[R] extract columns of a matrix/data frame

[Kyle.]

You can use the grep function to obtain the column indices matching  
your specified criteria.  For example,

 > r1<-c(1,2,3,7,1,3,2)
 > r2<-c(4,5,7,8,1,4,3)
 > test<-matrix(c(r1,r2),byrow=TRUE)
 > colnames(test)<-c("a1","a2","b1","b2","b3","c1","c2")
 > test
      a1 a2 b1 b2 b3 c1 c2
[1,]  1  2  3  7  1  3  2
[2,]  4  5  7  8  1  4  3
 > grep("a",(colnames(test)))
[1] 1 2
test.a<-test[,grep("a",(colnames(test)))]
test.a
      a1 a2
[1,]  1  2
[2,]  4  5



On Jul 31, 2007, at 10:35 AM, 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.
>
>
> ---------------------------------
>
>
> 	[[alternative HTML version deleted]]
>
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