vectorizing

[Catalina Mesina <cmesina@cs.vu.nl>]

Dear List,

I need to make a function like:

makefun-function(data,m,n){     
        mat <- matrix(0,m,n)
        u1 <- data[,1]
        u2<-data[,2]
        u3<-data[,3]
        for(i in (1:m)) {
                for(j in (1:n)) {
                        mat[i,j] <- sum(u1[(u2 == i & u3==j)])}}
        mat
}

How can I write a vectorized version of this function? The above function is 
slow for large data sets.

Thanks,
Catalina