With fortran I have always managed to be able to get identical results on the same computer with the same OS. You will have trouble if you switch OS or hardware, or try the same hardware and OS with different math libraries. All the real calculations need to be double, even intermediate variables. Also, I've had trouble with arrays not being initialized to double 0.0. If you initialize to single 0.0 the straggling bits can cause differences. You also need to be careful about conversion from integer to real, to do double conversion. I'm not sure about C, but I would guess there are some of the same problems.
Paul >-----Original Message----- >From: r-devel-boun...@r-project.org [mailto:r-devel-boun...@r- >project.org] On Behalf Of Renaud Gaujoux >Sent: September 10, 2010 6:47 AM >To: r-devel@r-project.org >Subject: [Rd] Non identical numerical results from R code vs C/C++ code? > >Hi, > >suppose you have two versions of the same algorithm: one in pure R, the >other one in C/C++ called via .Call(). >Assuming there is no bug in the implementations (i.e. they both do the >same thing), is there any well known reason why the C/C++ implementation >could return numerical results non identical to the one obtained from >the pure R code? (e.g. could it be rounding errors? please explain.) >Has anybody had a similar experience? > >By not identical, I mean very small differences (< 2.4 e-14), but enough >to have identical() returning FALSE. Maybe I should not bother, but I >want to be sure where the differences come from, at least by mere >curiosity. > >Briefly the R code perform multiple matrix product; the C code is an >optimization of those specific products via custom for loops, where >entries are not computed in the same order, etc... which improves both >memory usage and speed. The result is theoretically the same. > >Thank you, >Renaud > >-- >Renaud Gaujoux >Computational Biology - University of Cape Town >South Africa > > > > >### >UNIVERSITY OF CAPE TOWN > >This e-mail is subject to the UCT ICT policies and e-mail disclaimer >published on our website at >http://www.uct.ac.za/about/policies/emaildisclaimer/ or obtainable from >+27 21 650 4500. This e-mail is intended only for the person(s) to whom >it is addressed. If the e-mail has reached you in error, please notify >the author. If you are not the intended recipient of the e-mail you may >not use, disclose, copy, redirect or print the content. If this e-mail >is not related to the business of UCT it is sent by the sender in the >sender's individual capacity. > >### > >______________________________________________ >R-devel@r-project.org mailing list >https://stat.ethz.ch/mailman/listinfo/r-devel ==================================================================================== La version française suit le texte anglais. ------------------------------------------------------------------------------------ This email may contain privileged and/or confidential information, and the Bank of Canada does not waive any related rights. Any distribution, use, or copying of this email or the information it contains by other than the intended recipient is unauthorized. If you received this email in error please delete it immediately from your system and notify the sender promptly by email that you have done so. ------------------------------------------------------------------------------------ Le présent courriel peut contenir de l'information privilégiée ou confidentielle. La Banque du Canada ne renonce pas aux droits qui s'y rapportent. Toute diffusion, utilisation ou copie de ce courriel ou des renseignements qu'il contient par une personne autre que le ou les destinataires désignés est interdite. Si vous recevez ce courriel par erreur, veuillez le supprimer immédiatement et envoyer sans délai à l'expéditeur un message électronique pour l'aviser que vous avez éliminé de votre ordinateur toute copie du courriel reçu. ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
