Hi Gang Chen, If I have the right idea: for(zval in levels(myData$Z)) crossprod(as.matrix(myData[myData$Z==zval,c("X","Y")]))
Jim On Wed, Aug 24, 2016 at 8:03 AM, Gang Chen <gangch...@gmail.com> wrote: > 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. > > ______________________________________________ > R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. ______________________________________________ R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.