将日期转换为年份和周数并返回

Question

I want to convert a date to a combination of year and week number. Later I want to convert back to date again. I do not understand why it is not possible to arrive back at the same original date, even when I try all the different start of the week numbers:

library(dplyr)

df <- tidyr::expand_grid(
  date = as.Date(paste("2021 12 29"), "%Y %m %d") + 0:10,
  weekday = 0:6
)

df <- df %>%
  mutate(
    year = format(date, "%Y"),
    week = format(date, "%W"),
    newDate = as.Date(paste(year, week, weekday), format = "%Y %W %w")
  ) %>%
  tidyr::pivot_wider(
    id_cols = c("date"), names_from = "weekday", 
    values_from = "newDate"
  )

print(df)
# A tibble: 11 x 8
   date       `0`        `1`        `2`        `3`        `4`        `5`        `6`       
   <date>     <date>     <date>     <date>     <date>     <date>     <date>     <date>    
 1 2021-12-29 2021-12-26 2021-12-27 2021-12-28 2021-12-29 2021-12-30 2021-12-31 NA        
 2 2021-12-30 2021-12-26 2021-12-27 2021-12-28 2021-12-29 2021-12-30 2021-12-31 NA        
 3 2021-12-31 2021-12-26 2021-12-27 2021-12-28 2021-12-29 2021-12-30 2021-12-31 NA        
 4 2022-01-01 NA         NA         NA         NA         NA         NA         2022-01-01
 5 2022-01-02 NA         NA         NA         NA         NA         NA         2022-01-01
 6 2022-01-03 2022-01-02 2022-01-03 2022-01-04 2022-01-05 2022-01-06 2022-01-07 2022-01-08
 7 2022-01-04 2022-01-02 2022-01-03 2022-01-04 2022-01-05 2022-01-06 2022-01-07 2022-01-08
 8 2022-01-05 2022-01-02 2022-01-03 2022-01-04 2022-01-05 2022-01-06 2022-01-07 2022-01-08
 9 2022-01-06 2022-01-02 2022-01-03 2022-01-04 2022-01-05 2022-01-06 2022-01-07 2022-01-08
10 2022-01-07 2022-01-02 2022-01-03 2022-01-04 2022-01-05 2022-01-06 2022-01-07 2022-01-08
11 2022-01-08 2022-01-02 2022-01-03 2022-01-04 2022-01-05 2022-01-06 2022-01-07 2022-01-08

Why is there not a column that does not contains NAs?

Output of sessionInfo() :

R version 3.6.2 (2019-12-12)
Platform: x86_64-w64-mingw32/x64 (64-bit)
Running under: Windows 10 x64 (build 17763)

Matrix products: default

locale:
[1] LC_COLLATE=English_United Kingdom.1252  LC_CTYPE=English_United Kingdom.1252   
[3] LC_MONETARY=English_United Kingdom.1252 LC_NUMERIC=C                           
[5] LC_TIME=English_United Kingdom.1252    

Answer 1

Right now you are setting newDate to be equal to a specific year a specific month and then a range over a range of 7 days. Which is why newDate is currently:

"2021-01-03" "2021-01-04" "2021-01-05" "2021-01-06" "2021-01-07" "2021-01-01" "2021-01-02" NA

To get your desired output of returning the same date you initially had, you have to store the day as well and reference it in the creation of newDate. In short, do this:

date <- as.Date(paste("2021 01 03"), "%Y %m %d")
year <- format(date, "%Y")
week <- format(date, "%W")
day <- format(date, "%d")
newDate <- as.Date(paste(year, week, day), format = "%Y %W %w")

Answer 2

%G and %V solved my problem:

library(dplyr)

df <- tidyr::expand_grid(
  date = as.Date(paste("2021 12 26"), "%Y %m %d") + 0:10,
  weekday = 0:6
)

df <- df %>%
  mutate(
    year = format(date, "%G"),
    week = format(date, "%V"),
    newDate = as.Date(paste(year, week, weekday), format = "%Y %W %w")
  ) %>%
  tidyr::pivot_wider(
    id_cols = c("date", "year", "week"), names_from = "weekday", 
    values_from = "newDate"
  )

print(df)
# A tibble: 11 x 10
   date       year  week  `0`        `1`        `2`        `3`        `4`        `5`       
   <date>     <chr> <chr> <date>     <date>     <date>     <date>     <date>     <date>    
 1 2021-12-26 2021  51    2021-12-19 2021-12-20 2021-12-21 2021-12-22 2021-12-23 2021-12-24
 2 2021-12-27 2021  52    2021-12-26 2021-12-27 2021-12-28 2021-12-29 2021-12-30 2021-12-31
 3 2021-12-28 2021  52    2021-12-26 2021-12-27 2021-12-28 2021-12-29 2021-12-30 2021-12-31
 4 2021-12-29 2021  52    2021-12-26 2021-12-27 2021-12-28 2021-12-29 2021-12-30 2021-12-31
 5 2021-12-30 2021  52    2021-12-26 2021-12-27 2021-12-28 2021-12-29 2021-12-30 2021-12-31
 6 2021-12-31 2021  52    2021-12-26 2021-12-27 2021-12-28 2021-12-29 2021-12-30 2021-12-31
 7 2022-01-01 2021  52    2021-12-26 2021-12-27 2021-12-28 2021-12-29 2021-12-30 2021-12-31
 8 2022-01-02 2021  52    2021-12-26 2021-12-27 2021-12-28 2021-12-29 2021-12-30 2021-12-31
 9 2022-01-03 2022  01    2022-01-02 2022-01-03 2022-01-04 2022-01-05 2022-01-06 2022-01-07
10 2022-01-04 2022  01    2022-01-02 2022-01-03 2022-01-04 2022-01-05 2022-01-06 2022-01-07
11 2022-01-05 2022  01    2022-01-02 2022-01-03 2022-01-04 2022-01-05 2022-01-06 2022-01-07
# ... with 1 more variable: 6 <date>