Many thanks - this is very helpful indeed!

Best wishes,
Jeremy

On 15 Jun 2011, at 21:58, Daniel Malter [via R] wrote:

x<-c(1,1,1,1,1,2,2,2,2,2)
y<-rnorm(10)
z<-y+rnorm(10)
by(data.frame(y,z),factor(x),cor)

hth,
Daniel

jfdawson wrote:
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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