Newbie: Matrix indexing

index <- as.data.frame(table(index[,1],index[,2]))
index <- do.call("cbind",lapply(index, as.numeric)) # ensures numeric

occurence[index[,1:2]] <- index[,3]
occurence

-----Original Message-----
From: r-help-bounces at stat.math.ethz.ch 
[mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Pascal BLEUYARD
Sent: mardi 22 mars 2005 11:26
To: r-help at stat.math.ethz.ch
Subject: [R] Newbie: Matrix indexing


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.

--------------------------------------------------------------
--------------
Pascal BLEUYARD
Laboratoire de M?t?orologie Physique (LaMP)
OPGC, Universit? Blaise Pascal
24, avenue des Landais
63177 AUBIERE CEDEX
T?l  : 04 73 40 73 75
Fax : 04 73 40 51 36
M?l : P.Bleuyard at opgc.univ-bpclermont.fr