Simple operation on a subset of data

Say x=1,2 and y=4,5 I want to summarize z only if x=1 & y=4. I thought
that the way to do that would be to
write if((x=1) & (y=4)) summary(z)
butwhen I do this the result I get is for the whole data (irrespective
of the conditions imposed). Can anyone help?

There are actually worse problems with
   if((x=1) & (y=4)) summary(z) 
than that: you have just set x to 1 and y to 4.

You can do
    summary(z[(x==1) & (y==4)])
to get the answer you want (or various other things)

You really need to read the Introduction to R, which will tell you, among 
other things, what the = operator does.

 	-thomas