[R] problem with unlist POSIX date at midnight

[Prof Brian Ripley]

[Unstated, but a near repeat of a posting earlier today with the same 
subject, in response to unacknowledged help.]

The problem is not with unlist (as in your subject line) but your input to
strptime.

strptime() is intended to convert character strings, and

> as.character(acc.period[16])
[1] "2006-03-01"

is not in the format you specified.  Why are you using strptime() to 
convert objects of class POSIXct to class POSIXlt and not as.POSIXlt()?


On Wed, 17 Jan 2007, Hanneke Schuurmans wrote:

> Dear R-users,
>
> I use unlist of POSIX dates to extract the year, hour etc. With that I
> can search for files in my database which are in the form
> 'yyyymmddhh_synops.txt'
> However, I get stucked during midnight where unlist just gives NA's.
> The script is given below; the problem accurs at acc.period[16]
> (midnight). However when I write out the character, unlist works well.
> But as.character(acc.period[16]) is not the solution....
>
> > dates=seq(as.Date("2006-03-01"), as.Date("2006-11-01"), "days")
> > i=1
> > begin=paste(dates[i]-1,"09:00:00")
> > end=paste(dates[i],"08:00:00")
> > acc.period=seq(as.POSIXct(begin),as.POSIXct(end),"hour")
> >
> > unlist(strptime(acc.period[15],format="%Y-%m-%d %H:%M:%S"))
>  sec   min  hour  mday   mon  year  wday  yday isdst
>    0     0    23    28     1   106     2    58     0
> > unlist(strptime(acc.period[16],format="%Y-%m-%d %H:%M:%S"))
>  sec   min  hour  mday   mon  year  wday  yday isdst
>   NA    NA    NA    NA    NA    NA    NA    NA    -1
> > unclass(strptime(acc.period[16],format="%Y-%m-%d %H:%M:%S"))
> $sec
> [1] NA
>
> $min
> [1] NA
>
> $hour
> [1] NA
>
> $mday
> [1] NA
>
> $mon
> [1] NA
>
> $year
> [1] NA
>
> $wday
> [1] NA
>
> $yday
> [1] NA
>
> $isdst
> [1] -1
>
> >
> > a="2006-03-01 00:00:00"
> > unlist(strptime(a,format="%Y-%m-%d %H:%M:%S"))
>  sec   min  hour  mday   mon  year  wday  yday isdst
>    0     0     0     1     2   106     3    59     0
> >
> > sessionInfo()
> R version 2.4.0 (2006-10-03)
> i386-pc-mingw32
>
> locale:
> LC_COLLATE=English_United States.1252;LC_CTYPE=English_United
> States.1252;LC_MONETARY=English_United
> States.1252;LC_NUMERIC=C;LC_TIME=English_United States.1252
>
> attached base packages:
> [1] "methods"   "stats"     "graphics"  "grDevices" "utils"     "datasets"
> [7] "base"
>
> Thanks in advance!
>
> Hanneke
>
>

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595