[OpenBIOS] C's continue statement in Forth?

[Programmingkid]

On Aug 25, 2012, at 8:19 AM, openbios-request at openbios.org wrote:

>> I was wondering if there is a way to implement C's continue  
>> statement in Forth. I have this loop, and some times I only want to  
>> execute part of it, and then continue on to the next iteration  
>> without finishing the current iteration. Is there any way of doing  
>> this in OpenBIOS?
> 
> It certainly can be done.  But it sounds like you are making a
> word with a very big body; this is not the Forth way.  Instead,
> factor your words into smaller words, for example, pull the loop
> body (the part between WHILE and REPEAT) into a separate word.
> It then becomes trivial to do your "continue" (and it will also
> be much more readable!)
> 
>> begin
>> \ condition
>> while
>> 
>> \  conditional code
>> if
>> 	CONTINUE		\ skip rest of loop
>> then
>> 
>> \ rest of loop ...
>> 
>> repeat
> 
> You don't have anything in here that "increases" the condition
> (increases some counter, follows a pointer, whatever).  Typically
> you would have that just before the REPEAT, but your example makes
> it seem you have it between the BEGIN and WHILE.  So your code is:
> 
>    BEGIN cond WHILE smth IF CONTINUE THEN rest REPEAT
> 
> which you can write as
> 
>    BEGIN cond WHILE smth 0= IF rest THEN REPEAT
> 
> (and if you do have an "increment" just before the REPEAT, which
> you want to run on CONTINUE, you can put it between the THEN and
> REPEAT in the modified example).
> 
> But, let's build a CONTINUE like you described, just for fun.
> Let's look at the structure words you used:
> 
> BEGIN ( C: -- dest )
> WHILE ( C: dest -- orig dest )
> REPEAT ( C: orig dest -- )
> 
> You want to use your CONTINUE word between WHILE and REPEAT, and
> it should jump back to "dest"; at compile time, it should not drop
> the "dest" from the compilation stack.  So its stack diagram is
> 
> CONTINUE ( C: orig dest -- orig dest )
> 
> or just
> 
> CONTINUE ( C: dest -- dest )
> 
> For doing an unconditional jump back, there is AGAIN, which is
> 
> AGAIN ( C: dest -- )
> 
> That eats the dest from the compilation stack though, so we want
> to duplicate it first, using CS-DUP
> 
> CS-DUP ( C: dest -- dest dest )
> 
> Not every system has that; just make it from CS-PICK, like so:
> 
> : CS-DUP  0 CS-PICK ; IMMEDIATE
> 
> And then let's do CONTINUE itself:
> 
> : CONTINUE  CS-DUP POSTPONE AGAIN ; IMMEDIATE
> 
> Or you might want a ?CONTINUE which does the equivalent of
> IF CONTINUE THEN:
> 
> : ?CONTINUE  POSTPONE 0= CS-DUP POSTPONE UNTIL ; IMMEDIATE
> 
> 
> Hope this helps,
> 
> 
> Segher


You are right. My code was a little too long, so refactoring it fixed the problem. I guess I was still programming like a C programmer still. Thanks for the help.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.openbios.org/pipermail/openbios/attachments/20120827/7947de5a/attachment.html>