R: Create new variable based on date in other variable-CodePudding

I have a data frame that looks somewhat like this:

a = c(seq(as.Date("2020-08-01"), as.Date("2020-11-01"), by="months"), seq(as.Date("2021-08-01"), as.Date("2021-11-01"), by="months"),
  seq(as.Date("2022-08-01"), as.Date("2022-11-01"), by="months"))

b = rep(LETTERS[1:3], each = 4)

df = data_frame(ID = b, Date = a)

> df
  ID    Date      
 <chr> <date>    
 1 A     2020-08-01
 2 A     2020-09-01
 3 A     2020-10-01
 4 A     2020-11-01
 5 B     2021-08-01
 6 B     2021-09-01
 7 B     2021-10-01
 8 B     2021-11-01
 9 C     2022-08-01
10 C     2022-09-01
11 C     2022-10-01
12 C     2022-11-01

And I want to create a new variable that replaces Date with the smallest value in Date for each ID, the resulting data frame should look like this:

c = c(rep(as.Date("2020-08-01"), each = 4), rep(as.Date("2021-08-01"), each = 4), rep(as.Date("2022-08-01"), each = 4))

df$NewDate = c

> df
# A tibble: 12 × 3
   ID    Date       NewDate   
   <chr> <date>     <date>    
 1 A     2020-08-01 2020-08-01
 2 A     2020-09-01 2020-08-01
 3 A     2020-10-01 2020-08-01
 4 A     2020-11-01 2020-08-01
 5 B     2021-08-01 2021-08-01
 6 B     2021-09-01 2021-08-01
 7 B     2021-10-01 2021-08-01
 8 B     2021-11-01 2021-08-01
 9 C     2022-08-01 2022-08-01
10 C     2022-09-01 2022-08-01
11 C     2022-10-01 2022-08-01
12 C     2022-11-01 2022-08-01

Can someone please help me do it? Thank you very much in advance.

CodePudding user response：

Frist group, then mutate & min:

library(dplyr)
df %>%
  group_by(ID) %>%
  mutate(NewDate = min(Date)) %>%
  ungroup()

#> # A tibble: 12 × 3
#>    ID    Date       NewDate   
#>    <chr> <date>     <date>    
#>  1 A     2020-08-01 2020-08-01
#>  2 A     2020-09-01 2020-08-01
#>  3 A     2020-10-01 2020-08-01
#>  4 A     2020-11-01 2020-08-01
#>  5 B     2021-08-01 2021-08-01
#>  6 B     2021-09-01 2021-08-01
#>  7 B     2021-10-01 2021-08-01
#>  8 B     2021-11-01 2021-08-01
#>  9 C     2022-08-01 2022-08-01
#> 10 C     2022-09-01 2022-08-01
#> 11 C     2022-10-01 2022-08-01
#> 12 C     2022-11-01 2022-08-01