[R] Grouping data in dataframe

[Timo Schneider]

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