[R] Sum over indexed value

[smu]

P=data.frame(x=c(1,1,2,3,2,1),y=rnorm(6))
tapply(P$y,P$x,sum)

regards,
 stefan


On Mon, Nov 16, 2009 at 09:49:17AM -0800, Gunadi wrote:
> 
> I am sure this is easy but I am not finding a function to do this. 
> 
> I have two columns in a matrix. The first column contains multiple entries
> of numbers from 1 to 100 (i.e. 10 ones, 8 twos etc.). The second column
> contains unique numbers. I want to sum the numbers in column two based on
> the indexed values in column one (e.g. sum of all values in column two
> associated with the value 1 in column one). I would like two columns in
> return - the indexed value in column one (i.e. this time no duplicates) and
> the sum in column two. 
> 
> How do I do this? 
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