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    gt  week  2014-03-16     Sun  expecting 11  1  11  gt  week  2014-03-17     Mon  expecting 12  1  11  gt  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

User · Answer

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.

User · Answer

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  lt - seq as IDate  2013-12-20    as IDate  2014-01-20    1    dd  lt - seq as IDate  2013-12-01    as IDate  2014-03-31    1   dt  lt - 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  lt - function x       require data table      dt  lt - data table i   1 length x                        day   x                       weekday   weekdays x       offset  lt - data table weekday   c  Sunday    Monday    Tuesday    Wednesday                                          Thursday    Friday    Saturday                             offset   - 0 6       dt  lt - 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

User · Answer

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  lt - 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   lt -  date txt     this codes provides correct year-week numbers test  lt - test   gt           mutate date   as Date date txt  format     Y- m- d      gt           mutate yearweek   as integer strftime date  format     Y V       gt           mutate yearweek2   ifelse test   day date   gt  7  amp  substr yearweek  5  6      01                                    yes    yearweek   100                                   no     ifelse test   month date     1  amp  as integer substr yearweek  5  6    gt  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

User · Answer

Using only base  I wrote the following function   Note    Assumes Mon is day number 1 in the week First week is week 1 Returns 0 if week is 52 from last year   Fine-tune to suit your needs   findWeekNo  lt - function myDate       Find out the start day of week 1  that is the date of first Mon in the year   weekday  lt - 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  lt - ifelse weekday  1 1  9 - weekday      weekNo  lt - floor  as POSIXlt myDate  yday -  firstMon-1   7  1   return weekNo      findWeekNo  2017-01-15     2

User · Answer

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   gt  week function  x   yday x    7   1  lt environment  namespace lubridate gt    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

User · Answer

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

User · Answer

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

User · Answer

if you want to get the week number with the year use    Y-W V    e g    yearAndweeks  lt - strftime dates  format     Y-W V     so    gt  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

[r] How to get week numbers from dates?

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