data.table (version 1.10.0)

IDateTime: Integer based date class

Description

Date and time classes with integer storage for fast sorting and grouping. Still experimental!

Usage

as.IDate(x, ...)
"as.IDate"(x, ...)
"as.IDate"(x, ...)
"as.Date"(x, ...)
"as.POSIXct"(x, tz = "UTC", time = 0, ...)
"as.chron"(x, time = NULL, ...)
"round"(x, digits = c("weeks", "months", "quarters","years"), ...)
as.ITime(x, ...)
"as.ITime"(x, ...)
"as.POSIXct"(x, tz = "UTC", date = as.Date(Sys.time()), ...)
"as.chron"(x, date = NULL, ...)
"as.character"(x, ...)
"format"(x, ...)
IDateTime(x, ...)
"IDateTime"(x, ...)
second(x)
minute(x)
hour(x)
yday(x)
wday(x)
mday(x)
week(x)
isoweek(x)
month(x)
quarter(x)
year(x)

Arguments

x

an object

...

arguments to be passed to or from other methods. For as.IDate.default, arguments are passed to as.Date. For as.ITime.default, arguments are passed to as.POSIXlt.

tz

time zone (see strptime).

date

date object convertable with as.IDate.

time

time-of-day object convertable with as.ITime.

digits

really units; one of the units listed for rounding. May be abbreviated.

Value

For as.IDate, a class of IDate and Date with the date stored as the number of days since some origin.For as.ITime, a class of ITime stored as the number of seconds in the day.For IDateTime, a data table with columns idate and itime in IDate and ITime format.second, minute, hour, yday, wday, mday, week, month, quarter, and year return integer values for second, minute, hour, day of year, day of week, day of month, week, month, quarter, and year, respectively.These values are all taken directly from the POSIXlt representation of x, with the notable difference that while yday, wday, and mon are all 0-based, here they are 1-based.

Details

IDate is a date class derived from Date. It has the same internal representation as the Date class, except the storage mode is integer. IDate is a relatively simple wrapper, and it should work in almost all situations as a replacement for Date.

Functions that use Date objects generally work for IDate objects. This package provides specific methods for IDate objects for mean, cut, seq, c, rep, and split to return an IDate object.

ITime is a time-of-day class stored as the integer number of seconds in the day. as.ITime does not allow days longer than 24 hours. Because ITime is stored in seconds, you can add it to a POSIXct object, but you should not add it to a Date object.

Conversions to and from Date, POSIXct, and chron formats are provided.

ITime does not account for time zones. When converting ITime and IDate to POSIXct with as.POSIXct, a time zone may be specified.

In as.POSIXct methods for ITime and IDate, the second argument is required to be tz based on the generic template, but to make converting easier, the second arguement is interpreted as a date instead of a time zone if it is of type IDate or ITime. Therefore, you can use either of the following: as.POSIXct(time, date) or as.POSIXct(date, time).

IDateTime takes a date-time input and returns a data table with columns date and time.

Using integer storage allows dates and/or times to be used as data table keys. With positive integers with a range less than 100,000, grouping and sorting is fast because radix sorting can be used (see sort.list).

Several convenience functions like hour and quarter are provided to group or extract by hour, month, and other date-time intervals. as.POSIXlt is also useful. For example, as.POSIXlt(x)$mon is the integer month. The R base convenience functions weekdays, months, and quarters can also be used, but these return character values, so they must be converted to factors for use with data.table. isoweek is ISO 8601-consistent.

The round method for IDate's is useful for grouping and plotting. It can round to weeks, months, quarters, and years.

References

G. Grothendieck and T. Petzoldt, ``Date and Time Classes in R,'' R News, vol. 4, no. 1, June 2004.

H. Wickham, http://gist.github.com/10238.

ISO 8601, http://www.iso.org/iso/home/standards/iso8601.htm

See Also

as.Date, as.POSIXct, strptime, DateTimeClasses

Examples

# create IDate:
(d <- as.IDate("2001-01-01"))

# S4 coercion also works
identical(as.IDate("2001-01-01"), as("2001-01-01", "IDate"))

# create ITime:
(t <- as.ITime("10:45"))

# S4 coercion also works
identical(as.ITime("10:45"), as("10:45", "ITime"))

(t <- as.ITime("10:45:04"))

(t <- as.ITime("10:45:04", format = "%H:%M:%S"))

as.POSIXct("2001-01-01") + as.ITime("10:45")

datetime <- seq(as.POSIXct("2001-01-01"), as.POSIXct("2001-01-03"), by = "5 hour")
(af <- data.table(IDateTime(datetime), a = rep(1:2, 5), key = "a,idate,itime"))

af[, mean(a), by = "itime"]
af[, mean(a), by = list(hour = hour(itime))]
af[, mean(a), by = list(wday = factor(weekdays(idate)))]
af[, mean(a), by = list(wday = wday(idate))]

as.POSIXct(af$idate)
as.POSIXct(af$idate, time = af$itime)
as.POSIXct(af$idate, af$itime)
as.POSIXct(af$idate, time = af$itime, tz = "GMT")

as.POSIXct(af$itime, af$idate)
as.POSIXct(af$itime) # uses today's date

(seqdates <- seq(as.IDate("2001-01-01"), as.IDate("2001-08-03"), by = "3 weeks"))
round(seqdates, "months")

if (require(chron)) {
    as.chron(as.IDate("2000-01-01"))
    as.chron(as.ITime("10:45"))
    as.chron(as.IDate("2000-01-01"), as.ITime("10:45"))
    as.chron(as.ITime("10:45"), as.IDate("2000-01-01"))
    as.ITime(chron(times = "11:01:01"))
    IDateTime(chron("12/31/98","10:45:00"))
}