[R] group means of multi-way table?

[Simon Kiss]

hello, can someone tell me how to generate the means for a data frame  
that looks like this? My data frame has many more variables, but I  
won't bother you with those; these are the one's that I'm interested in.
Needless to say, z is the variable in which I'm interested. I'd like  
to find out the mean score of z for NDP managers, Conservative  
managers and Liberal managers and then for a few other configurations.
Ive played around with aggregate, tapply and by, but I can't get it to  
work.
Cordially,
Simon Kiss
mydata=data.frame(cbind(x,y,z))
mydata$x=as.factor(sample(c("labourers", "salaried", "managers"),  
size=300, replace=TRUE))
mydata$y=as.factor(sample(c("NDP", "Green", "Liberal",  
"Conservative"), size=300, replace=TRUE))
mydata$z=as.numeric(sample(1:4, size=300, replace=TRUE))

*********************************
Simon J. Kiss, PhD
Assistant Professor, Wilfrid Laurier University
223 Grand River Hall, 171 Colborne Street
Brantford, Ontario, Canada
N3T 2C9
Cell: +1 519 761 7606