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

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Another possibility is to transpose and use functions that assume the grouping is by rows, such as summaryBy in the doBy library. If you can easily work with your data this way, the code might be more clear.

> library ( doBy )
> databyrows <- rbind ( c ( 2, 2, 3, 4, 4, 4, 5, 5, 6), c(1, 1, 2, 1, 3, 3, 2, 1, 1) )
> databyrows
     [,1] [,2] [,3] [,4] [,5] [,6] [,7] [,8] [,9]
[1,]    2    2    3    4    4    4    5    5    6
[2,]    1    1    2    1    3    3    2    1    1
> databycols <- t ( databyrows)
> colnames ( databycols ) <- c("x", "y")
> databycols.df <- data.frame (databycols)
> # (summaryBy needs a data frame)
> # The FUN argument is set explicitly to its default for clarity.
> databycols.df
  x y
1 2 1
2 2 1
3 3 2
4 4 1
5 4 3
6 4 3
7 5 2
8 5 1
9 6 1
> sumbycols.df <- summaryBy ( y ~ x, data = databycols.df , FUN = sum, keep.names = TRUE )
> sumbycols.df
  x y
1 2 2
2 3 2
3 4 7
4 5 3
5 6 1
> sumbyrows.df <- t ( sumbycols.df )
> sumbyrows.df
  1 2 3 4 5
x 2 3 4 5 6
y 2 2 7 3 1
> 

----- Original message -----
From: "Eik Vettorazzi" <E.Vettorazzi at uke.uni-hamburg.de>
To: "Hans-Henning Gabriel" <hanshenning.gabriel at gmail.com>
Cc: r-help at r-project.org
Date: Tue, 07 Apr 2009 17:36:45 +0200
Subject: Re: [R] group by-like statement for 2-row matrix

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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