[R] aggregate

Gang Chen gangchen6 at gmail.com
Wed Aug 24 00:03:15 CEST 2016

This is a simple question: With a dataframe like the following

myData <- data.frame(X=c(1, 2, 3, 4), Y=c(4, 3, 2, 1), Z=c('A', 'A', 'B', 'B'))

how can I get the cross product between X and Y for each level of
factor Z? My difficulty is that I don't know how to deal with the fact
that crossprod() acts on two variables in this case.

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