Johannes Graumann
2011-Oct-05 11:29 UTC
[R] Vector-subsetting with ZERO - Is behavior changeable?
Dear All, I have trouble generizising some code.> index <- 0 > sapply(list(c(1,2,3),c(1,2),c(1)),function(x){x[max(length(x)-index,0)]})Will yield a wished for vector like so: [1] 3 2 1 But in this case (trying to select te second to last element in each vector of the list)> index <- 1 > sapply(list(c(1,2,3),c(1,2),c(1)),function(x){x[max(length(x)-index,0)]})I end up with [[1]] [1] 2 [[2]] [1] 1 [[3]] numeric(0) I would (massively) prefer something like [1] 2 1 NA My current implementation looks like> index <- 1 > unlist( > sapply( > list(c(1,2,3),c(1,2),c(1)), > function(x){ > value <- x[max(length(x)-index,0)] > if(identical(value,numeric(0))){return(NA)} else {return(value)} > } > ) > )[1] 2 1 NA Quite the inelegant eyesore. Any hints on how to do this better? Thanks, Joh
William Dunlap
2011-Oct-05 16:44 UTC
[R] Vector-subsetting with ZERO - Is behavior changeable?
You can use [1] on the output of FUN to ensure that exactly one value (perhaps NA from numeric(0)[1]) is returned. E.g. > index <- 1 > sapply(list(c(1,2,3),c(1,2),c(1)),function(x){x[max(length(x)-index,0)][1]}) [1] 2 1 NA I'll also put in a plug for vapply, which throws an error if FUN does not return what you expect it to: > vapply(list(c(1,2,3),c(1,2),c(1)),function(x){x[max(length(x)-index,0)]}, FUN.VALUE=numeric(1)) Error in vapply(list(c(1, 2, 3), c(1, 2), c(1)), function(x) { : values must be length 1, but FUN(X[[3]]) result is length 0 > vapply(list(c(1,2,3),c(1,2),c(1)),function(x){x[max(length(x)-index,0)][1]}, FUN.VALUE=numeric(1)) [1] 2 1 NA For long input vectors vapply can save a fair bit of memory and time over sapply. Bill Dunlap Spotfire, TIBCO Software wdunlap tibco.com> -----Original Message----- > From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf Of Johannes > Graumann > Sent: Wednesday, October 05, 2011 4:29 AM > To: r-help at stat.math.ethz.ch > Subject: [R] Vector-subsetting with ZERO - Is behavior changeable? > > Dear All, > > I have trouble generizising some code. > > > index <- 0 > > sapply(list(c(1,2,3),c(1,2),c(1)),function(x){x[max(length(x)-index,0)]}) > Will yield a wished for vector like so: > [1] 3 2 1 > > But in this case (trying to select te second to last element in each vector > of the list) > > index <- 1 > > sapply(list(c(1,2,3),c(1,2),c(1)),function(x){x[max(length(x)-index,0)]}) > I end up with > [[1]] > [1] 2 > > [[2]] > [1] 1 > > [[3]] > numeric(0) > > I would (massively) prefer something like > [1] 2 1 NA > > My current implementation looks like > > index <- 1 > > unlist( > > sapply( > > list(c(1,2,3),c(1,2),c(1)), > > function(x){ > > value <- x[max(length(x)-index,0)] > > if(identical(value,numeric(0))){return(NA)} else {return(value)} > > } > > ) > > ) > [1] 2 1 NA > > Quite the inelegant eyesore. > > Any hints on how to do this better? > > Thanks, Joh > > ______________________________________________ > R-help at 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.