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.
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