[R] Grouping and Computing

[Joerg Maeder]

Hello Alexander,

the function you are looking for is tapply and it works with dataframes
and matrixs.

here a small example
d <- data.frame(group=c(1,4,5,4,5,2,1),value=c(1,54,2,6,87,4,6))
tapply(d$value,d$group,mean)#arguments: the datas, the groups, the
function

will produce
   1    2    4    5 
 3.5  4.0 30.0 44.5 

you can also use factors to define the group, this has the advantage,
that you can give them a real name
d <-
data.frame(group=c('John','Jack','Joan','Jack','Jack','Joan','John'),value=c(1,54,2,6,87,4,6))
tapply(d$value,d$group,mean)

will produce
Jack Joan John 
49.0  3.0  3.5 

alexander.hener at gmx.de wrote:
> 
> Hi group,
> 
> To mention it in advance, I am an R newbie, and most likely, my question is
> more a mix of smaller, simpler tasks. Anyway, I got mixed up between by,
> select, aggregate, lapply etc.
> My problem is as follows :
> 
> I have read data in and transformed them into a matrix for no special reason
> so far. This matrix contains a column with regard to which I would like to
> group, i.e. one realisation specifies one group. Neither the number of
> occurences nor the value of these realisations is known in advance, which seems to
> be the mayor problem. For each group separately then, I would like to compute
> some aggregation function, namely the sum of a fraction of two columns. These
> sums should be kept in form of another vector.
> 
> My two questions are then
> 
> - Which object type (matrix, dataframe, list) lends itself to such a
> problem?
> - Do I have to create different objects for the groups, or can I compute the
> vector of sums directly? And how?
> 
> Thanks in advance
> 
> Alexander Hener
> 
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-- 
Joerg Maeder    .:|:||:..:.||.::   maeder at atmos.umnw.ethz.ch
Tel: +41 1 633 36 25   .:|:||:..:.||.::   
http://www.iac.ethz.ch/staff/maeder
PhD student at INSTITUTE FOR ATMOSPHERIC AND CLIMATE SCIENCE (IACETH)
ETH ZÜRICH Switzerland
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