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© Niklas 'Tanis of TCB' Malmqvist

 "By working faithfully eight hours a day you may eventually  get
to be a boss and work 12 hours a day."
                                                     Robert Frost


                YOUR SECOND GFA BASIC 3.XX MANUAL
                             - or -
        HOW I LEARNED TO STOP WORRYING AND LOVE GFA-BASIC
                             PART 18
              CHAPTER SEVENTEEN - PROGRAM DECISIONS
                          by Han Kempen

IF ... ENDIF

 If the value of a certain variable must fall in the range  min%-
max%, you could program that as follows using IF ... ENDIF:

     IF n%>max%
       n%=max%
     ELSE IF n%<min%
       n%=min%
     ENDIF

 In this case you could also use MAX and MIN:

     n%=MAX(MIN(n%,max%),min%)

 You probably test for two conditions by using AND:

     IF cond1! AND cond2!
       (...)                  ! both true
     ENDIF

 But separate testing is much faster:

     IF cond1!
       IF cond2!
         (...)                ! both true
       ENDIF
     ENDIF

 Multiple 'IF ...  ENDIF' tests that are mutually exclusive could
be replaced by 'ELSE IF' tests:

     IF cond1!
       (...)
     ELSE IF cond2!
       (...)
     ELSE IF cond3!
       (...)
     ELSE IF cond4!
       (...)
     ELSE IF cond5!
       (...)
     ENDIF

 Now  if one of the tests is TRUE,  the following tests  will  be
skipped. If you use multiple 'IF ... ENDIF' tests, all conditions
will always be tested.

ON ... GOSUB

 It's  important to understand when no Procedure is called in  an
ON .. GOSUB line, e.g.:

  ON p GOSUB p_1,p_2       ! Procedures always without parameters

 In this case variable p determines what happens as follows:

     p = 0          - no Procedure is called
     1 ? p < 2      - Procedure p_1 is called
     2 ? p < 3      - Procedure p_2 is called
     p ? 3          - no Procedure is called

SELECT

 Multiple  'ELSE IF' constructions can sometimes be  replaced  by
'SELECT'.
 At each CASE you can use integers,  strings or integer-variables
(byte|,  word&  or integer%) but not string-variables.  Only  the
first  four bytes of a string can be used.  The editor  will  not
accept something like CASE "test2", only CASE "test".

 If you use SELECT in a loop, you can exit the loop as follows:

     DO
       SELECT INKEY$
       (...)
       CASE CHR$(27)          ! user pressed <Esc>
         EXIT IF TRUE         ! exit the loop
       ENDSELECT
     LOOP

SELECT-bug

 In  a compiled program,  the code between SELECT  and  ENDSELECT
must not exceed 32K or the program will jump to the wrong  CASEs.
Actually  it's more likely that you will be struck  by  lightning
than that you'll ever encounter this bug.  Programmers using more
than  32K between SELECT and ENDSELECT obviously must  have  been
struck  by  lightning  (or  something  else)  already.   If   the
discoverer  of this bug is reading this,  I apologize  sincerely.
Let's hope he or she will not be struck by lightning again.

 Try the following listing:

     REPEAT
       INPUT t$
       SELECT t$
       CASE ""
         PRINT "empty string"
       DEFAULT
         PRINT "a string"
     UNTIL t$="q"

 If  you press <Return> immediately,  you enter the  null-string.
This  is  properly recognized in an interpreted  program,  but  a
compiled program runs amok. The null-string is not recognized and
strange things happen afterwards,  including showing a couple  of
bombs after you pressed "q". 

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The text of the articles is identical to the originals like they appeared in old ST NEWS issues. Please take into consideration that the author(s) was (were) a lot younger and less responsible back then. So bad jokes, bad English, youthful arrogance, insults, bravura, over-crediting and tastelessness should be taken with at least a grain of salt. Any contact and/or payment information, as well as deadlines/release dates of any kind should be regarded as outdated. Due to the fact that these pages are not actually contained in an Atari executable here, references to scroll texts, featured demo screens and hidden articles may also be irrelevant.