[R] aggregate

[Jim Lemon]

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 <gangchen6 at 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.
>
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