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

[Carlos J. Gil Bellosta]

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