[R] Finding minimum of time subset

[Tim Clark]

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