How to flag last friday or last day or month

Question

I have a data frame of dates and the day of the week

> head(data)
         day   weekday
1 2016-01-01    Friday
4 2016-01-04    Monday
5 2016-01-05   Tuesday
         


        

Answer 1

With the additional clarification and following the comment by @eminik the code below

library(data.table)
setDT(data)
data[, LastDayInMonth := day == max(day), by = .(year(day), month(day))]
data[, LastFridayInMonth := weekday == "Friday" & day == max(day), 
     by = .(year(day), month(day), weekdays(day))]

produces:

# show results (only relevant rows)
data[LastDayInMonth | LastFridayInMonth == TRUE]

          day  weekday LastDayInMonth LastFridayInMonth
1: 2016-01-29   Friday           TRUE              TRUE
2: 2016-02-26   Friday          FALSE              TRUE
3: 2016-02-29   Monday           TRUE             FALSE
4: 2016-03-25   Friday          FALSE              TRUE
5: 2016-03-31 Thursday           TRUE             FALSE
6: 2016-04-29   Friday           TRUE              TRUE
7: 2016-05-06   Friday          FALSE              TRUE
8: 2016-05-10  Tuesday           TRUE             FALSE

Edit: Code modified to account for change of years as requested by OP.

Note: weekdays returns a character vector of names in the locale in use. Therefore, the code only works if you are in an English locale. Otherwise, you may have to use Sys.setlocale(category = "LC_ALL", locale = "English") before.