[R] regular expressions with grep() and negative indexing
Tony Plate
tplate at acm.org
Wed Apr 25 17:39:23 CEST 2007
I use regexpr() instead of grep() in cases like this, e.g.:
x2[regexpr("exclude",x2)==-1]
(regexpr returns a vector of the same length as character vector given
it, so there's no problem with it returning a zero length vector)
-- Tony Plate
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
>
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