Thanks for all the suggestions! The tricky part isn't finding the common factors -- we knew how to do that, though not in so concise a fashion as some of these suggestions. It was finding all their products without what I (as a recovered Fortran programmer) would call "truly brute force." Several of these suggestions solve the problem nicely!
Gordon P.S. The numbers involved will never be very large -- these are dimensions of areas in which trees were sampled, in meters. They'll always be on the order of 50-100m or so on a side. -- Dr. Gordon A. Fox Voice: (813)974-7352 Fax: (813)974-3263 Dept. of Integrative Biology ((for US mail:)SCA 110) ((for FedEx etc:)NES 107) Univ. of South Florida 4202 E. Fowler Ave. Tampa, FL 33620, USA http://foxlab.cas.usf.edu "All the fun of sitting still, being quiet, writing down numbers. Yes, science has it all." -- Principal Skinner -----Original Message----- From: macra...@gmail.com [mailto:macra...@gmail.com] On Behalf Of Stavros Macrakis Sent: Wednesday, February 25, 2009 2:02 AM To: Fox, Gordon; r-help@r-project.org Subject: Re: [R] All the products of common factors Argh! The second (concise) version should have |, not & !!! -s On 2/24/09, Stavros Macrakis <macra...@alum.mit.edu> wrote: > "L'esprit de l'escalier" strikes again.... > > An even simpler statement of your original problem: > > Find the factors that A and B have in common. > > If A and B are fairly small (< 1e7, say), a very direct approach is: > > which( ! (A %% 1:min(A,B)) & !(B %% 1:min(A,B)) ) > > Is this "brute force"? Well, I suppose, but it is simple and direct > and fast enough for A=B=1e7 (5 sec). It doesn't involve factorization > into prime factors, GCDs, or combinations. > > If your goal is concision and not clarity or speed, you can do even better: > > which( !(A %% 1:B & B %% 1:A) ) > > which is still practical (though it gives a warning). > > How big do your A and B get, and how many different A's and B's do you > need to run this calculation for? > > -s > ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
