[R] Newbie: Matrix indexing

[Adaikalavan Ramasamy]

It would be a lot easier if try to do the reverse by generating the
table last. However you will need to coerce your numeric data into
factors first. See what happens with the first example when you omit the
factor coercion.

 mat <- matrix(1, nr=3, nc2)   # called 'index' in your example
 my.levels <- c(1, 2)
 table( factor( mat[ ,1], levels=my.levels), 
        factor( mat[ ,2], levels=my.levels) )
   
    1 2
  1 3 0
  2 0 0


Here is a slightly more interesting examples.

 mat <- matrix(c(1,1, 1,1, 1,2, 2,2, 1,2) , nr=5, nc=2, byrow=T)
 my.levels <- c(1, 2, 3)
 table( factor(mat[ ,1], levels=my.levels), 
        factor(mat[ ,2], levels=my.levels) )

    1 2 3
  1 2 2 0
  2 0 1 0
  3 0 0 0

Regards, Adai



On Tue, 2005-03-22 at 11:26 +0100, Pascal BLEUYARD wrote:
> 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 Météorologie Physique (LaMP)
> OPGC, Université Blaise Pascal
> 24, avenue des Landais
> 63177 AUBIERE CEDEX
> Tél  : 04 73 40 73 75
> Fax : 04 73 40 51 36
> Mél : P.Bleuyard at opgc.univ-bpclermont.fr
> 
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