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 apol

Answer 1

strftime is your friend...

? strftime # to see the help and the format

set.seed(1)
dates <- Sys.Date() + sample(1:365, size = 10)

R> dates
 [1] "2012-02-11" "2012-03-21" "2012-06-01" "2012-09-30"
 [5] "2012-01-18" "2012-09-25" "2012-10-11" "2012-06-30"
 [9] "2012-06-18" "2011-11-28"

# %j for julian day - number of the day since the 1st of january each year

R> strftime(dates, format = "%j") # or format(dates, format = "%j")
 [1] "042" "081" "153" "274" "018" "269" "285" "182" "170" "332"

R> strftime(dates, format = "%w")
 [1] "6" "3" "5" "0" "3" "2" "4" "6" "1" "1"

# my locale is in French so...
R> strftime(dates, format = "%A")
 [1] "samedi"   "mercredi" "vendredi" "dimanche" "mercredi"
 [6] "mardi"    "jeudi"    "samedi"   "lundi"    "lundi" 

By the way what do you mean by day, day within the month, the week or the year ?