Topic       : The GFA-Basic Compendium
Author      : GFA Systemtechnik GmbH
Version     : GFABasic.HYP v2.98 (12/31/2023)
Subject     : Documentation/Programming
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View Ref-FileIF condition [THEN]
  (instructions)
ELSE
  (instructions)
ENDIF

condition: bexp

These commands enable one to specify that a section of a program will only be
executed if a logical condition is met. The following example demonstrates
this:

    IF a=1 THEN
      PRINT "a is equal to 1"
      b=2
    ENDIF

In this case a=1 is the logical condition. The instructions in the lines
between IF and ENDIF are processed only if this logical condition is
met.  If it is untrue, then the program continues with the commands after the
ENDIF and the command THEN is not considered. It should be noted that it is
sufficient to use the form:

    IF a=1   instead of   IF a=1 THEN

The following construction is somewhat more complex:

    IF a=1
      PRINT "a is equal to 1"
    ELSE
      PRINT "a is not equal to 1,"
      PRINT "it is equal to ";a
    ENDIF

In this case shown above, the instructions between IF and ELSE are processed if
the logical condition after the IF is true. The program then continues
execution after the ENDIF. However, if the condition after IF is not fulfilled,
then the instructions between ELSE and ENDIF become effective. Again, program
execution is continued after the ENDIF.

Note: Any numerical expression, which is not equal to 0, i.e. is FALSE, is
      considered to be true. The logical value for TRUE is -1 but any non-zero
      value is considered to be equivalent.

Example:

    x=1
    IF x
      PRINT "x is true"
    ENDIF
    INPUT y
    IF x=9 OR ODD(y)
      PRINT "y is an odd number"
    ELSE
      PRINT "y is an even number"
    ENDIF

--> First the message 'x is true' is displayed and then a numeric input is
    requested. Since x cannot be 9, the text appears 'y is an odd number', if
    you have entered an odd number, otherwise 'y is an even' is displayed.

Memo: This fails compiled:
      IF FALSE  !the entire IF-ENDIF structure ends up missing!
      ENDIF
      IF 0      !same issue!
      ENDIF
      Seems ok in the interpreter.