[R] regular expressions with grep() and negative indexing

[Stephen Tucker]

Thanks guys for the suggestions guys- I come across this problem a lot but
now I have many solutions.

Thank you,

Stephen


--- Peter Dalgaard <P.Dalgaard at biostat.ku.dk> wrote:

> Peter Dalgaard wrote:
> > 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
> >
> >   
> ... which of course needs braces if typed on the command line
> 
> negIndex <- function(i) 
> {
>    if(length(i))
>        -i 
>    else 
>        TRUE
> }
> 
> And I should probably also have said that it works like this:
> 
> > x2 <- c("dolphin.0","dolphin.1")
> > x2[-grep("exclude",x2)]
> character(0)
> > x2[negIndex(grep("exclude",x2))]
> [1] "dolphin.0" "dolphin.1"
> 
> 
> 
> -- 
>    O__  ---- Peter Dalgaard             Øster Farimagsgade 5, Entr.B
>   c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
>  (*) \(*) -- University of Copenhagen   Denmark          Ph:  (+45)
> 35327918
> ~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)                  FAX: (+45)
> 35327907
> 
>