R获得最大日期字符串

Question

我想获得R中最多注明日期的国家/地区

我想获得其最大约会对象的国家/地区作为表格像这样：

Answer 1

library(dplyr)
library(lubridate)
df1 %>%
   group_by(person) %>%
   slice(which.max(dmy(date)))
   # if the format is %m.%d.%Y"
   # slice(which.max(mdy(date)))
# A tibble: 2 x 3
# Groups:   person [2]
#  person country date      
#  <chr>  <chr>   <chr>     
#1 a      germany 01.05.2020
#2 c      korea   01.01.2023

或使用data.table
library(data.table)
setDT(df1)[, .SD[which.max(as.IDate(date, "%d.%m.%Y"))], person]

数据

df1 <- structure(list(person = c("a", "a", "c", "c", "c"), country = c("usa", 
"germany", "france", "china", "korea"), date = c("01.01.2020", 
"01.05.2020", "01.01.2021", "01.01.2022", "01.01.2023")),
class = "data.frame", row.names = c(NA, 
-5L))

Answer 2

subset(transform(df, date = as.Date(date, "%d.%m.%Y")), 
                 date == ave(date, person, FUN = max))

#  person country       date
#2      a germany 2020-05-01
#5      c   korea 2023-01-01