[R] Correlations by subgroups

[jfdawson]

I'm hoping there is a simple answer to this - it seems that there should be,
but I can't figure it out.

I have a matrix/data frame with three variables of interest - V1, V2, V3.
One, V1, is a factor with x levels (x may be a large number); I want to
calculate  the correlation between the other two (i.e. cor(V2,V3)) for each
level, and store it as a vector of length x.

I should think this should be possible using a function like tapply, but I
cannot work out what the syntax would be - everything I have tried produces
either an error, or just repeats the correlations between V2 & V3 on the
whole matrix.

Could anyone suggest what I should be doing?

Many thanks,
Jeremy

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