[R] group by-like statement for 2-row matrix

Eik Vettorazzi E.Vettorazzi at uke.uni-hamburg.de
Tue Apr 7 17:36:45 CEST 2009


see ?aggregate

m<-rbind(sample(2:6,9,replace=T),sample(1:3,9,replace=T))
aggregate(m[2,],by=list(m[1,]),sum)

Hans-Henning Gabriel schrieb:
> Hi,
>
> my problem is as follows:
>
> I have a matrix of two rows like this:
>
> 2 2 3 4 4 4 5 5 6
> 1 1 2 1 3 3 2 1 1
>
> Can I apply something like "group by" in sql? What I want to achieve 
> is the some of second row for each unique entry of first row:
>
> 2 -> 2 (=1+1)
> 3 -> 2
> 4 -> 7 (=1+3+3)
> 5 -> 3 (=2+1)
> 6 -> 1
>
> Thanks!!
> Henning
>
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-- 
Eik Vettorazzi
Institut für Medizinische Biometrie und Epidemiologie
Universitätsklinikum Hamburg-Eppendorf

Martinistr. 52
20246 Hamburg

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