Hi,


On 2/2/07, Martin Olivier <martinol@ensam.inra.fr> wrote:
>
> Hi all,
>
> Suppose I have a vector x with numerical values.
> In y, I have a categorial variable : y<-c(1,1,..2,2,...30,30,30)
> x and y have the same length.
> I would like to compute the mean for x for the modality 1 to 30 in y.
> mean(x[y==1]),...,mean(x[y==30])


I hope I understand your question correctly, but is this what you are
looking for?

x <- runif(10000)
y <- sample(x=1:30, size=length(x), replace=TRUE)

one.possibility <- tapply(X=x, INDEX=list(my.categ.var=y), FUN=mean)

another.possibility <- by(data=x, INDICES=list(my.categ.var=y), FUN=mean)
one.possibility
another.possibility


Which one you prefer depends probably on your taste and what you intend to
do with the results.

HTH,
Roland

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