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VRF-Feedback_NoNo

Question asked by VRFuser on Feb 18, 1999
send q VRF-Feedback_NoNo ww vrf

from: Greg Goebel / HP-MXD
      greg_goebel@hp.com / 800-452-4844
      website:  ftp://fcext3.external.hp.com/dist/mxd/index.html
to:   VRF-David Howe
date: Friday, 19 February 1999 1441 MST

Sir:

Chicken and egg problem.  Use of feedback in this fashion can confuse the
VEE engine, since it makes inputs predicated on outputs.

There is a much cleaner way to do this:

   +---------+
   | Step 1  |
   +----+----+
        |
   +----+----+
   | Step 2  |
   +----+----+
        |
    +---+---+
    | Until |
    | Break +--------+
    +---+---+        |
        |       +----+----+
        |       | Step 3  |
        |       +----+----+
        |            |
        |       +----+----+  NO
        |       | Repeat? +------+  don't worry about the YES output,
        |       +---------+      |  loops by default.
        |                    +---+---+
        |                    | Break |
   +----+----+               +-------+
   | Step 4  |
   +---------+

What's the rule to recognize a bad VEE program in this respect?   "Don't use
feedback."

See the VEE Notebook on FCEXT3 (URL above) for a good control-flow tutorial
(if in need of some updating, immediately pending by the way).

[%%] regards -- gvg

--------------------------------- cut here ----------------------------------
>From: DAVID_HOWE@HP-Andover-om3.om.hp.com
Date: Fri, 19 Feb 1999 15:50:41 -0500
Subject: VRF - Feedback and Program Flow

     Hi, VRF,
    
     I'm having some trouble getting the correct program flow when I try to
     use "looping" structures.
    
     Basically, I a bunch of objects connected in series ("vertically"). 
     But, at some point, based on user input, I may want to "back up" in
     the program, and repeat a few steps, like this:
    
    
               Step 1
                 |
                 |
                 |
               Step 2
                 |   ------------
                 |   |          |
                 ---JCT--|      |
                         |      |
                       Step 3   |
                         |      |
                      Repeat?-Y-|
                         |
                         N
                         |
                       Step 4
                         |
                        End
    
     The problem is, when I try this, I get an error message from VEE that
     tell me "Illegal use of junctions inside of feedback cycles".  Can
     somebody out there give me some general guidance on what is legal and
     what is illegal in this situation?  My program certainly looks "legal"
     to me (of course, I'm sure that's the basic problem in the first
     place... ).  I'm using VEE 4.0 under UNIX.
    
     Thanks for your help,
     --Dave Howe



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