On 10/11/24 11:56, Rui Barradas wrote:
Hello,
A way to have different time zones is to store t1 and t2 in list,
which are vectors. Just not atomic vectors.
I think it complicates what should be simple, but here it is.
# create two lists
t1 <- lapply(c("2024-01-01 12:30", "2024-01-01 12:30"), as.POSIXlt, tz
= "GMT")
t2 <- lapply(c("2024-01-01 12:30", "2024-01-01 12:30"), as.POSIXlt, tz
= "CET")
# this works but is it a wanted way of making simple computations?
Map(`-`, t1, t2)
#> [[1]]
#> Time difference of 1 hours
#>
#> [[2]]
#> Time difference of 1 hours
# mapply default is to simplify the result, if possible
mapply(`-`, t1, t2)
#> [1] 1 1
t1 <- lapply(c("2024-01-01 12:30", "2024-01-01 12:30"), as.POSIXlt, tz
= "GMT")
t2 <- lapply(c("2024-01-01 12:30", "2024-01-01 12:30"), as.POSIXlt, tz
= "CET")
# as documented in ?mapply > sapply, all attributes are lost,
# after simplification the class attribute follows the hierarchy
# NULL < raw < logical < integer < double < complex < character < list
< expression
mapply(`-`, t1, t2) |> str()
#> num [1:2] 1 1
# now change only one member of the list t1
attr(t1[[2]], "tzone") <- attr(t2[[2]], "tzone")
# t1 has two different time zones and the Map/mapply loops
# above still give the expected results
Map(`-`, t1, t2)
#> [[1]]
#> Time difference of 1 hours
#>
#> [[2]]
#> Time difference of 0 secs
mapply(`-`, t1, t2)
#> [1] 1 0
Hope this helps,
Rui Barradas
Thanks. That would basically mean writing a separate class for date-time
objects. If it was for a specific application where it is important to
keep the time zones, this might be a good solution. However, the dates
are output from a generic package and I would like to output something
that is practical for users. Such a list of dates as you are proposing
would not be supported by a lot of existing packages. Even something
like plotting with the dates on the x-axis would not work. Perhaps a
POSIXct object with an extra attribute containing the time zones would
then be easier: by default everything would then be in, for example, UTC.
As I mentioned in the previous mail, I currently have a solution that
converts everything to local time. But that means loosing the individual
time zone information.
Best,
Jan
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