clamp Value

DEF FN clamp$(value$, minVal$, maxVal$)
    IF value$ < minVal$ THEN
        RETURN minVal$
    END IF
    IF value$ > maxVal$ THEN
        RETURN maxVal$
    END IF
    RETURN value$
END DEF

PRINT FN clamp$(150, 0, 100)   ' Output: 100
PRINT FN clamp$(-50, 0, 100)   ' Output: 0
PRINT FN clamp$(50, 0, 100)    ' Output: 50

Distance Formula

DEF FN distance$(x1$, y1$, x2$, y2$)
    LET dx$ = x2$ - x1$
    LET dy$ = y2$ - y1$
    RETURN SQR(dx$ * dx$ + dy$ * dy$)
END DEF

PRINT FN distance$(0, 0, 3, 4)   ' Output: 5

Is Even/Odd

DEF FN isEven$(n$)
    RETURN MOD(n$, 2) = 0
END DEF

DEF FN isOdd$(n$)
    RETURN MOD(n$, 2) = 1
END DEF

FOR i$ = 1 TO 5
    IF FN isEven$(i$) THEN
        PRINT i$; " is even"
    ELSE
        PRINT i$; " is odd"
    END IF
NEXT

Order of code

Before code can call a user-created function, it must be loaded from the source code.

Wrong

Code below causes error: Error: Line 1: 'ADD' is not a valid variable name (must end with $ or #) or keyword

' incorrect way
PRINT FN ADD$(1, 2)

DEF FN ADD($a$, b$)
	RETURN a$ + b$
END DEF

Right

Code below is valid

' correct way
DEF FN ADD$(a$, b$)
	RETURN a$ + b$
END DEF

PRINT FN ADD$(1, 2)

Tip

If you have several of your own functions, write them in their own file and include them in the code using INCLUDE right at the beginning of your program.