On Mon, Sep 6, 2010 at 3:29 PM, Kennedy <[email protected]> wrote:
> 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
Use library(plyr), convert to data frame, do ddply, convert back to
matrix if you want. I'm surprised mmply doesn't do it, but I dont
think it does:
> Md=data.frame(M)
> ddply(Md,c(1,2),function(r){sum(r[,3])})
X1 X2 V1
1 1 2 6
2 1 3 4
3 2 4 11
4 3 4 7
5 3 5 17
6 3 6 10
plyr is on CRAN and that's the third time today I've told someone to use it.
Barry
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