Another approach is to use the reshape package
--Assuming your data.frame is called xx
------------------------------------------
libarary(reshape)
mm <- melt(xx, id=c("Size")) ; mm
cast(mm, Size ~variable, median)
--------------------------------------
--- On Tue, 7/14/09, Timo Schneider <timo.schnei...@s2004.tu-chemnitz.de> wrote:
> From: Timo Schneider <timo.schnei...@s2004.tu-chemnitz.de>
> Subject: [R] Grouping data in dataframe
> To: "r-help@r-project.org" <r-help@r-project.org>
> Received: Tuesday, July 14, 2009, 11:56 PM
> Hello,
>
> I have a dataframe (obtained from read.table()) which looks
> like
>
>
> ExpA ExpB ExpC Size
> 1 12 23
> 33 1
> 2 12 24
> 29 1
> 3 10 22
> 34 1
> 4 25 50
> 60 2
> 5 24 53
> 62 2
> 6 21 49
> 61 2
>
> now I want to take all rows that have the same value in the
> "Size"
> column and apply a function to the columns of these rows
> (for example
> median()). The result should be a new dataframe with the
> medians of the
> groups, like this:
>
>
> ExpA ExpB ExpC Size
> 1 12 23
> 34 1
> 2 24 50
> 61 2
>
> I tried to play with the functions by() and tapply() but I
> didn't get
> the results I wanted so far, so any help on this would be
> great!
>
> The reason why I am having this problem: (I explain this
> just to make
> sure I don't do something against the nature of R.)
>
> I am doing 3 simillar experiments, A,B,C and I change a
> parameter in the
> experiment (size). Every experiment is done multiple times
> and I need
> the median or average over all experiments that are the
> same. Should I
> preprocess my data files so that they are completely
> different? Or is it
> fine the way it is and I just overlooked the simple
> solution to the
> problem described above?
>
> Regards,
> Timo
>
> ______________________________________________
> R-help@r-project.org
> mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained,
> reproducible code.
>
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