[R] Merging levels of matrix dimensions

[Anders Nielsen]

Hey 

A first try might be something like: 


cfsum<-function(X, cf)
{
ret<-matrix(NA, ncol=nlevels(cf), nrow=nrow(X))
for(j in 1:nlevels(cf))ret[,j]<-apply(as.matrix(X[,
cf==levels(cf)[j]]),1,sum)
return(ret)
}

A<-matrix(1:25, 5, 5)
col.f<-as.factor(c("A","B","C","C","C"))

cfsum(A, col.f)


After this we can use the same function on the transposed matrix to 
add some of the rows. 

Anders


On Tue, 28 Nov 2000, BXC (Bendix Carstensen) wrote:

> If I have a matrix I can sum some of the rows by hand:
> > M
>      [,1] [,2] [,3]
> [1,]    1    6   11
> [2,]    2    7   12
> [3,]    3    8   13
> [4,]    4    9   14
> [5,]    5   10   15
> > rbind(M[1,],M[2,]+M[3,],M[4,]+M[5,])
>      [,1] [,2] [,3]
> [1,]    1    6   11
> [2,]    5   15   25
> [3,]    9   19   29
> 
> This is a trivial special case of the following problem that I
> wuold like to have a solution to:
> 
> Consider a matix m, and suppose I have factors f.row and f.rcol 
> of lengths dim(m), respectively.
> These factors induce a division of m into submatrices.
> How do I get a nlevels(f.row) by nlevels(f.col) matrix where each
> entry is the sum (or any old function) of the elements in 
> each submatrix?
> 
> Of course there is an array-generalization of this problem as well.
> 
> Any help would be appreaciated.
> 
> Bendix
> 
> ----------------------
> Bendix Carstensen
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