subsetting a dataframe

["Data Analytics Corp." <dataanalytics@earthlink.net>]

Hi,

I wrote a function, say foo, part of which subsets a dataframe.  Let's 
say the dataframe is df with columns
x   y   z
1   1   1
1   1   0
1   0   1
1   0   0
0   1   1
0   1   0
0   0   1
0   0   0

and I want the subset such that x == 1.  I normally just use

df[df$x == 1, ]

But what if I want to include the subsetting variable in an argument 
list so that I could switch between x, y and z depending on the 
problem?  Let the argument be sub and the call be, say, foo(sub = "x") 
or foo(sub = "z").  I can't figure out what to write in place of df[df$x 
== 1,].  Does anyone know how to do this?
Thanks,

Walt Paczkowski