[R] Conditional sampling?

[Olga Lyashevska]

Hello,

I have a dataframe consisting of two columns.

 > col1<-factor(c("a","a","b","b","c","c"))
 > col2<-factor(c("a","b","c","d","e","f"))
 > somedf<-data.frame(col1,col2)
 > somedf
     col1   col2
1      a      d
2      a      e
3      b      f
4      b      g
5      c      h
6      c      i

 > sample(col1,2,replace=T)
[1] b c
Levels: a b c

Now I want to sample from col2, but I want to restrict a vector I am  
going to sample from to only those elements that correspond to col1
That is, I want to take a sample from f, g, h, i.  Elements  
corresponding to level a need to be dropped.

    col1    col2

3      b      f
4      b      g
5      c      h
6      c      i

Any ideas?
Thanks in advance!