converting a data-frame by a defined rule

Hi,

I have a data frame with several columns.

Now I want to transfer the data into a new variable (also a data  
frame), but I only want a part of the data, defined by a rule ...


for example; I have following data frame:

row1 	row2 	row3
x		2		3
x		1		4
y		5		3
y		2		3

I know want a data frame, only with lines containing x in row1.

I know how to do that for one row    (f <- d$row2[d$row1=="x"]).
But how can I do that for all rows in one step?


So that I get this for f:

row1 	row2 	row3
x		2		3
x		1		4