Very interesting. I had never really thought about 'optimizing' an array, but it certainly makes good sense. I appreciate the pointers. Thanks again. --- Joe Pluta <joepluta@PlutaBrothers.com> wrote: > 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: email@example.com > > [mailto:firstname.lastname@example.org]On Behalf Of > Richard Reeve > > Sent: Sunday, December 02, 2001 5:33 AM > > To: email@example.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 > > > > > > _______________________________________________ > > > This is the Midrange Systems Technical > Discussion > > > (MIDRANGE-L) mailing list > > > To post a message email: MIDRANGE-L@midrange.com > > > To subscribe, unsubscribe, or change list > options, > > > visit: > > > > > > http://lists.midrange.com/cgi-bin/listinfo/midrange-l > > > or email: MIDRANGE-Lfirstname.lastname@example.org > > > Before posting, please take a moment to review > the > > > archives > > > at http://archive.midrange.com/midrange-l. > > > > > > > > > ===== > > > > > > __________________________________________________ > > Do You Yahoo!? > > Buy the perfect holiday gifts at Yahoo! Shopping. > > http://shopping.yahoo.com > > _______________________________________________ > > This is the Midrange Systems Technical Discussion > (MIDRANGE-L) > > mailing list > > To post a message email: MIDRANGE-L@midrange.com > > To subscribe, unsubscribe, or change list options, > > visit: > http://lists.midrange.com/cgi-bin/listinfo/midrange-l > > or email: MIDRANGE-Lemail@example.com > > Before posting, please take a moment to review the > archives > > at http://archive.midrange.com/midrange-l. > > _______________________________________________ > This is the Midrange Systems Technical Discussion > (MIDRANGE-L) mailing list > To post a message email: MIDRANGE-L@midrange.com > To subscribe, unsubscribe, or change list options, > visit: > http://lists.midrange.com/cgi-bin/listinfo/midrange-l > or email: MIDRANGE-Lfirstname.lastname@example.org > Before posting, please take a moment to review the > archives > at http://archive.midrange.com/midrange-l. > ===== __________________________________________________ Do You Yahoo!? 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