RE: array handling -- MIDRANGE-L

No, you're right on-base here, Rich.  You can significantly reduce your
overhead this way; I use caching a lot for small tables.  Since the number
of entries is so small, I don't think we'll be able to significantly reduce
your processing time by the backloading technique, but let me explain it and
see if you're interested in implementing it.

A LOOKUP operation has to do "N" compare operations.  The worst case is an
unsorted array where there is no hit; in this case, "N" is the number of
entries in the array.  (Of course, if you're getting REALLY pedantic, it's
actually doing twice that many compares, along with a number of increments
equal to the size of the array, but I think you get the picture.)

As your array gets larger, this can add up to a lot of cycles.  There are
two ways to reduce the number of compares.  First is to order the array -
that is, make sure the array is in sequence.  Knowing that, the compiler can
generate a much more efficient LOOKUP algorithm (using a binary scan), where
the maximum number of compares is actually ((number of entries) log 2).  For
a 20 element array, that's five compares, or a potential savings of 75%.  On
larger arrays, the savings is even more significant.  The only downside is
making sure that all the unused entries at the end of your array have "high
values", such as X'FF'.

Now you run into the problem of unused elements.  Let's say you make your
array 1000 elements long, but you only use 20.  In that case, most of the
time is spent chceking unused elements.  Even with an ascending array, you
still have to do 10 compares, twice the number required to handle 32
entries.  One of the nice things about the LOOKUP operation in RPG is that
you can specify a starting position in the array; it will only scan the
elements from that index forward.  So, what you do is you load the array
BACKWARD, starting with the last position in the array.  Then, whenever you
do a LOOKUP, you start with the last position that you loaded.  Depending on
the number of entries and the size of your array, you can probably see that
this can significantly reduce your processing.

So what's the best for you?  In your case, since the number of elements
doesn't fluctuate very much, you should be able to tune your array size to
be close to the number of entries you need.  You may occasionally have to
resize your array, and you MAY want to put a check in your maintenance
program to notify you if someone tries to add more records to the file than
your cache program expects.  But the backloading technique is probably
overkill - it's more relevant when you have to define a really BIG array
that may only have a small number of elements in it.

On the other hand, since you ARE doing this for performance reasons, making
your array ascending may be a smart move.  Regardless of the size, ascending
arrays allows the compiler to generate the far more efficient binary search
algorithm, which should reduce processing even in your case.  Simply add the
ASCEND keyword to the D-spec defining the array.  Then, before you load your
array, do a MOVEA *HIVAL to preload the array with X'FF':

D ARY1            S              2    DIM(20) ASCEND
(...)
C                   MOVEA     *HIVAL        ARY1

Hope this helps a little.

Joe Pluta
www.plutabrothers.com


> -----Original Message-----
> From: midrange-l-admin@midrange.com
> [mailto:midrange-l-admin@midrange.com]On Behalf Of Richard Reeve
> Sent: Sunday, December 02, 2001 5:33 AM
> To: midrange-l@midrange.com
> Subject: RE: array handling
>
>
> Joe,
>
>      You hit the nail on the head.  I am doing this
> for performance reasons.  There are currently less
> than 20 hold codes (much less actually, currently
> using 4 so I defined the array(s) as 20.  Without
> doing something along the lines that I described then
> I would have to go out and read the table file for
> each hold transaction rather than loading the array
> one time and doing a lookup for each hold transaction
> (to get corresponding code).  I should always get a
> hit on the table (after loading).
>
> Please let me know if you think that I am missing
> something.....
>
> --- Joe Pluta <joepluta@PlutaBrothers.com> wrote:
> > Richard, one quick question: are you doing this for
> > performance reasons?
> > That is, are you caching data in order to avoid
> > doing reads on the file?
> >
> > If not, I'd be interested to know why you are doing
> > this, since it can cause
> > maintenance headaches down the road - you must
> > change the program if the
> > number of entries ever exceeds the size of your
> > array.
> >
> > If you *are* doing this for performance, will there
> > be a significant number
> > of times when you get a "miss" rather than a "hit"?
> > If the answer is yes,
> > you might want to "backload" the arrays; this can
> > substantially reduce your
> > processing time.  Let me know and I'll go into this
> > a little further.
> >
> > Joe
> >
> > > -----Original Message-----
> > > From: Richard Reeve
> > >
> > > Thanks to all who responded.  This list is
> > certainly a
> > > life saver.
> > >
> > > Happy Holidays to all.
> > >
> > > Rich
> >
> > _______________________________________________
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