And there is no real magic in which(). You already know that V[I] where 'I' is a logical vector the length of 'V' extracts the values in 'V' corresponding to the TRUE's in 'I'. Replace 'V' with the vector made by seq_along(I), (== 1, 2, ..., length(I)), and you have the essentials of which(I).
which(I) does add an extra twist - it considers missing values in I to be the same as FALSE's: seq_along(I)[ !is.na(I) & I ] Bill Dunlap Spotfire, TIBCO Software wdunlap tibco.com > -----Original Message----- > From: r-help-boun...@r-project.org [mailto:r-help-boun...@r-project.org] On > Behalf > Of Bert Gunter > Sent: Friday, June 28, 2013 5:01 PM > To: Julio Sergio > Cc: r-h...@stat.math.ethz.ch > Subject: Re: [R] Transforming boolean subsetting into index subsetting > > ?which > > -- Bert > > On Fri, Jun 28, 2013 at 2:58 PM, Julio Sergio <julioser...@gmail.com> wrote: > > One of the techniques to subset, a vector for instance, is the following: > > > > V <- c(18, 8, 5, 41, 8, 7) > > V > > ## [1] 18 8 5 41 8 7 > > ( I <- abs(V - 9) <= 3 ) > > ## [1] FALSE TRUE FALSE FALSE TRUE TRUE > > V[I] > > ## [1] 8 8 7 > > > > However, sometimes we are interested in the indexes of the elements where > > the > > condition holds. Is there an easy way to transform the I vector into an > > indexes vector similar to: I == c(2,5,6) ? > > > > I know that I can traverse the I vector with a for() loop collecting the > > indexes, I just wonder if such an operation can be avoided. > > > > Thanks, > > > > -Sergio. > > > > ______________________________________________ > > 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. > > > > -- > > Bert Gunter > Genentech Nonclinical Biostatistics > > Internal Contact Info: > Phone: 467-7374 > Website: > http://pharmadevelopment.roche.com/index/pdb/pdb-functional-groups/pdb- > biostatistics/pdb-ncb-home.htm > > ______________________________________________ > 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.
