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

[jim holtman]

something like this should work;

yourMean <- lapply(seq(nrow(period)), function(.indx){
    mean(met_h$AT_h[(met_h$DateTime >= period$Start[.indx]) &
(met_h$DateTime <= period$End[.indx])])
})


On Fri, Nov 14, 2008 at 3:24 PM, Kara Przeczek <przeczek at unbc.ca> wrote:
> 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
>
>
>
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> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>



-- 
Jim Holtman
Cincinnati, OH
+1 513 646 9390

What is the problem that you are trying to solve?