RE: Opinions Wanted: List API loop logic

[CWilt@xxxxxxxxxxxx]

Scott,

I've always done it using your original method.  I'll be following this to
see what everybody comes up with.

I had brainstorm, but like many it was nice sounds and lights..but little of
substance. <grin>

But I'll throw it out it case it leads somebody else in another direction.

Basis: hey we can do pointer arithmetic right?

     c                   eval      p_Limit = p_ListHeader + DataOffset

     c                                     + ( NumEntries * ( EntrySize - 1
) ) 
 

     c                   for       p_ListEntry = p_ListHeader + DataOffset

     c                                  to p_Limit

     c                                  by EntrySize

 

     c***       ... at this point, the JOBL0100 data structure contains

     c***       ... one of the entries of the list.

                         //do something

     c                   endfor



But the FOR op-code didn't appreciate my genius!

Charles


> -----Original Message-----
> From: Scott Klement [mailto:klemscot@xxxxxxxxxxxx]
> Sent: Thursday, May 20, 2004 6:59 PM
> To: rpg400-l@xxxxxxxxxxxx
> Subject: Opinions Wanted: List API loop logic
> 
> 
> 
> First, some background:
> 
> In this morning's "Club Tech iSeries Programming Tips" newsletter, I
> listed the following code snippet:
> 
>      c                   eval      p_ListEntry = p_ListHeader 
> + DataOffset
> 
>      c                   for       EntryNo = 1 to NumEntries
> 
>      c***       ... at this point, the JOBL0100 data 
> structure contains
>      c***       ... one of the entries of the list.
> 
>      c                   eval      p_ListEntry = p_ListEntry 
> + EntrySize
>      c                   endfor
> 
> This code is intended to run through a list of entries in a 
> user space as
> a demonstration of how to use the list APIs.
> 
> Now, because I have my pointer incremented at the end of the 
> loop, it'll
> always do it one extra time.  Barbara tells me that it's possible that
> p_ListEntry could be set to an illegal location, causing an 
> MCH0601 error.
> (Before reading her note, I had thought that as long as I 
> didn't use the
> data that the pointer points to after the loop ends, I was safe.)
> 
> Even if the user space is set to auto-extend, it could still cause a
> problem because there's a limit to how large a user space 
> might be, and I
> might happen to hit that limit exactly.  (okay, it's not all 
> that likely,
> but still possible...)
> 
> Now my question:
> 
> Which of the following code samples do you think is the best 
> way to do it,
> and why?
> 
> This is the way I used to write this sort of loop, but I've found that
> maintenance programmers don't like the way the pointer logic works, it
> confuses them, plus it's ugly when wrapped to two-lines:
> 
>       c                   eval      p_ListStart = 
> p_ListHeader + DataOffset
> 
>       c                   for       EntryNo = 1 to NumEntries
>       c                   eval      p_ListEntry = p_ListStart +
>       c                                     ( EntrySize * 
> (EntryNo - 1) )
> 
>       c***       ... at this point, the JOBL0100 data 
> structure contains
>       c***       ... one of the entries of the list.
> 
>       c                   endfor
> 
> 
> Here's another alternative, where the pointer work is done at 
> the start of
> the loop... but since it already starts on the first entry of 
> the list,
> it's necessary to skip the pointer math the first time 
> through.  I don't
> know how intuitive it is to do it this way:
> 
>     c                   eval      p_ListEntry = p_ListHeader 
> + DataOffset
> 
>     c                   for       EntryNo = 1 to NumEntries
> 
>     c                   if        EntryNo <> 1
>     c                   eval      p_ListEntry = p_ListEntry + 
> EntrySize
>     c                   endif
> 
>     c***       ... at this point, the JOBL0100 data structure contains
>     c***       ... one of the entries of the list.
> 
>     c                   endfor
> 
> Finally, here's a similar example to the last one, except 
> that it sets the
> first entry inside the loop as well as the others:
> 
> 
>     c                   for       EntryNo = 1 to NumEntries
> 
>     c                   if        EntryNo = 1
>     c                   eval      p_ListEntry = p_ListHeader 
> + DataOffset
>     c                   else
>     c                   eval      p_ListEntry = p_ListEntry + 
> EntrySize
>     c                   endif
> 
>     c***       ... at this point, the JOBL0100 data structure contains
>     c***       ... one of the entries of the list.
> 
>     c                   endfor
> 
> So, what does everyone think?  Which one is the most intuitive of the
> bunch?  Or, does anyone have a better idea?
> 
> Thanks!
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