[R] can I use one data frame to subset data from another?

[Kara Przeczek]

I am new to R and have been struggling with the following problem. I apologize if there is an obvious answer that I have missed.
 
Problem:
I have a set of hourly temperature data that I would like to average over a number of different hours. Example, I would like the average temperature between 11:00 am on April 1 2008 and 12:00 pm on April 12 2008, and the average temperature between 2:00 pm on April 1 2008 and 2:00 pm on April 12 2008, and so on. I have two different data frames. The first has the air temperature and date/time data (length = 7188) the other has the start and end date/time data which I would like to average over (length = 36).
name: "met_h"
   DateTime               AT_h
  09/21/2007 100     3.66850
  09/21/2007 200     3.63925
  09/21/2007 300     3.39800
  09/21/2007 400     3.38000
  09/21/2007 500     3.11425
  09/21/2007 600     2.89850
  09/21/2007 700     2.65600
  09/21/2007 800     2.08900
  09/21/2007 900     2.17325
  09/21/2007 1000   3.22350
   .....

name: "period"
   Start                    End
  4/1/2008 11:00     4/12/2008 12:00
  4/1/2008 14:00     4/12/2008 14:00
  4/1/2008 17:00     4/12/2008 16:00
  4/1/2008 19:00     4/12/2008 17:00
 4/12/2008 13:00    4/20/2008 12:00
 4/12/2008 15:00    4/20/2008 15:00
 4/12/2008 17:00    4/20/2008 17:00
 4/12/2008 18:00    4/20/2008 19:00
 4/20/2008 13:00    4/27/2008 12:00
 4/20/2008 16:00    4/27/2008 13:00
 .....

I first converted the date time columns using strptime: 
strptime(met_h$DateTime, format="%m/%d/%Y %H:%M")
 
I am attempting to use the second data frame to select the date to average in the first data frame: 
mean(met_h$AT_h[met_h$DateTime>=(period$Start) & met_h$DateTime<=(period$End)])

This gets me many error messages. Firstly because the reference data frame "period" is not the same length as the temperature data frame "met_h". However, I want to create an output of the same length as "period". I would like to use the "period" data frame to pick out certain data from the larger "met_h".
 
Thank you very much for your time!
Kara
 
 
 
Kara Przeczek
M.Sc. Candidate  NRES - Environmental Science
University of Northern B.C.
3333 University Way
Prince George B.C   V2N 4Z9
Phone: (250) 960-5427
przeczek at unbc.ca