[R] Sum efficiently from large matrix according to re-occuring levels of factor?

[hadley wickham]

On Sun, Jul 20, 2008 at 4:16 PM, Ralph S. <ruffel1 at hotmail.com> wrote:
>
> Hi,
>
> I am trying to calculate the sum for each occurrence of the level of a factor in a very large matrix. In addition, I want to save that sum together with the information of the level of the factor and the level of a second factor.
>
> My matrix looks like this:
>
> x<-matrix(c(1,1,1,2,2,3,3,1,1,7,7,7,4,4,2,2,7,7,1,1,1,1,1,1,2,5,5),9,3)
>
> I want to sum according to the levels in the first column and save the sum with the information of the level in the first and the second column in a new matrix.
>
> That is, I want output in the matrix of form:
>
> 1 7 3
> 2 4 2
> 3 2 3
> 1 7 10
>

Why that and not:

1 7 13
2 4 2
3 2 3

?

Hadley

-- 
http://had.co.nz/