Hi,
Assuming I understood the question correctly  you can use the "by" function
from the base package i.e:
by(MyData[,4],MyData[,2],mean)
where the factor is in the second column and the numerical data is in the
fourth column.
cheers,
Achilleas

On Thu, May 12, 2011 at 11:20 AM, Fabrice Tourre <fabrice.ciup@gmail.com>wrote:

> Dear list,
> I have dataframe, the second column is groups factor, each group has
> 10 items. The data as fellow.
> chr10   rs9971029   71916552    0.1
> chr10   rs9971029   71916553    0.4
> chr10   rs9971029   71916554    0.3
> chr10   rs9971029   71916555    0.9
> chr10   rs9971029   71916556    1
> chr10   rs9971029   71916557    2
> chr10   rs9971029   71916558    4
> chr10   rs9971029   71916559    0.8
> chr10   rs9971029   71916560    0.9
> chr10   rs9971029   71916561    0.8
> chr10   rs9971030   71916726    0.6
> chr10   rs9971030   71916727    0.5
> chr10   rs9971030   71916728    0.4
> chr10   rs9971030   71916729    0.7
> chr10   rs9971030   71916730    0
> chr10   rs9971030   71916731    0
> chr10   rs9971030   71916732    0.6
> chr10   rs9971030   71916733    0.8
> chr10   rs9971030   71916734    0.9
> chr10   rs9971030   71916735    1
>
> I want to get a average of each item based on the group factor. So at
> last I want to get a vector which length is 10.
> The value calculated as this:
>
> (0.1+0.6)/2
> (0.4+0.5)/2
> …
> (0.8+1)/2
>
> Thank you very much in advance.
>
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