On Wed, Aug 15, 2012 at 5:32 AM, Federico Calboli <f.calb...@imperial.ac.uk> wrote: > Hi, > > I am subsetting a matrix thus: > > test > [,1] [,2] [,3] > [1,] 1 7 13 > [2,] 2 8 14 > [3,] 3 9 15 > [4,] 4 10 16 > [5,] 5 11 17 > [6,] 6 12 18 > > test[cbind(c(1,3,5), c(2,1,3))] > [1] 7 3 17 > > This works fine, and is the equivalent of c(test[1,2], test[3,1], test[5,3]). > cbind(c(1,3,5), c(2,1,3)) would obviously look like: > > [,1] [,2] > [1,] 1 2 > [2,] 3 1 > [3,] 5 3 > > > My question is, since the subsetting is effectively extracting the values by > taking one line at a time in the cbind() coordinates matrix, is a for() loop > at work here? Or is the subsetting action doing something smarter than just > extracting each value reading one set of coordinates at a time? This is > purely an academic question, but I'm very curious about the answer.
Canonically, see $(R_HOME)/src/main/subset.c lines 159ff and $(R_HOME)/src/main/subscript.c lines 316ff. More directly, R does something akin to arrayInd() but at the C level -- so yes, a for loop, but a good one ;-) -- and then regular subsetting. Cheers, Michael > > Best wishes, > > Federico > > > > -- > Federico C. F. Calboli > Neuroepidemiology and Ageing Research > Imperial College, St. Mary's Campus > Norfolk Place, London W2 1PG > > Tel +44 (0)20 75941602 Fax +44 (0)20 75943193 > > f.calboli [.a.t] imperial.ac.uk > f.calboli [.a.t] gmail.com > > ______________________________________________ > 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. ______________________________________________ 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.
