[R] how to subset rows using regular expression patterns

[Peter Dalgaard]

"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.

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