[R] Preserving the class of POSIXt objects

[Kenn Konstabel]

Dear all,

I am working with a list of objects each of which contains two POSIXct
objects (say, $Start and $End) and a number of different data in
addition to that. Now an easy way to extract Start times of all object
could be sapply(x, "[", "Start") but this converts them all to
numeric, and so does sapply(x, "[[", "Start"). lapply preserves the
class but is obviously a list rather than an atomic vector but using
unlist is a way of getting rid of the class again. I know the vector
can be converted back using as.POSIXct and that one can set the class
attribute back to "POSIXct" but is there an easier trick that I can't
just figure out? Thanks in advance for any suggestions.

A reproducible example:

foo <- as.list(Sys.time()+1:10)
sapply(foo, "[", 1)

I've also noticed that trunc.POSIXt does something unexpected (to me)
in some cases:

foo <- as.POSIXct("2011-03-25 21:03:20")+1:10
length(foo) ## obviously 10
length(trunc(foo, "day"))  # 1!

This makes it impossible to use [ and length() with trunc'ed POSIXct
vectors (and it is a little unexpected too that the result of using
trunc with a POSIX_ct_ object is a POSIX_lt_ object) but the cure
seems easy: in the code of [.POSIXt,

##               old           ##  new
            x$sec <- 0      ##  x$sec[] <-  0
            x$min <- 0L    ##   x$min[] <- 0L
            x$hour <- 0L   ##   x$hour[] <- 0L

After these changes, [ and length would work "correctly" (in my
possibly naive understanding), and length(trunc(foo, "day")) would
return 10 in the example above.


Best regards,

Kenn

Kenn Konstabel
National Institute for Health Development
Hiiu 42
Tallinn
Estonia