[R] Aggregate certain rows in a matrix

[Dimitris Rizopoulos]

one way is the following:

M <- cbind(c(1,1,1,1,2,2,3,3,3,3), c(2,2,2,3,4,4,4,5,5,6),
     c(1,2,3,4,5,6,7,8,9,10))

ind <- do.call(paste, c(as.data.frame(M[, 1:2], sep = "\r")))
M[, 3] <- ave(M[, 3], ind, FUN = "sum")
unique(M)


I hope it helps.

Best,
Dimitris


On 9/6/2010 4:29 PM, Kennedy wrote:
>
> 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
>
>
>

-- 
Dimitris Rizopoulos
Assistant Professor
Department of Biostatistics
Erasmus University Medical Center

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