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

[Peter Dalgaard]

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
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