[R] how to subset rows using regular expression patterns

[zhihua li]

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!

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Re: [R] how to subset rows using regular expression patterns

[Peter Dalgaard]

zhihua li [EMAIL PROTECTED] 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 Øster Farimagsgade 5, Entr.B
  c/ /'_ --- Dept. of Biostatistics PO Box 2099, 1014 Cph. K
 (*) \(*) -- University of Copenhagen   Denmark  Ph:  (+45) 35327918
~~ - ([EMAIL PROTECTED])  FAX: (+45) 35327907

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Re: [R] how to subset rows using regular expression patterns

[paul sorenson]

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!

__
R-help@stat.math.ethz.ch mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html