[R] Aggregate certain rows in a matrix

[Kennedy]

Hi,

I have a matrix that looks like this 

  a <- c(1,1,1,1,2,2,3,3,3,3)
  b <- c(2,2,2,3,4,4,4,5,5,6)
  c <- c(1,2,3,4,5,6,7,8,9,10)
  M <- matrix(nr=10,nc=3)
  M[,1] <- a
  M[,2] <- b
  M[,3] <- c

> M
      [,1] [,2] [,3]
 [1,]    1    2    1
 [2,]    1    2    2
 [3,]    1    2    3
 [4,]    1    3    4
 [5,]    2    4    5
 [6,]    2    4    6
 [7,]    3    4    7
 [8,]    3    5    8
 [9,]    3    5    9
[10,]    3    6   10

I want to reduce the matrix according to the following: If the values of the
two first columns are the same in two or more rows the values in the third
column of the corresponding rows should be added and only one of the rows
should be keept. Hence the matrix M above should look like this

  1 2 6
  1 3 4
  2 4 11
  3 4 7
  3 5 17
  3 6 10


Thank you

Henrik



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