R help - Dec 2005 - how to subset rows using regular expression patterns

hi netters,

i have a dataframe A with several columns(variables). the elements of 
column M are character strings. so 
A$M=c("ab","abc","bcd","ac","abcd","fg",....."fl").

i wanna extract all the rows where A$M match some regular expression 
pattern.
for a simple example, let the pattern be just "ab", i wanna subset the
rows
where A$M="ab" or "abc" or "abcd" or
"abXX".........

i know i can write a loop,using some regular expression pattern functions 
like grep row by row. but when A's size is pretty large, it's
inefficient.
could anyone give me a hint about a faster code?

thanks a lot!

"zhihua li" <lzhtom at hotmail.com> writes:
> hi netters,
> 
> i have a dataframe A with several columns(variables). the elements of
> column M are character strings. so
>
A$M=c("ab","abc","bcd","ac","abcd","fg",....."fl").
> 
> i wanna extract all the rows where A$M match some regular expression
> pattern.
> for a simple example, let the pattern be just "ab", i wanna
subset the
> rows where A$M="ab" or "abc" or "abcd" or
"abXX".........
> 
> i know i can write a loop,using some regular expression pattern
> functions like grep row by row. but when A's size is pretty large,
> it's inefficient. could anyone give me a hint about a faster code?
> 
> thanks a lot!
Notice that grep() returns an index vector, so

A[grep(pattern, A$M),]

or

subset(A, grep(pattern, M))

should do it.

-- 
   O__  ---- Peter Dalgaard             ??ster Farimagsgade 5, Entr.B
  c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
 (*) \(*) -- University of Copenhagen   Denmark          Ph:  (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)                  FAX: (+45) 35327907

Something like

A[grep('^ab', as.vector(A$M)),]

might work

zhihua li wrote:
> hi netters,
> 
> i have a dataframe A with several columns(variables). the elements of 
> column M are character strings. so 
>
A$M=c("ab","abc","bcd","ac","abcd","fg",....."fl").
> 
> i wanna extract all the rows where A$M match some regular expression 
> pattern.
> for a simple example, let the pattern be just "ab", i wanna
subset the
> rows where A$M="ab" or "abc" or "abcd" or
"abXX".........
> 
> i know i can write a loop,using some regular expression pattern 
> functions like grep row by row. but when A's size is pretty large,
it's
> inefficient. could anyone give me a hint about a faster code?
> 
> thanks a lot!