[R] extract columns of a matrix/data frame

[Chuck Cleland]

yuvika wrote:
> Hello all,
>    
>   I have a matrix whose column names look like
>    
>   a1  a2  b1  b2  b3  c1 c2
>   1   2    3    7    1    3   2
>   4   6    7    8    1    4   3
>    
>   Now, I can have any number of a's. not just two as shown above and same goes for b's and c's.  I need to extract all the a's columns and put them in another matrix, extract all b's columns and put them in some matrix and same goes for "c". How can I identify such pattern and get subsets of this matrix depending on columns names?
>    
>   I will appreciate a quick reply.
>   Thanks  a lot.

mymat <- matrix(runif(60), ncol=6)

colnames(mymat) <- c("a1","a2","b1","b2","c1","c2")

mymat
              a1         a2          b1         b2         c1         c2
 [1,] 0.73623481 0.25204019 0.332436396 0.36629507 0.39517285 0.62491949
 [2,] 0.48867382 0.20933245 0.511805497 0.03142542 0.82168732 0.20550784
 [3,] 0.89198874 0.24477456 0.629644977 0.23442137 0.17828551 0.29640615
 [4,] 0.99222414 0.49044514 0.571213786 0.91068115 0.09484414 0.78108139
 [5,] 0.66615787 0.13183354 0.004350679 0.32443025 0.38742483 0.76044740
 [6,] 0.06642704 0.96257552 0.189716240 0.83969989 0.53470898 0.28319039
 [7,] 0.31172264 0.20201281 0.577353264 0.62082694 0.31649255 0.40977000
 [8,] 0.52890283 0.46576510 0.107363256 0.72534897 0.12038182 0.06295499
 [9,] 0.55292555 0.76459699 0.212533012 0.73275529 0.98008863 0.85302931
[10,] 0.84320369 0.09958472 0.158443155 0.92321443 0.50935938 0.08514859

mymat[,grep("^a", colnames(mymat))]
              a1         a2
 [1,] 0.73623481 0.25204019
 [2,] 0.48867382 0.20933245
 [3,] 0.89198874 0.24477456
 [4,] 0.99222414 0.49044514
 [5,] 0.66615787 0.13183354
 [6,] 0.06642704 0.96257552
 [7,] 0.31172264 0.20201281
 [8,] 0.52890283 0.46576510
 [9,] 0.55292555 0.76459699
[10,] 0.84320369 0.09958472

?grep

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-- 
Chuck Cleland, Ph.D.
NDRI, Inc.
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