[R] how to subset rows using regular expression patterns

Peter Dalgaard p.dalgaard at biostat.ku.dk
Sat Dec 3 12:59:09 CET 2005

"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),]


subset(A, grep(pattern, M))

should do it.

   O__  ---- Peter Dalgaard             Øster 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

More information about the R-help mailing list