Logo Questions Linux Laravel Mysql Ubuntu Git Menu
 

How to convert an ambiguous datetime column in data.table without using strptime?

My data.table has a column with an "ambiguous" datetime format: "12/1/2016 15:30". How can I convert this datetime to a format R recognizes in a data.table without using strptime() and getting the warning message for initially converting to POSIXlt. The process works but the warning makes me think there is another way.

My data table:

my_dates <- c("12/1/2016 15:30", "12/1/2016 15:31", "12/1/2016 15:32")
this <- c("a", "b", "c")
that <- c(1, 2, 3)

my_table <- data.table(my_dates, this, that)

my_table
   my_dates          this that
1: 12/1/2016 15:30    1    a
2: 12/1/2016 15:31    2    b
3: 12/1/2016 15:32    3    c

my_table[, my_dates := as.POSIXct(strptime(my_dates, "%m/%d/%Y %H:%M"))]

Warning message:
In strptime(my_dates, "%m/%d/%Y %H:%M") :
POSIXlt column type detected and converted to POSIXct. We do not
recommend use of POSIXlt at all because it uses 40 bytes to store one date.

So, it works but I bet there is a technique that I have simply overlooked to avoid this warning. I used my_table[, dates:= as.POSIXct(dates)] but this drops the time portion of the datetime. I have also tried my_table[, dates:= as.POSIXct(dates, "%m/%d/%Y %H:%M")] and the time is dropped as well and warnings are retruned.

Thank you for advice.

like image 542
Dodge Avatar asked Jan 06 '17 18:01

Dodge


1 Answers

We need the format argument and it can be converted using as.POSIXct alone without resorting to strptime/as.POSIXct

my_table[, my_dates := as.POSIXct(my_dates, format = "%m/%d/%Y %H:%M")]
my_table
#              my_dates this that
#1: 2016-12-01 15:30:00    a    1
#2: 2016-12-01 15:31:00    b    2
#3: 2016-12-01 15:32:00    c    3

The reason for the warning is because of the order of arguments in as.POSIXct/strptime. If we check ?as.POSIXct, the usage is

as.POSIXct(x, tz = "", ...)

That means, without specifying the format, it thinks that the second argument is for tz

like image 145
akrun Avatar answered Oct 10 '22 21:10

akrun