ave and grouping

Dear list,

# I have a DF like this:
sleep$b   <- c(rep(8,10), rep(9,10))
sleep$me  <- with(sleep, ave(extra, group, FUN = mean))
sleep

# I would like to create a new variable
# holding the b-th value of group 1 and 2.

# This is not what I want, it takes always the '8' from group '1'
# and not the '9'
sleep$gr  <- with(sleep, ave(extra, group, FUN = function(x) x[ b[1] ]))
sleep

Thanks for any help!
Patrick