In terms of speed Toby's original idea was actually the fastest. Here they are decreasing order of the largest timing in each row of system.time. I also tried it with a 100x10 matrix and got almost the same order:
> library(reshape) > system.time(for(i in 1:1000) iapply(a, 1, "/", a[1,])) [1] 11.51 0.01 18.65 NA NA > system.time(for(i in 1:1000) t(apply(a, 1, "/", a[1,]))) [1] 0.83 0.00 1.36 NA NA > system.time(for(i in 1:1000) sweep(a, 2, a[1,], "/")) [1] 0.27 0.00 0.39 NA NA > system.time(for(i in 1:1000) a/outer(rep(1, nrow(a)), a[1,])) [1] 0.23 0.00 0.39 NA NA > system.time(for(i in 1:1000) a %*% diag(1/a[1,])) [1] 0.25 0.00 0.38 NA NA > system.time(for(i in 1:1000) a/rep(a[1,], each = nrow(a))) [1] 0.09 0.00 0.16 NA NA > system.time(for(i in 1:1000) t(t(a)/a[1,])) [1] 0.10 0.00 0.13 NA NA > system.time(for(i in 1:1000) a/matrix(a[1,], nrow(a), ncol(a), byrow = TRUE)) [1] 0.05 0.00 0.12 NA NA > On 9/6/06, Rolf Turner <[EMAIL PROTECTED]> wrote: > Prof. Brian Ripley wrote: > > > On Wed, 6 Sep 2006, Christos Hatzis wrote: > > > > > See ?sweep > > > > > > sweep(a, 2, a[1,],"/") > > > > That is less efficient than > > > > a/rep(a[1,], each=nrow(a)) > > *My* first instinct was to use > > t(t(a)/a[1,]) > > (which has not heretofore been suggested). > > This seems to be more efficient still (at least in respect of Prof. > Grothendieck's toy example) by between 20 and 25 percent: > > > a <- matrix(1:24,4) > > system.time(for(i in 1:1000) junk <- a / rep(a[1,], each = 4)) > [1] 0.690 0.080 1.051 0.000 0.000 > > system.time(for(i in 1:1000) junk <- t(t(a)/a[1,])) > [1] 0.520 0.120 0.647 0.000 0.000 > > system.time(for(i in 1:10000) junk <- a / rep(a[1,], each = 4)) > [1] 7.08 0.99 10.08 0.00 0.00 > > system.time(for(i in 1:10000) junk <- t(t(a)/a[1,])) > [1] 5.530 0.940 7.856 0.000 0.000 > > cheers, > > Rolf Turner > > ______________________________________________ > [email protected] 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. > ______________________________________________ [email protected] 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.
