Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uwm.edu!rpi!zaphod.mps.ohio-state.edu!usc!snorkelwacker!bloom-beacon!eru!luth!sunic!mcsun!unido!balu!gamber@cosmo.UUCP
From: gamber@cosmo.UUCP (Johannes Teich)
Newsgroups: comp.lang.forth
Subject: +FOR...NEXT
Keywords: loops
Message-ID: <5628@balu.UUCP>
Date: 8 May 90 10:49:05 GMT
Sender: news@balu.UUCP
Reply-To: gamber@cosmo.UUCP (Johannes Teich)
Organization: CosmoNet, D-3000 Hannover 1, FRG
Lines: 48

Forthers,
I'm glad to have them both running:
 
              +--------+                     +--------+
       FOR    |  For   |              ?DO    |  ?Do   |
              +--------+                     +--------+
              |  offs  |--.                  |  offs  |--.
              +--------+  |                  +--------+  |
              |        |<-|-.                |        |<-|-.
              |        |  | |                |        |  | |
       LEAVE  | Leave  ---|-|-.       LEAVE  | Leave  ---|-|-.
              |        |  | | |              |        |  | | |
       LEAVE  | Leave  ---|-|-.       LEAVE  | Leave  ---|-|-.
              |        |  | | |              |        |  | | |
              +--------+  | | |              +--------+  | | |
       NEXT   |  Next  |<-' | |       LOOP   |  Loop  |  | | |
              +--------+    | |              +--------+  | | |
              |  offs  |----' |              |  offs  |--|-' |
              +--------+      |              +--------+  |   |
              |        |<-----'              |        |<-' <-'
 
 
        FOR ... I ... LEAVE ... LEAVE ... NEXT
        ?DO ... I ... LEAVE ... LEAVE ... LOOP   ( or DO and/or +LOOP )
 
There are 3 values on the return stack: jump-addr reference loop-index.
The jump-addr is for LEAVE . The reference is for FORTH-83 & speed.
The word  I  does a subtraction:  R> DUP R@ - SWAP >R  (it could be an
addition like in Zen as well).
 
The reference is always zero for FOR...NEXT , but must be present because
of LEAVE , I and J .  R@ is the same as  I , then, but quicker.
 
Now I think of something like  n +FOR  which sets the reference to n.
 
                       10 4 +FOR ... NEXT         -->   9 8 7 6 5 4
doing the same as:     4 9 DO ... -1 +LOOP ( FORTH-83 )
 
The reference value would no longer be waisted.
What do others think about +FOR or whatever it should be called?
 
I prefer offsets over absolut addresses (as used in Pygmy), for I plan to
shift the code in memory, and metacompiling becomes a little simpler, too.
Btw, my 0 FOR...NEXT loops zero times. (In addition, cmFOR is available.)
 
   cheers,  --Hannes                               | gamber@cosmo.uucp |
                                                   | fido 2:507/414.20 |