[R] question on indexing of a matrix

[Roger Levy]

Peter Dalgaard <p.dalgaard at biostat.ku.dk> writes:

> Roger Bivand <Roger.Bivand at nhh.no> writes:
> 
> > On 12 Feb 2005, Roger Levy wrote:
> > 
> > > Hi,
> > > 
> > > I have a k-level factor F of length n that I would like to use to
> > > extract from an n-by-k matrix M a vector V such that
> > > 
> > >   V[i] = M[i,as.numeric(F)[i]]
> > > 
> > > I don't currently understand how to do that in R -- can anyone explain
> > > to me how to do so?
> > 
> > This may be an answer:
> > 
> > > k <- 3
> > > n <- 10
> > > M <- matrix(1:(k*n), ncol=k, nrow=n)
> > > M
> >       [,1] [,2] [,3]
> >  [1,]    1   11   21
> >  [2,]    2   12   22
> >  [3,]    3   13   23
> >  [4,]    4   14   24
> >  [5,]    5   15   25
> >  [6,]    6   16   26
> >  [7,]    7   17   27
> >  [8,]    8   18   28
> >  [9,]    9   19   29
> > [10,]   10   20   30
> > > set.seed(1234)
> > > K <- factor(sample(1:3, n, replace=TRUE))
> > > K
> >  [1] 1 2 2 2 3 2 1 1 2 2
> > Levels: 1 2 3
> > > Kmm <- model.matrix(terms(~ K-1))
> > > Kmm
> >    K1 K2 K3
> > 1   1  0  0
> > 2   0  1  0
> > 3   0  1  0
> > 4   0  1  0
> > 5   0  0  1
> > 6   0  1  0
> > 7   1  0  0
> > 8   1  0  0
> > 9   0  1  0
> > 10  0  1  0
> > attr(,"assign")
> > [1] 1 1 1
> > attr(,"contrasts")
> > attr(,"contrasts")$K
> > [1] "contr.treatment"
> > 
> > > rowSums(M * Kmm)
> >  1  2  3  4  5  6  7  8  9 10 
> >  1 12 13 14 25 16  7  8 19 20 
> > 
> > My understanding is that you want to extract the K[i]th column from the 
> > [i]th row of M, so the model matrix of K should zero out the values you 
> > don't want. Using something like this in a function would mean checking K 
> > carefully for the number of levels actually present and their coding. 
> 
> This looks simpler:
> 
> >  M[cbind(seq(along=K),as.numeric(K))]
>  [1]  1 12 13 14 25 16  7  8 19 20

Thank you!  This does the trick.

Best,

Roger