Re: A Reap Sow question

*To*: mathgroup at smc.vnet.net*Subject*: [mg65232] Re: A Reap Sow question*From*: albert <awnl at arcor.de>*Date*: Sun, 19 Mar 2006 03:19:09 -0500 (EST)*References*: <200603170513.AAA09771@smc.vnet.net> <4EF26559-3634-461D-9E5E-6522C06A6751@mimuw.edu.pl> <dvgsf0$9qe$1@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

Hi JÃ¡nos, > So, my situation is similar to Trurl's. Right now I am appending to > a list and as I am adding more and more items to it by using > AppendTo, I have to pull back some earlier members of the list, > because they are needed to create the new members of the list. This > method works satisfactory for the first 60 - 80 members of the list > and my G5 duo core constructor - although it is NOTHING compared to > Trurl - is happy to churn out them in less than a minute. As soon as > I have about 110 items, it takes half an hour to put one new item > into it. So I was thinking to give up on AppendTo and use Reap Sow > to collect the list elements, but on the same time I needed to look > into the list for earlier elements, because I needed them for > construction. I'm not sure if I fully understood what you want and I also am not sure whether my following suggestion will be a solution concerning speed, but since AppendTo is a known speed-killer it might be worth a try. Have you tried to use DownValues instead of a real list? That has some overhead ,too, but as far as my experience goes, you won't suffer much if the number of entries gets big, at least not for a few hundert entries. To find the last entry you might need to store the current max index in an additional variable. That is, instead of using: (* initialization *) collection = {} (* append element *) AppendTo[collection,element] use: (* initialization *) $maxindex = 0 container[$maxindex++] = element accessing "list" elements is straightforward then: collection[[i]] will be container[i] and Last[collection] will be container[$maxindex] of course you could encapsulate all this in some functions that mimic the regular list interface (e.g. by defining functions part[], appendto[], first[] and last[]) if it turns out to solve your problem. It might also well be that there are better ways to do it. Anyway it will be possible to access previously defined entries in a while loop unlike when using the Reap-Sow version and my guess is it will be considerably faster than AppendTo... hth, albert

**References**:**A Reap Sow question***From:*János <janos.lobb@yale.edu>