On Mon, 5 Jun 2006, Marc Schwartz (via MN) wrote:
> Based upon an offlist communication this morning, I am somewhat confused
> (more than I usually am on most Monday mornings...) about the use of
> grep() with factors as the 'x' argument.
> ...
> > grep("[a-z]", letters)
> [1] 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22
> [23] 23 24 25 26
>
> > grep("[a-z]", factor(letters))
> numeric(0)
I was recently surprised by this also. In addition, if
R's grep did support factors in this way, what sort of
object (factor or character) should it return when value=T?
I recently changed Splus's grep to return a character vector in
that case.
Splus> grep("[def]", letters[26:1])
[1] 21 22 23
Splus> grep("[def]", factor(letters[26:1], levels=letters[26:1]))
[1] 21 22 23
Splus> grep("[def]", letters[26:1], value=T)
[1] "f" "e" "d"
Splus> grep("[def]", factor(letters[26:1], levels=letters[26:1]), value=T)
[1] "f" "e" "d"
Splus> class(.Last.value)
[1] "character"
R does this when grepping an integer vector.
R> grep("1", 0:11, value=T)
[1] "1" "10" "11"
help(grep) says it returns "the matching elements themselves", but
doesn't say if "themselves" means before or after the conversion to
character.
----------------------------------------------------------------------------
Bill Dunlap
Insightful Corporation
bill at insightful dot com
360-428-8146
"All statements in this message represent the opinions of the author and do
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