Andrey wrote on 10/13/2011 08:40:21 AM:
>
> Dear All,
>
> For a vector, I use this
>
> xu<-1:20
> t<-rep((1:4),each=5)
> tapply(xu,t,mean)
> 1 2 3 4
> 3 8 13 18
>
> and for a matrix the only way I may guess is:
> > xu
> [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8]
> [1,] 1 4 3 2 1 4 3 2
> [2,] 2 1 4 3 2 1 4 3
> [3,] 3 2 1 4 3 2 1 4
> [4,] 1 4 3 2 1 4 3 2
> [5,] 2 1 4 3 2 1 4 3
> [6,] 3 2 1 4 3 2 1 4
> > t
> [1] 1 1 1 2 2 2
> > y
> [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8]
> [1,] 0 0 0 0 0 0 0 0
> [2,] 0 0 0 0 0 0 0 0
>
> > for (i in 1:dim(xu)[2]) y[,i]<-tapply(xu[,i],t,mean)
> > y
> [,1] [,2] [,3] [,4] [,5] [,6] [,7]
> [,8]
> [1,] 2 2.333333 2.666667 3 2 2.333333 2.666667 3
> [2,] 2 2.333333 2.666667 3 2 2.333333 2.666667 3
>
> I do not like the need to create a matrix (y) for the result.
> Is there a better way?
>
> Thanks,
> Andrei.
apply(xu, 2, tapply, t, mean)
Jean
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