invert argument in grep

On 11/10/2006 12:52 PM, Romain Francois wrote:

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()) ]


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

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.)

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.)

Duncan Murdoch

Hi,

It sounds like a nice thing to have. I would still prefer to type :

R> grep ( "dark", grep("blue", colors(), value = TRUE, invert=TRUE), 
value = TRUE )  

That's good for intersecting two searches, but not for other boolean 
combinations.

My main point was that inversion isn't the only boolean operation you 
may want, but R has perfectly good powerful boolean operators, so 
installing a limited subset of boolean algebra into grep() is probably 
the wrong approach.

What about a way to pass more than one regular expression then be 
able to call :

R> grep( c("dark", "blue"), colors(), value = TRUE, invert = c(TRUE, 
FALSE)

Again, it covers & and !, but it misses other boolean operators.

I usually use that kind of shortcuts that are easy to remember.

vgrep <- function(...) grep(..., value = TRUE)
igrep <- function(...) grep(..., invert = TRUE)
ivgrep <- vigrep <- function(...) grep(..., invert = TRUE, value = TRUE)

If you're willing to write these, then it's easy to write igrep 
without an invert arg to grep:

igrep <- function(pat, x, ...)
   setdiff(1:length(x), grep(pat, x, value = FALSE, ...))

ivgrep would also be easy, except for the weird semantics of 
value=TRUE pointed out by Brian:  but it could still be written with a 
little bit of care.

Duncan Murdoch

What about things like the arguments `after` and `before` in unix 
grep. That could be used when grepping inside a function :

R> grep("plot\\.", body(plot.default) , value= TRUE)
[1] "localWindow <- function(..., col, bg, pch, cex, lty, lwd) 
plot.window(...)"
[2] "plot.new()"
[3] "plot.xy(xy, type, ...)"


when this could be useful  (possibly).

R> # grep("plot\\.", plot.default, after = 2, value = TRUE)
R> tmp <- tempfile(); sink(tmp) ; print(body(plot.default)); sink(); 
system( paste( "grep -A2 plot\\. ", tmp) )
    localWindow <- function(..., col, bg, pch, cex, lty, lwd) 
plot.window(...)
    localTitle <- function(..., col, bg, pch, cex, lty, lwd) title(...)
    xlabel <- if (!missing(x))
-- 
    plot.new()
    localWindow(xlim, ylim, log, asp, ...)
    panel.first
    plot.xy(xy, type, ...)
    panel.last
    if (axes) {
-- 
    if (frame.plot)
        localBox(...)
    if (ann)


BTW, if I call :

R> grep("plot\\.", plot.default)
Error in as.character(x) : cannot coerce to vector

What about adding that line at the beginning of grep, or something 
else to be able to do as.character on a function ?

if(is.function(x)) x <- body(x)


Cheers,

Romain

Cheers,

Romain