[R] Newbie: Matrix indexing

[Gabor Grothendieck]

Pascal BLEUYARD <p.bleuyard <at> opgc.univ-bpclermont.fr> writes:

: 
: Hi all,
: 
:   I need to compute some "occurence matrix": given a zero matrix and a set
: of paired indexes, I want to store the number of occurences of each paired
: index in a matrix. The paired indexes are stores as an index matrix. I
: prefere not to use loops for performances purpose.
: 
:   Here follows a dummy example:
: 
: > occurence <- matrix(0, 2, 2); data
:      [,1] [,2]
: [1,]    0    0
: [2,]    0    0
: >
: > index <- matrix(1, 3, 2); index
:      [,1] [,2]
: [1,]    1    1
: [2,]    1    1
: [3,]    1    1
: >
: > occurence[index] <- occurence[index] + 1
: 
:   I was expecting the folowing result:
: 
: > occurence
:      [,1] [,2]
: [1,]    3    0
: [2,]    0    0
: 
:   I get instead:
: 
: > occurence
:      [,1] [,2]
: [1,]    1    0
: [2,]    0    0
: 
:   I guess that there is some "hidden copy" involved but I wanted to know if
: there is an efficient workaround (not using some loop structure). I thought
: "factors" could do the job but I didn't manage to use them for that problem.

Turn your index matrix into a data frame so you can use lapply on it.
Then convert each of the two columns to a two-level factor.  Now you 
can use table on the result:

   table(lapply(as.data.frame(index), factor, lev = 1:2))