assign POSIXlt date to data.table column

Question

data.table 1.9.6 throws an error when you try to assign a POSIXlt date to a column

ex:

dt <- data.table(strdate = c("20140101", "20140103"))
dt[, date := strptime(strdate, "%Y%m%d")]
Warning message:
  In `[.data.table`(dt, , `:=`(date, strptime(strdate, "%Y%m%d"))) :
  Supplied 11 items to be assigned to 2 items of column 'date' (9 unused)

dt
    strdate date
1: 20140101  0,0
2: 20140103  0,0

However, a data frame works

df <- as.data.frame(dt)
df$date <- strptime(df$strdate, "%Y%m%d")
df
   strdate       date
1 20140101 2014-01-01
2 20140103 2014-01-03

Is there something about data.table that makes it unable to handle this data type?

Take a look at `unclass(strptime(dt$strdate, "%Y%m%d"))` - `POSIXlt` is actually a list object. Also see - http://stackoverflow.com/questions/21487614/error-creating-r-data-table-with-date-time-posixlt — thelatemail, Nov 14 '16 at 23:00
This is documented. Just use `POSIXct` instead. `data.table` has fine converters as `as.IDate()` and `as.ITime()`. See the answer I just added. — Dirk Eddelbuettel, Nov 14 '16 at 23:12

Dirk Eddelbuettel · Accepted Answer · 2016-11-14T23:24:02.057

Just use POSIXct which you, inter alia, get for free from the anytime package.

R> dt <- data.table(strdate = c("20140101", "20140103"))
R> dt
    strdate
1: 20140101
2: 20140103
R> library(anytime)
R> dt[, date := anytime(strdate)]   # creates POSIXct, use anydate() for Date
R> dt
    strdate       date
1: 20140101 2014-01-01
2: 20140103 2014-01-03
R>

The column now contains proper date(time) representation which you can still compute on:

R> dt[, newdate := as.IDate(date) + 7]
R> dt
    strdate       date    newdate
1: 20140101 2014-01-01 2014-01-08
2: 20140103 2014-01-03 2014-01-10
R>

Or, if you want dates, just use the Date type, eg via anydate():

R> dt <- data.table(strdate = c("20140101", "20140103"))
R> dt[, date := anydate(strdate)]
R> dt[, newdate := date + 7]
R> dt
    strdate       date    newdate
1: 20140101 2014-01-01 2014-01-08
2: 20140103 2014-01-03 2014-01-10
R>

score 1 · Answer 2 · edited May 23 '17 at 12:30

1

Here's a workaround using lubridate:

dt$date <- as.POSIXct(lubridate::ymd(strptime(dt$strdate, "%Y%m%d")))

    strdate       date
1: 20140101 2014-01-01
2: 20140103 2014-01-03

Please also see this post on why we use POSIXct instead of POSIXlt with data.table.

edited May 23 '17 at 12:30

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answered Nov 14 '16 at 23:06

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assign POSIXlt date to data.table column

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