[R] "group by" functionality in R

[hadley wickham]

On Wed, Oct 1, 2008 at 8:58 AM, Max Rausch <maxrausch at gmail.com> wrote:
> I have a data frame with the following information
>
>         day         hour    volume
> 1  2003-07-18   10      836700
> 2  2003-07-18   11      375000
> 3  2003-07-18   12      60000
> 4  2003-07-18    8       1020000
> 5  2003-07-18    9       390000
>
> I have been trying create a new data frame with the following
>
>         day  total_daily_volume
>
> I can accomplish this by bringing the data to excel and using a pivot table
> but I would like to be able to do in R. I have been searching for several
> hours but I can not figure out how to  sum the  the volume of each day.

You might want to have a look at the plyr (http://had.co.nz/plyr) and
reshape (http://had.co.nz/reshape) packages which provide similar
functionality to pivot tables in excel.

Hadley

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