RE: array handling -- MIDRANGE-L

You can send to richreeve@yahoo.com....

--- Booth Martin <Booth@MartinVT.com> wrote:
> Richard, then come to our Vermont User Group meeting
> next time around, at
> least.  Let me know your e-mail address and I'll add
> you to the invite list.
>  Plus, ought to be lots of ways to have a dinner
> some time along.
>
>
---------------------------------------------------------
> Booth Martin http://www.MartinVT.com
> Booth@MartinVT.com
>
---------------------------------------------------------
> -------Original Message-------
> From: midrange-l@midrange.com
> Date: Sunday, December 02, 2001 04:03:24 PM
> To: midrange-l@midrange.com
> Subject: RE: array handling
> Thanks for the pointers. It's always a bit rough
> when
> the client brings you in for one thing and winds up
> asking for something else once you get on site. One
> can't really refuse, especially in this economy.
> I'll be here in VT for at least 6 months (can't miss
> the ski season!) and possibly up to 2 years so we've
> got plenty of time to exchange war stories. Let me
> know when is good for you.
> --- Booth Martin <Booth@MartinVT.com> wrote:
> > If you are going to sort a few elements into a
> large
> > array in ascending
> > order you can also set a counter and increment it
> > with each addition to the
> > array as you load it. Then after the array is
> > loaded, sort the array, and
> > set your counter's value at the (array size -
> > counted value). At that point
> > you now have a valid initial value for your
> look-up.
> >
> > In the example that Joe used, then the look up
> would
> > begin at element (1000
> > - 20) or at element 980, so there'd be 979
> elements
> > ignored in the lookup
> > process.
> >
> > Vermont eh, Rich? great place. If you're around
> > this week I'll take you
> > out to dinner some night and we can talk AS/400
> war
> > stories. :)
> >
> >
>
---------------------------------------------------------
> > Booth Martin http://www.MartinVT.com
> > Booth@MartinVT.com
> >
>
---------------------------------------------------------
> >
> > -------Original Message-------
> > From: midrange-l@midrange.com
> > Date: Sunday, December 02, 2001 07:16:44 AM
> > To: midrange-l@midrange.com
> > Subject: RE: array handling
> > 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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