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ZX Basic allows function and subroutines declarations. This feature is new to ZX Basic. Sinclair BASIC allowed limited function definitions using DEF FN.

A function is a subroutine (like GO SUB) that is invoked directly. The subroutine returns a value than can be used later. BASIC language already has some predefined functions, like SIN, COS, PEEK or LEN. The user is now allowed to define his/her own functions.


Basic function declaration is:

FUNCTION <function name>[(<paramlist>)] [AS <type>]


A simple function declaration:

span style="color: #808080;"> REM 'This function receives and returns a byte
FUNCTION PlusOne(x AS Byte) AS Byte
    RETURN x + 1
REM 'Using the function
PRINT x; " plus one is "; PlusOne(x)

If AS <type> is omitted, the function is supposed to return a FLOAT.

span style="color: #808080;"> REM 'This function returns a float number because its type has been omitted.
REM 'Also, the x parameter will be converted to float,
REM 'because it's type has been omitted too
FUNCTION Square(x)
    RETURN x^2
REM 'Using the function
PRINT "Square of "; x; " is "; Square(x)


Recursion is a programming technique in which a function calls itself. A classical recursion example is the factorial function:

FUNCTION Factorial(x)
    IF x < 2 THEN RETURN x: END IF
    RETURN Factorial(x - 1) * x

However, not using types explicitly might have a negative impact on performance. Better redefine it using data types. Factorial is usually defined on unsigned integers and also returns an unsigned integer. Also, keep in mind that factorial numbers tends to grow up very quickly (e.g. Factorial of 10 is 3628800), so ULONG type (32 bits unsigned) seems to be the most suitable for this function.

This version is faster (just the 1st line is changed):

FUNCTION Factorial(x AS Ulong) AS Ulong
    IF x < 2 THEN RETURN x: END IF
    RETURN Factorial(x - 1) * x

Memory Optimization

If you invoke zxbasic using -O1 (or higher) optimization flag the compiler will detect and ignore unused functions (thus saving memory space). It will also issue a warning (perhaps you forgot to call it?), that can be ignored.