[R] Selecting all rows of factors which have at least one positive value?

[Stephan Lindner]

Dear all,

I'm trying to select from a dataframe all rows which correspond to a
factor (the id variable) for which there exists at least one positive
value of a certain variable. As an example:

x <- data.frame(matrix(c(rep(11,4),rep(12,3),rep(13,3),rep(0,3),1,rep(0,4),rep(1,2)),ncol=2))

> x


   X1 X2
1  11  0
2  11  0
3  11  0
4  11  1
5  12  0
6  12  0
7  12  0
8  13  0
9  13  1
10 13  1 


and I want to select all rows pertaining to factor levels of X1 for
which exists at least one "1" for X2. To be clear, I want rows 1:4
(since there exists at least one observation for X1==11 for which
X2==1) and rows 8:10 (likewise). 

It is easy to obtain the corresponding factor levels (i.e.,
unique(x$X1[x$X2==1])), but I got stalled selecting the corresponding
rows. I tried grep, but then I have to loop and concatenate the
resulting vector. Any ideas?


Thanks a lot!


	Stephan





-- 
-----------------------
Stephan Lindner
University of Michigan