R help - Aug 2007 - About grep

Hi,everyone.

I have a problem when using the grep.
for example:
a <- c("aa","aba","abac")
b<- c("ab","aba")

I want to match the whole word,so
grep("^aba$",a)
it returns 2

but when I used it a more useful way:
grep("^b[2]$",a),
it doesn't work at all, it can't find it, returning integer(0).

How can I chang the format in the second way?

Thanks.

-- 
Shao

	[[alternative HTML version deleted]]

try

grep(paste("^",b[2],"$",sep=""),a)


your version will match "b2":
> grep("^b[2]$",c("b","b2","b3"))
[1] 2


--- Shao <xshining at gmail.com> wrote:
> Hi,everyone.
> 
> I have a problem when using the grep.
> for example:
> a <- c("aa","aba","abac")
> b<- c("ab","aba")
> 
> I want to match the whole word,so
> grep("^aba$",a)
> it returns 2
> 
> but when I used it a more useful way:
> grep("^b[2]$",a),
> it doesn't work at all, it can't find it, returning integer(0).
> 
> How can I chang the format in the second way?
> 
> Thanks.
> 
> -- 
> Shao
> 
> 	[[alternative HTML version deleted]]
> 
> ______________________________________________
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> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
> 


       
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Shao wrote:
> 
> Hi,everyone.
> 
> I have a problem when using the grep.
> for example:
> a <- c("aa","aba","abac")
> b<- c("ab","aba")
> 
> I want to match the whole word,so
> grep("^aba$",a)
> it returns 2
> 
> but when I used it a more useful way:
> grep("^b[2]$",a),
> it doesn't work at all, it can't find it, returning integer(0).
> 
> How can I chang the format in the second way?
> 
> Thanks.
> 
The regexp "^b[2]$" matches only two words: "b2" and
"b" (in your present
grep call with defaults
extended = TRUE,  perl = FALSE, fixed = FALSE).

They don't present in a, that's why grep returns integer(0).
If you want to find the word "b[2]" (the expression similar to an
array
indexing operator), either use fixed=TRUE and remove any regexp markup (^
and $)
> a<- c("aa","aba","abac","b[2]")
> grep("b[2]",a,fixed=TRUE)
[1] 4

, or use escapes
> grep("^b\\[2\\]$",a)
[1] 4

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On 07-Aug-07 04:20:27, Shao wrote:
> Hi,everyone.
> 
> I have a problem when using the grep.
> for example:
> a <- c("aa","aba","abac")
> b<- c("ab","aba")
> 
> I want to match the whole word,so
> grep("^aba$",a)
> it returns 2
> 
> but when I used it a more useful way:
> grep("^b[2]$",a),
> it doesn't work at all, it can't find it, returning integer(0).
> 
> How can I chang the format in the second way?
> 
> Thanks.
> 
> -- 
> Shao
The problem is that in the string "^b[2]$" the element b[2] of b
is not evaluated, but simply the successive characters ^ b [ 2 ] $
are passed to grep as a character string, which of course is not
found. You can construct a character string with the value of b[2]
in it by using paste():

grep(paste("^",b[2],"$",sep=""),a)
[1] 2

Ted.


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Date: 07-Aug-07                                       Time: 07:43:14
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