R devel - Oct 2010 - Unexpected behabiour of min, tapply and POSIXct/POSIXlt classes?

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 :
#   n?mero de items para para sustituir no es un m?ltiplo 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

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 :
> # ? n?mero de items para para sustituir no es un m?ltiplo 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
-------------------------------
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