[R] regular expressions with grep() and negative indexing
Peter Dalgaard
P.Dalgaard at biostat.ku.dk
Wed Apr 25 12:35:44 CEST 2007
Stephen Tucker wrote:
> Dear R-helpers,
>
> Does anyone know how to use regular expressions to return vector elements
> that don't contain a word? For instance, if I have a vector
> x <- c("seal.0","seal.1-exclude")
> I'd like to get back the elements which do not contain the word "exclude",
> using something like (I know this doesn't work) but:
> grep("[^(exclude)]",x)
>
> I can use
> x[-grep("exclude",x)]
> for this case but then if I use this expression in a recursive function, it
> will not work for instances in which the vector contains no elements with
> that word. For instance, if I have
> x2 <- c("dolphin.0","dolphin.1")
> then
> x2[-grep("exclude",x2)]
> will give me 'character(0)'
>
> I know I can accomplish this in several steps, for instance:
> myfunc <- function(x) {
> iexclude <- grep("exclude",x)
> if(length(iexclude) > 0) x2 <- x[-iexclude] else x2 <- x
> # do stuff with x2 <...?
> }
>
> But this is embedded in a much larger function and I am trying to minimize
> intermediate variable assignment (perhaps a futile effort). But if anyone
> knows of an easy solution, I'd appreciate a tip.
>
It has come up a couple of times before, and yes, it is a bit of a pain.
Probably the quickest way out is
negIndex <- function(i)
if(length(i))
-i
else
TRUE
--
O__ ---- Peter Dalgaard Øster Farimagsgade 5, Entr.B
c/ /'_ --- Dept. of Biostatistics PO Box 2099, 1014 Cph. K
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~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk) FAX: (+45) 35327907
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