[R] Grouping and Computing

[alexander.hener@gmx.de]

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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