[R] need more help.... Re: iterating over a data frame the R way?

[donahchoo at me.com]

Okay, I don't know if I'm blocking because of my other programming  
language experience of if I'm just being dense.

I have a data.frame with high,open,low,last,day, and start_time time  
columns.  What I want to do is get all the rows with the same day and  
process each of those columns.  If I was able to run a query against  
the data.frame I would be want something like:

result = all the rows where the day is equal to 2007-12-05

Then I would want get max(result["high"]), min(result["low"])

How do I do this?  I tired sqldf, but its complaining about  
libtcl0.5.8 not being found.  I'm on OS X 10.5.8 and haven't yet tired  
to install a new version of tcl in /usr/local.

Here's the dataframe:

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