How to get week numbers from dates?

Question

Looking for a function in R to convert dates into week numbers (of year) I went for week from package data.table. However, I observed some strange behaviour:

> week("2014-03-16") # Sun, expecting 11
[1] 11
> week("2014-03-17") # Mon, expecting 12
[1] 11
> week("2014-03-18") # Tue, expecting 12
[1] 12

Why is the week number switching to 12 on tuesday, instead of monday? What am I missing? (Timezone should be irrelevant as there are just dates?!)

Other suggestions for (base) R functions are appreciated as well.

Answer 1

Base package

Using the function strftime passing the argument %V to obtain the week of the year as decimal number (01–53) as defined in ISO 8601. (More details in the documentarion: ?strftime)

strftime(c("2014-03-16", "2014-03-17","2014-03-18", "2014-01-01"), format = "%V")

Output:

[1] "11" "12" "12" "01"

Answer 2

if you try with lubridate:

library(lubridate)
lubridate::week(ymd("2014-03-16", "2014-03-17","2014-03-18", '2014-01-01'))

[1] 11 11 12  1

The pattern is the same. Try isoweek

lubridate::isoweek(ymd("2014-03-16", "2014-03-17","2014-03-18", '2014-01-01'))
[1] 11 12 12  1

Answer 3

Actually, I think you may have discovered a bug in the week(...) function, or at least an error in the documentation. Hopefully someone will jump in and explain why I am wrong.

Looking at the code:

library(lubridate)
> week
function (x) 
yday(x)%/%7 + 1
<environment: namespace:lubridate>

The documentation states:

Weeks is the number of complete seven day periods that have occured between the date and January 1st, plus one.

But since Jan 1 is the first day of the year (not the zeroth), the first "week" will be a six day period. The code should (??) be

(yday(x)-1)%/%7 + 1

NB: You are using week(...) in the data.table package, which is the same code as lubridate::week except it coerces everything to integer rather than numeric for efficiency. So this function has the same problem (??).

Answer 4

if you want to get the week number with the year use: "%Y-W%V":

e.g    yearAndweeks <- strftime(dates, format = "%Y-W%V")

so

> strftime(c("2014-03-16", "2014-03-17","2014-03-18", "2014-01-01"), format = "%Y-W%V")

becomes:

[1] "2014-W11" "2014-W12" "2014-W12" "2014-W01"

Answer 5

I think the problem is that the week calculation somehow uses the first day of the year. I don't understand the internal mechanics, but you can see what I mean with this example:

library(data.table)

dd <- seq(as.IDate("2013-12-20"), as.IDate("2014-01-20"), 1)
# dd <- seq(as.IDate("2013-12-01"), as.IDate("2014-03-31"), 1)

dt <- data.table(i = 1:length(dd),
                 day = dd,
                 weekday = weekdays(dd),
                 day_rounded = round(dd, "weeks"))
## Now let's add the weekdays for the "rounded" date
dt[ , weekday_rounded := weekdays(day_rounded)]
## This seems to make internal sense with the "week" calculation
dt[ , weeknumber := week(day)]
dt 

    i        day   weekday day_rounded weekday_rounded weeknumber
1:  1 2013-12-20    Friday  2013-12-17         Tuesday         51
2:  2 2013-12-21  Saturday  2013-12-17         Tuesday         51
3:  3 2013-12-22    Sunday  2013-12-17         Tuesday         51
4:  4 2013-12-23    Monday  2013-12-24         Tuesday         52
5:  5 2013-12-24   Tuesday  2013-12-24         Tuesday         52
6:  6 2013-12-25 Wednesday  2013-12-24         Tuesday         52
7:  7 2013-12-26  Thursday  2013-12-24         Tuesday         52
8:  8 2013-12-27    Friday  2013-12-24         Tuesday         52
9:  9 2013-12-28  Saturday  2013-12-24         Tuesday         52
10: 10 2013-12-29    Sunday  2013-12-24         Tuesday         52
11: 11 2013-12-30    Monday  2013-12-31         Tuesday         53
12: 12 2013-12-31   Tuesday  2013-12-31         Tuesday         53
13: 13 2014-01-01 Wednesday  2014-01-01       Wednesday          1
14: 14 2014-01-02  Thursday  2014-01-01       Wednesday          1
15: 15 2014-01-03    Friday  2014-01-01       Wednesday          1
16: 16 2014-01-04  Saturday  2014-01-01       Wednesday          1
17: 17 2014-01-05    Sunday  2014-01-01       Wednesday          1
18: 18 2014-01-06    Monday  2014-01-01       Wednesday          1
19: 19 2014-01-07   Tuesday  2014-01-08       Wednesday          2
20: 20 2014-01-08 Wednesday  2014-01-08       Wednesday          2
21: 21 2014-01-09  Thursday  2014-01-08       Wednesday          2
22: 22 2014-01-10    Friday  2014-01-08       Wednesday          2
23: 23 2014-01-11  Saturday  2014-01-08       Wednesday          2
24: 24 2014-01-12    Sunday  2014-01-08       Wednesday          2
25: 25 2014-01-13    Monday  2014-01-08       Wednesday          2
26: 26 2014-01-14   Tuesday  2014-01-15       Wednesday          3
27: 27 2014-01-15 Wednesday  2014-01-15       Wednesday          3
28: 28 2014-01-16  Thursday  2014-01-15       Wednesday          3
29: 29 2014-01-17    Friday  2014-01-15       Wednesday          3
30: 30 2014-01-18  Saturday  2014-01-15       Wednesday          3
31: 31 2014-01-19    Sunday  2014-01-15       Wednesday          3
32: 32 2014-01-20    Monday  2014-01-15       Wednesday          3
     i        day   weekday day_rounded weekday_rounded weeknumber

My workaround is this function: https://github.com/geneorama/geneorama/blob/master/R/round_weeks.R

round_weeks <- function(x){
    require(data.table)
    dt <- data.table(i = 1:length(x),
                     day = x,
                     weekday = weekdays(x))
    offset <- data.table(weekday = c('Sunday', 'Monday', 'Tuesday', 'Wednesday', 
                                     'Thursday', 'Friday', 'Saturday'),
                         offset = -(0:6))
    dt <- merge(dt, offset, by="weekday")
    dt[ , day_adj := day + offset]
    setkey(dt, i)
    return(dt[ , day_adj])
}

Of course, you can easily change the offset to make Monday first or whatever. The best way to do this would be to add an offset to the offset... but I haven't done that yet.

I provided a link to my simple geneorama package, but please don't rely on it too much because it's likely to change and not very documented.

Answer 6

I understand the need for packages in certain situations, but the base language is so elegant and so proven (and debugged and optimized).

Why not:

dt <- as.Date("2014-03-16")
dt2 <- as.POSIXlt(dt)
dt2$yday
[1] 74

And then your choice whether the first week of the year is zero (as in indexing in C) or 1 (as in indexing in R).

No packages to learn, update, worry about bugs in.

Answer 7

If you want to get the week number with the year, Grant Shannon's solution using strftime works, but you need to make some corrections for the dates around january 1st. For instance, 2016-01-03 (yyyy-mm-dd) is week 53 of year 2015, not 2016. And 2018-12-31 is week 1 of 2019, not of 2018. This codes provides some examples and a solution. In column "yearweek" the years are sometimes wrong, in "yearweek2" they are corrected (rows 2 and 5).

library(dplyr)
library(lubridate)

# create a testset
test <- data.frame(matrix(data = c("2015-12-31",
                                   "2016-01-03",
                                   "2016-01-04",
                                   "2018-12-30",
                                   "2018-12-31",
                                   "2019-01-01") , ncol=1, nrow = 6 ))
# add a colname
colnames(test) <- "date_txt"

# this codes provides correct year-week numbers
test <- test %>%
        mutate(date = as.Date(date_txt, format = "%Y-%m-%d")) %>%
        mutate(yearweek = as.integer(strftime(date, format = "%Y%V"))) %>%
        mutate(yearweek2 = ifelse(test = day(date) > 7 & substr(yearweek, 5, 6) == '01',
                                 yes  = yearweek + 100,
                                 no   = ifelse(test = month(date) == 1 & as.integer(substr(yearweek, 5, 6)) > 51,
                                               yes  = yearweek - 100,
                                               no   = yearweek)))
# print the result
print(test)

    date_txt       date yearweek yearweek2
1 2015-12-31 2015-12-31   201553    201553
2 2016-01-03 2016-01-03   201653    201553
3 2016-01-04 2016-01-04   201601    201601
4 2018-12-30 2018-12-30   201852    201852
5 2018-12-31 2018-12-31   201801    201901
6 2019-01-01 2019-01-01   201901    201901

Answer 8

Using only base, I wrote the following function.

Note:

1. Assumes Mon is day number 1 in the week
2. First week is week 1
3. Returns 0 if week is 52 from last year

Fine-tune to suit your needs.

findWeekNo <- function(myDate){
  # Find out the start day of week 1; that is the date of first Mon in the year
  weekday <- switch(weekdays(as.Date(paste(format(as.Date(myDate),"%Y"),"01-01", sep = "-"))),
                    "Monday"={1},
                    "Tuesday"={2},
                    "Wednesday"={3},
                    "Thursday"={4},
                    "Friday"={5},
                    "Saturday"={6},
                    "Sunday"={7}
  )

  firstMon <- ifelse(weekday==1,1, 9 - weekday )

  weekNo <- floor((as.POSIXlt(myDate)$yday - (firstMon-1))/7)+1
  return(weekNo)
}


findWeekNo("2017-01-15") # 2