[R] Finding minimum of time subset

[Tim Clark]

Jim,

That works great!  However, would you please explain what the '[' and the 1 do in the sapply function?  I understand that you are cutting x by quarter, then creating a list of x that is split based on those cuts.  I just don't understand what "[" means in this contex, or what the number one at the end does.

Thanks for you help,

Tim



Tim Clark
Department of Zoology 
University of Hawaii


--- On Fri, 8/14/09, jim holtman <jholtman at gmail.com> wrote:

> From: jim holtman <jholtman at gmail.com>
> Subject: Re: [R] Finding minimum of time subset
> To: "Tim Clark" <mudiver1200 at yahoo.com>
> Cc: r-help at r-project.org
> Date: Friday, August 14, 2009, 6:18 AM
> Here is one way to do it:
> 
> >
> mytime<-c("12:00:00","12:00:05","12:15:05","12:15:06","12:20:00","12:30:01","12:45:01","13:00:00","13:15:02")
> > # you might want a date on your data
> > x <- as.POSIXct(mytime, format="%H:%M:%S")
> > # create quarter hour intervals for the data range
> > quarter <- seq(trunc(min(x), 'days'), trunc(max(x)
> + 86400, 'days'), by='15 min') # add 86400 to add a day for
> truncation
> > # cut the data by quarter hours and then take the
> first value in each group
> > x.s <- sapply(split(x, cut(x, breaks=quarter),
> drop=TRUE), '[', 1)
> > # lost the 'class' for some reason; put it back
> > class(x.s) <- c("POSIXt", "POSIXct")
> > # the answer
> > x.s
>       2009-08-14 12:00:00   
>    2009-08-14 12:15:00   
>    2009-08-14
> 12:30:00       2009-08-14
> 12:45:00       2009-08-14 13:00:00
> "2009-08-14 12:00:00 EDT" "2009-08-14 12:15:05 EDT"
> "2009-08-14
> 12:30:01 EDT" "2009-08-14 12:45:01 EDT" "2009-08-14
> 13:00:00 EDT"
>       2009-08-14 13:15:00
> "2009-08-14 13:15:02 EDT"
> >
> 
> 
> On Thu, Aug 13, 2009 at 4:10 PM, Tim Clark<mudiver1200 at yahoo.com>
> wrote:
> > Dear List,
> >
> > I have a data frame of data taken every few seconds.
>  I would like to subset the data to retain only the data
> taken on the quarter hour, and as close to the quarter hour
> as possible.  So far I have figured out how to subset the
> data to the quarter hour, but not how to keep only the
> minimum time for each quarter hour.
> >
> > For example:
> >
> mytime<-c("12:00:00","12:00:05","12:15:05","12:15:06","12:20:00","12:30:01","12:45:01","13:00:00","13:15:02")
> >
> subtime<-grep(pattern="[[:digit:]]+[[:punct:]]00[[:punct:]][[:digit:]]+|[[:digit:]]+[[:punct:]]15[[:punct:]][[:digit:]]+|[[:digit:]]+[[:punct:]]30[[:punct:]][[:digit:]]+|[[:digit:]]+[[:punct:]]45[[:punct:]][[:digit:]]+",mytime)
> > mytime[subtime]
> >
> > [1] "12:00:00" "12:00:05" "12:15:05" "12:15:06"
> "12:30:01" "12:45:01" "13:00:00" "13:15:02"
> >
> > This gives me the data taken at quarter hour intervals
> (removes 12:20:00) but I am still left with multiple values
> at the quarter hours.
> >
> > I would like to obtain:
> >
> > "12:00:00" "12:15:05" "12:30:01" "12:45:01" "13:00:00"
> "13:15:02"
> >
> > Thanks!
> >
> > Tim
> >
> >
> >
> >
> > Tim Clark
> > Department of Zoology
> > University of Hawaii
> >
> > ______________________________________________
> > R-help at r-project.org
> mailing list
> > https://stat.ethz.ch/mailman/listinfo/r-help
> > 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?
>