[R] vector-factor operation

[Berwin A Turlach]

G'day Murray,

>>>>> "MJ" == Murray Jorgensen <maj at stats.waikato.ac.nz> writes:

    MJ> I found myself wanting to average a vector [vec] within each
    MJ> level of a factor [Fac], returning a vector of the same length
    MJ> as vec.
I presume that the vector that you want as result should not just have
the same length as vec, should it? :-)

    MJ> But there must be another way to do this, and it would be good
    MJ> to be able to apply other functions than mean() in this way.
Something like:

> Fac <- sample(gl(2,4))
> Fac
[1] 2 1 1 2 2 1 1 2
Levels: 1 2
> vec <- rnorm(length(Fac))
> tapply(vec, Fac, mean)
         1          2 
-0.6435816 -0.9267021 
> tapply(vec, Fac, mean)[Fac]
         2          1          1          2          2          1          1 
-0.9267021 -0.6435816 -0.6435816 -0.9267021 -0.9267021 -0.6435816 -0.6435816 
         2 
-0.9267021 
> tapply(vec, Fac, sum)[Fac]
        2         1         1         2         2         1         1         2 
-3.706808 -2.574326 -2.574326 -3.706808 -3.706808 -2.574326 -2.574326 -3.706808


Cheers,

        Berwin

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