Re: Opinions Wanted: List API loop logic

["Keith Carpenter" <CarpCon@xxxxxxx>]

Scott,

Interesting point about the potential MCH0601 error.

Here's another variation on your examples.

c                   eval      p_ListEntry = p_ListHeader + DataOffset
                                                                            
- EntrySize

 c                   for       EntryNo = 1 to NumEntries
c                   eval      p_ListEntry = p_ListEntry + EntrySize

 c***       ... at this point, the JOBL0100 data structure contains
c***       ... one of the entries of the list.

c                   endfor


Keith



----- Original Message ----- 
From: "Scott Klement" <klemscot@xxxxxxxxxxxx>
To: <rpg400-l@xxxxxxxxxxxx>
Sent: Thursday, May 20, 2004 3:58 PM
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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