On Sep 6, 2010, at 10:47 AM, Dimitris Rizopoulos wrote:
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 had been working on a similar approach with ave( ,paste(), sum)
inside a datafrmae, but I liked your approach of setting up the
results of the paste operation as a vector outside of M. (Skips the
dataframe operation I was using.) The above solution is "destructive",
so I constructed this similar alternative that returns the results
without altering M:
> cbind(M, ave(M[ , 3], list(M[,1], M[,2]), FUN=sum))[
!duplicated(M[,1:2]),
c(1,2,4)]
[,1] [,2] [,3]
[1,] 1 2 6
[2,] 1 3 4
[3,] 2 4 11
[4,] 3 4 7
[5,] 3 5 17
[6,] 3 6 10
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
Address: PO Box 2040, 3000 CA Rotterdam, the Netherlands
Tel: +31/(0)10/7043478
Fax: +31/(0)10/7043014
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