[R] lubridate:ymd_hm and coercion of class POSIXct. Smooth way to restore the date format.

[R. Michael Weylandt]

Well, it might take some time to work out a lubridate solution that
works, but how about good old base R?

as.POSIXct(x$datetime)

Michael

On Fri, Mar 30, 2012 at 10:22 AM, Henrik Pärn <henrik.parn at bio.ntnu.no> wrote:
> Dear all,
>
> I wish to create a POSIXct variable from date and time variables using the ymd_hm function in package lubridate. In some cases data for time is missing, which causes a problem for ymd_hm. I wish to find a smooth way to handle this.
>
>
> # Some example data:
> x <- data.frame(date =  c("2011-09-22", "2011-07-28"), time = c("15:00", NA))
> x
>
> # paste date and time together to a format that ymd_hm recognizes
> x$datetime <- with(x, ifelse(!is.na(time), paste(date, time, sep = " "), NA))
> x
>
> # try ymd_hm on rows with complete data on date and time.
> # install.packages("lubridate")
> # library(lubridate)
> ymd_hm(x$datetime[!is.na(x$time)])
> #Looks OK
>
> # try on the whole data frame:
> ymd_hm(x$datetime)
> # missing data causes error. Fair enough.
>
> # try to allocate the result to the data frame on rows with complete data
> x$datetime2[!is.na(x$time)] <- ymd_hm(x$datetime[!is.na(x$time)])
> x
>
> class(ymd_hm(x$datetime[!is.na(x$time)]))
> class(x$datetime2)
> # POSIXct is coerced to numeric. Fair enough
>
> # try to pre-allocate a vector and convert it to class POSIXct:
> x$datetime3 <- NA
> class(x$datetime3) <- "POSIXct"
> x
>
> x$datetime3[!is.na(x$time)] <- ymd_hm(x$datetime[!is.na(x$time)])
> x
>
> # something happened with the time (15:00 vs 17:00). A time zone issue?
> tz(x$datetime3[!is.na(x$time)])
> Sys.timezone()
>
> x$datetime3 <- with_tz(x$datetime3, "CEST")
> x
> # warnings, but correct time
>
> # another 'random' try
> x$datetime3 <- NA
> x
> class(x$datetime3) <- "POSIXct"
> x$datetime3[!is.na(x$time)] <- ymd_hm(x$datetime[!is.na(x$time)])
> x
>
> tz(x$datetime3)
> x$datetime3 <- with_tz(x$datetime3, "UTC")
> x
>
> # works, but I don't understand why.
> #------------------------------------------
>
> sessionInfo()
> R version 2.14.2 (2012-02-29)
> Platform: i386-pc-mingw32/i386 (32-bit)
>
> locale:
> [1] LC_COLLATE=Norwegian (Bokmål)_Norway.1252  LC_CTYPE=Norwegian (Bokmål)_Norway.1252    LC_MONETARY=Norwegian (Bokmål)_Norway.1252 LC_NUMERIC=C
> [5] LC_TIME=Norwegian (Bokmål)_Norway.1252
>
> attached base packages:
> [1] grDevices datasets  splines   graphics  stats     tcltk     utils     methods   base
>
> other attached packages:
> [1] lubridate_1.1.0  svSocket_0.9-51  TinnR_1.0.3      R2HTML_2.2       Hmisc_3.9-2      survival_2.36-12
>
> loaded via a namespace (and not attached):
> [1] cluster_1.14.2 grid_2.14.2    lattice_0.20-0 plyr_1.7.1     stringr_0.6    svMisc_0.9-63  tools_2.14.2
>
> Windows 7
> #------------------------------------------
>
> I am sure I am missing something. Can anyone point me to a smooth way to handle the time zones (if that is *the* problem here...)?
>
> Thanks a lot in advance for taking your time.
>
>
> Best regards,
>
>
> Henrik
>
> --
>
> Henrik Pärn
>
> Centre for Conservation Biology
>
> Department of Biology
>
> Norwegian University of Science and Technology
>
> NO-7491 Trondheim
>
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>
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