[Rd] invert argument in grep

Prof Brian Ripley ripley at stats.ox.ac.uk
Fri Nov 10 12:28:47 CET 2006

On Fri, 10 Nov 2006, Duncan Murdoch wrote:

> On 11/9/2006 5:14 AM, Romain Francois wrote:
>> Hello,
>> What about an `invert` argument in grep, to return elements that are
>> *not* matching a regular expression :
>> R> grep("pink", colors(), invert = TRUE, value = TRUE)
>> would essentially return the same as :
>> R> colors() [ - grep("pink", colors()) ]

Note that grep("pat", x, value = TRUE) is not the same as x[grep("pat", x)],
as the help page carefully points out.  (I think it would be better 
if it were.)

>> I'm attaching the files that I modified (against today's tarball) for
>> that purpose.

(BTW, sending whole files makes it difficult to see the changes and even 
harder to merge them; please use diffs.  From a quick look the changes 
were very incomplete, as the internal functions were changed and there 
were no changed C files.)

> I think a more generally useful change would be to be able to return a
> logical vector with TRUE for a match and FALSE for a non-match, so a
> simple !grep(...) does the inversion.  (This is motivated by the recent
> R-help discussion of the fact that x[-selection] doesn't always invert
> the selection when it's a vector of indices.)

I don't think that is pertinent here, as the indices are always a vector 
of positive integers.

> A way to do that without expanding the argument list would be to allow
> value="logical"
> as well as value=TRUE and value=FALSE.
> This would make boolean operations easy, e.g.
> colors()[grep("dark", colors(), value="logical")
>       & !grep("blue", colors(), value="logical")]
> to select the colors that contain "dark" but not "blue". (In this case
> the RE to select that subset is rather simple because "dark" always
> precedes "blue", but if that wasn't true, it would be a lot messier.)

That might be worthwhile, but it is relatively simple to change positive 
integer indices to logical ones and v.v.

My personal take is that having 'value=TRUE' was already a complication 
not worth having, and implementing it at C level was an efficiency tweak 
not worth the maintenance effort (and also means that '[' methods are not 

Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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