在R中迭代POSIXlt

Question

Imagine I have an input vector of POSIXct objects:

times.input <- c(
  as.POSIXct('2013-01-01 00:00:00', tz='GMT'),
  as.POSIXct('2013-01-01 00:00:01', tz='GMT')
)
class(times.input)
# [1] "POSIXct" "POSIXt" 

Some of the functions (like round or trunc ) transform POSIXct to POSIXlt :

unique.dates <- unique(trunc(times.input, 'days')))
class(times.rounded)
# [1] "POSIXlt" "POSIXt"

Then, if we try to iterate or lapply this object, strange things happen:

lapply(X=unique.dates, FUN=print)
# [1] 0 0
# [1] 0 0
# [1] 4 4
# [1] 1 1
# [1] 0 0
# [1] 113 113
# [1] 2 2
# [1] 0 0
# [1] 0 0

It seems like POSIXlt is stored internally vertically as a list of vectors, and iteration goes over internal structure instead of iterating over each object.

I know that converting to POSIXct helps in this case, because POSIXct is stored as an integer internally, but I consider this solution to be ugly.

So my question is: given a POSIXlt[2] data input, how to perform for loop or lapply over POSIXct objects?

Answer 1

It's happening cause lapply works on list and you're right about storing, after help("DateTimeClasses") :

Class "POSIXlt" is a named list of vectors (...)

If you really need to work with POSIXlt I propose convert to list, eg:

times.input.L <- split(times.input, seq_along(times.input))
lapply(times.input.L, print)
[1] "2013-01-01 01:00:00 CET"
[1] "2013-01-01 01:00:01 CET"