[R] iterating over a data frame the R way?

[donahchoo at me.com]

Hi,

I have a data frame that was create by issuing a select against my  
sqlite database.  I want to get each row from the data frame and use  
the each of the column values.  The data frame looks like this:

start_time    end_time
09:30:00      10:00:00
10:00:01      10:30:00
etc

Can a point me to a tutorial/example of doing this?  I the other  
programming languages I'm familiar with I would just loop over the  
frame and access the elements, but I believe that's not the R way and  
can't find examples of doing this.

Thanks!