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

From: Ralph S. <ruffel1_at_hotmail.com>
Date: Sun, 20 Jul 2008 23:16:08 +0000

Hi,

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

The important thing is, that a factor level such as "1" in the example can re-occur many times. There are no regularities on the number of re-occurences etc.

How could I do this efficiently (the matrix is large:>10^6 rows)?

Many thanks!!

-Ralph

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