[R] dataframe subset

[Bernhard Baumgartner]

I have a dataframe with a column, say "x" consisting of values, each 
value appearing different times, e.g.
x: 1,1,1,1,2,2,4,4,4,9,10,10,10,10,10 ...
and a vector, including e.g.:
y: 2,9,10,...
I need a subset of the dataframe: all rows where x is equal to one of 
the values in y. Currently I use a loop for this, but because x and y 
are large this is very slow. 
Is there any idea how to solve this problem faster?
Thank you,
Bernhard