[Rd] Unexpected behabiour of min, tapply and POSIXct/POSIXlt classes?

[Joris Meys]

Hi Carlos,

this definitely does not belong to r-devel, just ask it on the normal
help list or on www.stackoverflow.com next time.

If you read the help file, this behaviour is exactly as expected :
---start quote---
Note that if the return value has a class (e.g. an object of class
"Date") the class is discarded.
---end quote---

see ?tapply.

So you do:
tapply( my.times, my.period, min , simplify=F)

There's your class.

Cheers
Joris





On Thu, Oct 28, 2010 at 7:11 PM, Carlos J. Gil Bellosta
<cgb at datanalytics.com> wrote:
> Hello,
>
> I found rather surprising the behaviour of POSIXct and POSIXlt classes
> when combined with min and tapply.
>
> The details can be deduced from the script below:
>
> ############# Start of the script ####################
>
> before <- Sys.time()
>
> Sys.sleep( 1 )
>
> now1 <- now2 <- Sys.time()
>
> my.times <- c( before,  now1, now2 )
>
> class( my.times )                               ## [1] "POSIXct" "POSIXt"
>
> min( my.times )                                 ## [1] "2010-10-28 18:52:17 CEST"
>
> ### So far, so good... but:
>
> my.period <- c( "a", "b", "b" )
> tapply( my.times, my.period, min )
>
> ##          a          b
> ## 1288284737 1288284780
>
> ## Where did my POSIXct class go?
>
>
> my.times.lt <- as.POSIXlt( my.times )
>
> min( my.times.lt )                              ## [1] "2010-10-28 18:52:17 CEST"; good!
>
> tapply( my.times.lt, my.period, min )
>
> # $a
> # [1] 17.449
> #
> # $b
> # [1] 52
> #
> # Mensajes de aviso perdidos
> # In ansmat[index] <- ans :
> #   número de items para para sustituir no es un múltiplo de la
> longitud del reemplazo
> #
> # ¿?  :(
>
>
> ############# End of the script ####################
>
> Here are the details of my session (as reported by sessionInfo):
>
> R version 2.12.0 (2010-10-15)
> Platform: i386-pc-mingw32/i386 (32-bit)
>
> locale:
> [1] LC_COLLATE=Spanish_Spain.1252  LC_CTYPE=Spanish_Spain.1252
> LC_MONETARY=Spanish_Spain.1252 LC_NUMERIC=C
> [5] LC_TIME=Spanish_Spain.1252
>
> attached base packages:
> [1] stats     graphics  grDevices utils     datasets  methods   base
>
> other attached packages:
> [1] RODBC_1.3-2 plotrix_3.0
>
> loaded via a namespace (and not attached):
> [1] tools_2.12.0
>
> I am running R on a Windows XP Box.
>
>
> Thank you very much,
>
> Carlos J. Gil Bellosta
> http://www.datanalytics.com
>
> ______________________________________________
> R-devel at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
>



-- 
Joris Meys
Statistical consultant

Ghent University
Faculty of Bioscience Engineering
Department of Applied mathematics, biometrics and process control

tel : +32 9 264 59 87
Joris.Meys at Ugent.be
-------------------------------
Disclaimer : http://helpdesk.ugent.be/e-maildisclaimer.php