RE: Infinit loop in /free

["Steve Richter" <srichter@xxxxxxxxxxxxx>]

well I just had to test this ( comparing a count to a 31 digit packed
decimal hival number seems a bit wasteful ) ...

the following million iteration loop runs for 63 milliseconds on a 170/2386
( 400+ cpw ):
d conLoopCx       c                   1000000
d conTrue         c                   '1'
d BgnTs           s               z
d EndTs           s               z
d Ix              s             10i 0

  BgnTs       = %timestamp( ) ;
  dow         conTrue ;
    Ix        = Ix + 1 ;
    if        Ix > conLoopCx ;
      leave ;
    endif ;
  enddo ;
  EndTs       = %timestamp( ) ;

and this do *hival version runs for 390 milliseconds
    BgnTs       = %timestamp( ) ;
    Ix          = 0 ;
  /end-free
 C                   do        *hival
 c                   eval      Ix = Ix + 1
 c                   if        ix > conLoopCx
 c                   leave
 c                   endif
 c                   enddo
  /free
   EndTs       = %timestamp( ) ;

-Steve


-----Original Message-----
From: rpg400-l-bounces@xxxxxxxxxxxx
[mailto:rpg400-l-bounces@xxxxxxxxxxxx]On Behalf Of Hans Boldt
Sent: Friday, November 21, 2003 9:17 AM
To: rpg400-l@xxxxxxxxxxxx
Subject: Re: Infinit loop in /free


Steve Richter wrote:
>
>>I have used in fixed format RPG the statements Do *HIVAL for an infinit
>
> loop
>
>>and, for instance,  DO 7 for a fixed loop. I can't find a nice
translations
>>into /free for these statements. I don't like, for instance DOW 1=1, or
>>constructions in which I have to use a variable.
>
>
> doesnt "do *hival" mean to loop a large number of times? maybe 4 billion
> times?
>
> I would guess that that kind of a loop requires the compiler to add to a
> count field and compare the count to *hival on each iteration.  If so,
then
> that is more overhead that "dow 1".
>

First, technically (and philosophically) there's no such thing as an
"infinite loop". But that's a discussion better left to another time and
place. ;-)

But to the question at hand, which is, does "DO *HIVAL" loop an
indeterminate number of times? Well, technically, yes. At least, if you
compile using TRUNCNBR(*YES), which is the default. By default, decimal
arithmetic on fixed-form calcs (but not the expression calcs) truncates
on overflow. That means that when the implicit loop index reaches the
value of *HIVAL, rather than overflowing when it's incremented again, it
gets truncated. Thus, the implicit loop index never exceeds the limit,
and the loop keeps churning away. (At least until the operator notices
and cancels the job!)

In general though, iterating for *HIVAL number of times takes a long
long time. (Decimal *HIVAL on V5R2 is 10**31-1.) And if *HIVAL
iterations seems to take an eternity on a V5R2 machine, it will seem
even longer on V5R3! ;-)

But in my opinion, "DO *HIVAL" is bad style. Other techniques, like the
"DOU FOREVER;" mentioned by someone else, are preferable.

Cheers! Hans


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