R help - Nov 2004 - Unable to understand strptime() behaviour

R V2.0.1 on Windows XP.

I have read the help pages on strptime() over and over, but can't
understand why strptime() is producing the following results.

   > v <- format("2002-11-31", format="%Y-%m-%d")
   > v
[1] "2002-11-31"
   > factor(v, levels=v)
[1] 2002-11-31
Levels: 2002-11-31
   > x <- strptime("2002-11-31", format="%Y-%m-%d")
   > x
[1] "2002-12-01"
   > factor(x, levels=x)
[1] <NA> <NA> <NA> <NA> <NA> <NA> <NA>
<NA> <NA>
Levels: 2002-12-01 NA NA NA NA NA NA NA NA

Tim C

The result of strptime is a 9-element list.  From the help page

Value:

      'strptime' turns character representations into an object of class
      '"POSIXlt"'.

See Also:

      DateTimeClasses for details of the date-time classes; 'locales' to
      query or set a locale.

Try ?DateTimeClasses.


On Wed, 1 Dec 2004, Tim Churches wrote:
> R V2.0.1 on Windows XP.
>
> I have read the help pages on strptime() over and over, but can't
> understand why strptime() is producing the following results.
>
>  > v <- format("2002-11-31", format="%Y-%m-%d")
>  > v
> [1] "2002-11-31"
>  > factor(v, levels=v)
> [1] 2002-11-31
> Levels: 2002-11-31
>  > x <- strptime("2002-11-31", format="%Y-%m-%d")
>  > x
> [1] "2002-12-01"
>  > factor(x, levels=x)
> [1] <NA> <NA> <NA> <NA> <NA> <NA>
<NA> <NA> <NA>
> Levels: 2002-12-01 NA NA NA NA NA NA NA NA
I can't understand why you are trying to turn a classed object into a 
factor.

-- 
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

Try to say:

class(x)
unclass(x)

and it will dawn on you what goes on.

----------------------
Bendix Carstensen
Senior Statistician
Steno Diabetes Center
Niels Steensens Vej 2
DK-2820 Gentofte
Denmark
tel: +45 44 43 87 38
mob: +45 30 75 87 38
fax: +45 44 43 07 06
bxc at steno.dk
www.biostat.ku.dk/~bxc
----------------------


> -----Original Message-----
> From: r-help-bounces at stat.math.ethz.ch 
> [mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Tim Churches
> Sent: Tuesday, November 30, 2004 11:37 PM
> To: r-help at stat.math.ethz.ch
> Subject: [R] Unable to understand strptime() behaviour
> 
> 
> R V2.0.1 on Windows XP.
> 
> I have read the help pages on strptime() over and over, but 
> can't understand why strptime() is producing the following results.
> 
>    > v <- format("2002-11-31", format="%Y-%m-%d")
>    > v
> [1] "2002-11-31"
>    > factor(v, levels=v)
> [1] 2002-11-31
> Levels: 2002-11-31
>    > x <- strptime("2002-11-31",
format="%Y-%m-%d")
>    > x
> [1] "2002-12-01"
>    > factor(x, levels=x)
> [1] <NA> <NA> <NA> <NA> <NA> <NA>
<NA> <NA> <NA>
> Levels: 2002-12-01 NA NA NA NA NA NA NA NA
> 
> Tim C
> 
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list 
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read 
> the posting guide! http://www.R-project.org/posting-guide.html
>

Tim,

which aspect of the results?  November has 30 days :)  What is your goal?

(I'm guessing) try:

x <- as.POSIXct(strptime("2002-11-31",
format="%Y-%m-%d"))

see e.g.

https://stat.ethz.ch/pipermail/r-help/2003-May/032823.html

I hope that this helps,

Andrew.

On Wed, Dec 01, 2004 at 09:36:37AM +1100, Tim Churches
wrote:
> R V2.0.1 on Windows XP.
> 
> I have read the help pages on strptime() over and over, but can't
> understand why strptime() is producing the following results.
> 
>   > v <- format("2002-11-31", format="%Y-%m-%d")
>   > v
> [1] "2002-11-31"
>   > factor(v, levels=v)
> [1] 2002-11-31
> Levels: 2002-11-31
>   > x <- strptime("2002-11-31",
format="%Y-%m-%d")
>   > x
> [1] "2002-12-01"
>   > factor(x, levels=x)
> [1] <NA> <NA> <NA> <NA> <NA> <NA>
<NA> <NA> <NA>
> Levels: 2002-12-01 NA NA NA NA NA NA NA NA
> 
> Tim C
> 
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide! 
> http://www.R-project.org/posting-guide.html
-- 
Andrew Robinson                      Ph: 208 885 7115
Department of Forest Resources       Fa: 208 885 6226
University of Idaho                  E : andrewr at uidaho.edu
PO Box 441133                        W : http://www.uidaho.edu/~andrewr
Moscow ID 83843                      Or: http://www.biometrics.uidaho.edu
No statement above necessarily represents my employer's opinion.

Hi Tim

On 1 Dec 2004 at 9:36, Tim Churches wrote:
> R V2.0.1 on Windows XP.
> 
> I have read the help pages on strptime() over and over, but can't
> understand why strptime() is producing the following results.
> 
>    > v <- format("2002-11-31", format="%Y-%m-%d")
>    > v
> [1] "2002-11-31"
>    > factor(v, levels=v)
> [1] 2002-11-31
> Levels: 2002-11-31
>    > x <- strptime("2002-11-31",
format="%Y-%m-%d")
>    > x
> [1] "2002-12-01"
>    > factor(x, levels=x)
> [1] <NA> <NA> <NA> <NA> <NA> <NA>
<NA> <NA> <NA>
> Levels: 2002-12-01 NA NA NA NA NA NA NA NA

This is due to difference in POSIXlt and POSIXct
see
> length(factor(x, levels=x))
[1] 9
> y<-as.POSIXct(x)
> factor(y, levels=y)
[1] 2002-12-01
Levels: 2002-12-01

I do now the result but I do not now the intimate details about 
classes.

Cheers
Petr

> 
> Tim C
> 
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide!
> http://www.R-project.org/posting-guide.html
Petr Pikal
petr.pikal at precheza.cz