[R] Aggregate certain rows in a matrix

[David Winsemius]

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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David Winsemius, MD
West Hartford, CT