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
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