Topic       : The GFA-Basic Compendium
Author      : GFA Systemtechnik GmbH
Version     : GFABasic.HYP v2.98 (12/31/2023)
Subject     : Documentation/Programming
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Index Size  : 28056
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Compiled on : Atari
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View Ref-FileFOR c=b TO e [STEP s]
  (instructions)
NEXT i
FOR c=b DOWNTO e
  (instructions)
NEXT i

c: avar
b, e, s: aexp

The FOR-NEXT command (also called a counting loop) is used for repeated
processing a group of instructions between the commands FOR and NEXT. For this
purpose the count variable c is incremented (or decremented), beginning with
the initial value b, as far as the end value 'e'.

The instructions in the loop body are gone through, until NEXT is reached.
There the count variable c is increased by the value s given after STEP. If no
STEP value is given, then an increment of one is assumed. Next c is checked to
see whether it has exceeded the value e. If this is the case, the loop is
exited and the program continued with the command after the NEXT. If not, the
program loops back again to the first instruction, this process is repeated,
until c is larger than e. A consequence of this exit method is that c, after
the loop is left, is equal to e+s, i.e. the first value which exceeds the abort
criterion. Also, the contents of a FOR-NEXT loop will always be gone through at
least once.

Note: In place of a STEP value of -1 the command DOWNTO can be used instead
      of TO. However, with DOWNTO the use of STEP is not possible.

In general, for the count variable c one should use integer variables, because
thereby the loop can be processed more quickly than with floating-point
variables. This is naturally not possible with non-integer STEP increments. In
place of the command NEXT followed by the count variable, the command ENDFOR i%
can be used, but the Interpreter automatically replaces it with NEXT i%.

Examples:

    FOR c=1 TO 10
      PRINT c'
    NEXT c
    FOR c=-1 DOWNTO -10
      PRINT c'
    NEXT c

--> Displays on the screen, the numbers:
    1 2 3 4 5 6 7 8 9 10 -1 -2 -3 -4 -5 -6 -7 -8 -9 -10

    a$="T*e*s*t*w*o*r*d"
    FOR j=1 TO LEN(a$) STEP 2
      PRINT MID$(a$,j,1);
    NEXT j

--> Displays the word 'Testword' on the screen.

Memo: For compiler options see section 'FOR-NEXT Loop Checking'.

      When counting down the counter will end up -1 lower than the set limit.
      When counting up the counter will end up +1 higher than the set limit.
      This can be exploited:
         FOR i&=0 to 32
           PRINT i&
           EXIT IF i&=16
         NEXT
         IF i&=32+1
           PRINT "completed all iterations"
         ELSE
           PRINT "terminated prematurely"
         ENDIF

      A FOR-NEXT loop going backwards with a byte| counter using STEP will
      completely fail when compiled. Such a loop will work in the interpreter
      however. Somehow the loop ends up missing entirely when compiled!

      FOR i|=200 TO 100 STEP -1  !does not work compiled
        PRINT i|;",";
      NEXT i|
      FOR i|=200 DOWNTO 100      !this works compiled
        PRINT i|;",";
      NEXT i|

      FOR-NEXT loops using floats can possibly issue error code #8 if FRE()
      goes too low. One would expect error code #91, however it's never
      actually used anywhere, including the library.