[R] Sum of Probabilities in a matrix...

[Rolf Turner]

On 02/10/11 14:06, Jim Silverton wrote:
> Hi all,
> I have 2 columns in a mtrix, one of which is a column of probabilities and
> the other is simply a vector of integers. I want to sum all the
> probabilities with the same integer value and put it in a new column.
> For example,
> If my matrix is:
>
> 0.98   2
> 0.2     1
> 0.01   2
> 0.5     1
> 0.6     6
>
>
> Then I should get:
> 0.98   2    0.99
> 0.2     1    0.70
> 0.01   2    0.99
> 0.5     1    0.70
> 0.6     6    0.60
>
> Any help is greatly appreciated.

Suppose your matrix is called "m".  Execute:

 > ttt <- tapply(m[,1],m[,2],sum)
 > m <- cbind(m,ttt[match(m[,2],names(ttt))])
 > dimnames(m) <- NULL # To tidy up a bit.

You get:
 > m
      [,1] [,2] [,3]
[1,] 0.98    2 0.99
[2,] 0.20    1 0.70
[3,] 0.01    2 0.99
[4,] 0.50    1 0.70
[5,] 0.60    6 0.60

Easy-peasy.

     cheers,

         Rolf Turner