Steve,
Just below are some examples that I hope will help.
With regard to what you've tried, I don't see any reason for using with(),
or the select argument to subset(). They both look unnecessary to me.
## examples of subsetting date-time values
## create fake data
tmp <- seq(as.POSIXct('2011-08-01 13:00'), as.POSIXct('2011-08-05 03:00'),
len=42)
df <- data.frame(tm=tmp, x=seq(42))
## subset examples
## on or before the 2nd at 01:30
df1 <- subset(df, tm <= as.POSIXct('2011-08-02 1:30'))
## everying on the 3rd
df2 <- subset(df, format(tm,'%d')=='03')
## everything in hours 11am through 1pm inclusive
df3 <- subset(df, format(tm,'%H') %in% c('11','12','13'))
## 11 am through 3:59 pm on the 2nd
df4 <- subset(df, tm >= as.POSIXct('2011-08-02 11:00') & tm <=
as.POSIXct('2011-08-02 15:59'))
## just for reference, a sequence of every 15 minutes
tmp <- seq(as.POSIXct('2011-08-01 13:00'), as.POSIXct('2011-08-02 03:00'),
by='15 min')
Note that all comparisons use POSIXct class objects, converting character
to POSIXct where needed. As Luke mentioned, if the character strings are
in standard format, 'yyyy-mm-dd HH:MM:SS', just use as.POSIXct() without
any additional args.
-Don
--
Don MacQueen
Lawrence Livermore National Laboratory
7000 East Ave., L-627
Livermore, CA 94550
925-423-1062
On 9/8/11 1:03 PM, "Steve E." <se...@vt.edu> wrote:
>Dear R Community,
>
>I am new to R, and have a question that I suspect may be quite simple but
>is
>proving a formidable roadblock for me. I have a large data set that
>includes water-quality measurements collected over many 24-hour periods.
>The date and time of sample collection are in a combined Date/Time field
>in
>the format yyyy-mm-dd hh:mm:ss. I need to be able to subset the data for
>analysis of different date and time windows. Thus far, I have tried
>casting
>the Date/Time field using several approaches, such as:
>
>DataSet$NewDateTime <- strptime(DataSet$DateTime, '%Y-%m-%d %H:%M:%S')
>DataSet$NewDateTime <- as.POSIXlt(strptime(DataSet$DateTime, '%Y-%m-%d
>%H:%M:S'))
>
>These instructions seem to cast the NewDateTime field correctly (at least
>it
>appears to be in the correct format, and I assume R sees the field as a
>date
>and a time) but I am then unable to subset the data using instructions
>such
>as:
>
>with(DataSet, subset(DataSet, DataSet$NewDateTime < '2004-08-05
>14:15:00'))
>DataSubset <- subset(DataSet, DataSet$NewDateTime < '2004-08-05 14:00:00',
>select = DataSet)
>
>I have tried also separating the date and time fields in the input file,
>and
>casting with instructions such as:
>
>DataSet$NewTime <- strptime(DataSet$Time, '%H:%M:%S')
>DataSet$NewTime <- as.POSIXct(strptime(DataSet$Time, '%H:%M:%S'))
>
>but these seem to generate a NewTime field that contains today's date +
>the
>time data, and also will not subset based on date/time.
>
>I appreciate greatly any help and advice,
>Steve
>
>--
>View this message in context:
>http://r.789695.n4.nabble.com/help-subsetting-data-based-on-date-AND-time-
>tp3799933p3799933.html
>Sent from the R help mailing list archive at Nabble.com.
>
>______________________________________________
>R-help@r-project.org mailing list
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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.
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