[R] "group by" functionality in R

[Max Rausch]

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.

Thanks,

Max