R: How to judge Date in the same week? - r

Question

I want create a new colume to represent which date are in the same week.
A data.table DATE_SET contains Date information, like:
DATA_SET<- data.table(transday = seq(from = (Sys.Date()-64), to = Sys.Date(), by = 1))
For example, '2017-03-01' and '2017-03-02' are in the same week, '2017-03-01' and '2017-03-08' both Wednesday, but they are not in the same week.
If "2016-01-01" is the first week in 2016, "2017-01-01" is the first week in 2017, the value is 1, but they are not in the same week. So i want the unique value to pecify "a same week".

Answer 1

The answer to this question depends strongly on
the definition of the first day of the week (usually Sunday or Monday) and
the numbering of the weeks within the year (starting with the first Sunday, Monday, or Thursday of the year, or on 1st January, etc).
A selection of different options can be seen from the example below:
dates isoweek day week_iso week_US week_UK DT_week DT_iso lub_week lub_iso cut.Date
2015-12-25 2015-W52 5 2015-W52 51 51 52 52 52 52 2015-12-21
2015-12-26 2015-W52 6 2015-W52 51 51 52 52 52 52 2015-12-21
2015-12-27 2015-W52 7 2015-W52 52 51 52 52 52 52 2015-12-21
2015-12-28 2015-W53 1 2015-W53 52 52 52 53 52 53 2015-12-28
2015-12-29 2015-W53 2 2015-W53 52 52 52 53 52 53 2015-12-28
2015-12-30 2015-W53 3 2015-W53 52 52 53 53 52 53 2015-12-28
2015-12-31 2015-W53 4 2015-W53 52 52 53 53 53 53 2015-12-28
2016-01-01 2015-W53 5 2015-W53 00 00 1 53 1 53 2015-12-28
2016-01-02 2015-W53 6 2015-W53 00 00 1 53 1 53 2015-12-28
2016-01-03 2015-W53 7 2015-W53 01 00 1 53 1 53 2015-12-28
2016-01-04 2016-W01 1 2016-W01 01 01 1 1 1 1 2016-01-04
2016-01-05 2016-W01 2 2016-W01 01 01 1 1 1 1 2016-01-04
2016-01-06 2016-W01 3 2016-W01 01 01 1 1 1 1 2016-01-04
2016-01-07 2016-W01 4 2016-W01 01 01 2 1 1 1 2016-01-04
2016-01-08 2016-W01 5 2016-W01 01 01 2 1 2 1 2016-01-04
which is created by this code:
library(data.table)
dates <- as.Date("2016-01-01") + (-7:7)
print(data.table(
dates,
isoweek = ISOweek::ISOweek(dates),
day = ISOweek::ISOweekday(dates),
week_iso = format(dates, "%G-W%V"),
week_US = format(dates, "%U"),
week_UK = format(dates, "%W"),
DT_week = data.table::week(dates),
DT_iso = data.table::isoweek(dates),
lub_week = lubridate::week(dates),
lub_iso = lubridate::isoweek(dates),
cut.Date = cut.Date(dates, "week")
), row.names = FALSE)
The format YYYY-Www used in some of the columns is one of the ISO 8601 week formats. It includes the year which is required to distinguish different weeks in different years as requested by the OP.
The ISO week definition is the only format which ensures that each week always consists of 7 days, also across New Year. The other definitions may start or end the year with "weeks" with less than 7 days. Due to the seamless partioning of the year, the ISO week-numbering year is slightly different from the traditional Gregorian calendar year, e.g., 2016-01-01 belongs to the last ISO week 53 of 2015 (2015-W53).
As suggested here, cut.Date() might be the best option for the OP.
Disclosure: I'm maintainer of the ISOweek package which was published at a time when strptime() did not recognize the %G and %V format specifications for output in the Windows versions of R. (Still today they aren't recognized on input).

Answer 2

You can use the week() function of the lubridate package in R.
library(lubridate)
DATA_SET$week <- week(DATA_SET$transday)
This will give you a new column week. Dates within the same week will have same week number.