Thanks to you all. I stand corrected Ted and Manuela:) I am just an end user and trying to pick up from such forums. Many thanks sirs.
On Mon, Jul 12, 2010 at 5:45 PM, Huso, Manuela <manuela.h...@oregonstate.edu > wrote: > Using Ted Harding's example: > > news1o <- runif(1000000) > s2o <- runif(1000000) > > pt1 <- proc.time() > s <- numeric(length(news1o))-1 # Set all of s to -1 > s[news1o>s2o] <-1 # Change to 1 only those values of s > # for which news1o>s2o > pt2<- proc.time() > pt2-pt1 # Takes even less time... > # user system elapsed > # 0.04 0.00 0.05 > > >::<>::<>::<>::<>::<>::<>::<>::<>::<>::<>::<>::< > Please note: I will be out of the office and out > of email contact from 7/11-7/25/2010 > >::<>::<>::<>::<>::<>::<>::<>::<>::<>::<>::<>::< > Manuela Huso > Consulting Statistician > 201H Richardson Hall > Department of Forest Ecosystems and Society > Oregon State University > Corvallis, OR 97331 > ph: 541-737-6232 > fx: 541-737-1393 > > > -----Original Message----- > From: r-help-boun...@r-project.org [mailto:r-help-boun...@r-project.org] > On Behalf Of Ted Harding > Sent: Monday, July 12, 2010 9:36 AM > To: r-help@r-project.org > Cc: Raghu > Subject: Re: [R] in continuation with the earlier R puzzle > > On 12-Jul-10 14:09:30, Raghu wrote: > > When I just run a for loop it works. But if I am going to > > run a for loop every time for large vectors I might as well > > use C or any other language. > > The reason R is powerful is becasue it can handle large vectors > > without each element being manipulated? Please let me know where > > I am wrong. > > > > for(i in 1:length(news1o)){ > > + if(news1o[i]>s2o[i]) > > + s[i]<-1 > > + else > > + s[i]<--1 > > + } > > > > -- > > 'Raghu' > > Many operations over the whole length of vectors can be done > in "vectorised" form, in which an entire vector is changed > in one operation based on the values of the separate elemnts > of other vectors, also all take into account in a single > operation. What happens "behind to scenes" is that the single > element by element operations are performed by a function > in a precompiled (usually from C) library. Hence R already > does what you are suggesting as a "might as well" alternative! > > Below is an example, using long vectors. The first case is a > copy of your R loop above (with some additional initialisation > of the vectors). The second achieves the same result in the > "vectorised" form. > > news1o <- runif(1000000) > s2o <- runif(1000000) > s <- numeric(length(news1o)) > > proc.time() > # user system elapsed > # 1.728 0.680 450.257 > for(i in 1:length(news1o)){ ### Using a loop > if(news1o[i]>s2o[i]) > s[i]<- 1 > else > s[i]<- (-1) > } > proc.time() > # user system elapsed > # 11.184 0.756 460.340 > s2 <- 2*(news1o > s2o) - 1 ### Vectorised > proc.time() > # user system elapsed > # 11.348 0.852 460.663 > > sum(s2 != s) > # [1] 0 ### Results identical > > Result: The loop took (11.184 - 1.728) = 9.456 seconds, > Vectorised, it took (11.348 - 11.184) = 0.164 seconds. > > Loop/Vector = (11.184 - 1.728)/(11.348 - 11.184) = 57.65854 > > i.e. nearly 60 times as long. > > Ted. > > -------------------------------------------------------------------- > E-Mail: (Ted Harding) <ted.hard...@manchester.ac.uk> > Fax-to-email: +44 (0)870 094 0861 > Date: 12-Jul-10 Time: 17:36:07 > ------------------------------ XFMail ------------------------------ > > ______________________________________________ > 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. > -- 'Raghu' [[alternative HTML version deleted]] ______________________________________________ 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.
