How can I group days into weeks?

Question

In R, how can I associate between a vector of dates (days) and weeks?

Edit: "weeks" should be the week index within the date range and not within the year. My apologies for the ambiguity.

Answer 1

For each date, get the week of the year it belongs to by formatting it via format() using the %U of %W format placeholders. %U treats Sunday as the first day of the week, whereas %W considers Monday to be the first day of the week. Here is an example:

now <- as.Date(Sys.time())
dates <- seq(now, now + 25, by = "1 day") 

dat <- data.frame(Dates = dates, Week = format(dates, format = "%W"))
head(dat, 10)

Which gives:

> head(dat, 10)
        Dates Week
1  2011-11-06   44
2  2011-11-07   45
3  2011-11-08   45
4  2011-11-09   45
5  2011-11-10   45
6  2011-11-11   45
7  2011-11-12   45
8  2011-11-13   45
9  2011-11-14   46
10 2011-11-15   46

Although it is unclear from your question exactly what you want to do, a way to match this vector of dates (well, my data frame dat above) is via merge(). Say we have a vector of weeks we want to match against and some associated data in data frame weekdat:

weekdat <- data.frame(Week = 44:50, Price = c(10, 20, 25, 30, 20, 15, 10))

Then we can link dat with weekdat using merge() as so:

> merge(dat, weekdat)
   Week      Dates Price
1    44 2011-11-06    10
2    45 2011-11-07    20
3    45 2011-11-08    20
4    45 2011-11-09    20
5    45 2011-11-10    20
6    45 2011-11-11    20
7    45 2011-11-12    20
8    45 2011-11-13    20
9    46 2011-11-14    25
10   46 2011-11-15    25
11   46 2011-11-16    25
12   46 2011-11-17    25
13   46 2011-11-18    25
14   46 2011-11-19    25
15   46 2011-11-20    25
16   47 2011-11-21    30
17   47 2011-11-22    30
18   47 2011-11-23    30
19   47 2011-11-24    30
20   47 2011-11-25    30
21   47 2011-11-26    30
22   47 2011-11-27    30
23   48 2011-11-28    20
24   48 2011-11-29    20
25   48 2011-11-30    20
26   48 2011-12-01    20

There are other ways of matching or linking dates to weeks, but the above should give you something to work from.

Answer 2

Do you just want the number of completed 7 day intervals since the first record?

dvec <- as.Date("2001-04-01")+0:90
dweek <- as.numeric(dvec-dvec[1]) %/% 7
dweek[1:21]
# [1] 0 0 0 0 0 0 0 1 1 1 1 1 1 1 2 2 2 2 2 2 2