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

require(data.table)

lfom<-function(d){
  x<-seq(min(d),max(d),by=1)
  fri<-which(wday(x)==6)
  mend<-which(mday(x)==1)-1
  d %in%  x[fri[findInterval(mend,fri)]]
}

ldom<-function(d){
  x<-seq(min(d),max(d),by=1)
  mend<-which(mday(x)==1)-1
  d %in%  x[mend]
}


data <-data.frame(
  day = seq(as.Date("2014-01-01"), as.Date("2016-05-10"), "day"), 
  weekday = weekdays(seq(as.Date("2014-01-01"), as.Date("2016-05-10"), "day"))
)
excludeDays <- c("Saturday", "Sunday")
data <- data.table(subset(data, !weekdays(data$day) %in% excludeDays)) #exclude weekend
data$lfom<-lfom(data$day)
data$ldom<-ldom(data$day)

Answer 2

library(lubridate)
library(dplyr)
data%>% 
  mutate(year = year(day),month= month(day)) %>%
  group_by(year, month) %>% 
  mutate(LastDayInMonth = max(day)==day)%>% 
  arrange(day) %>%
  ungroup() %>% 
  group_by(year, month, weekday) %>%
  mutate(LastWeekInMonth = row_number() ==  n(), 
         LastFriayInMonth = weekday =="Freitag" & LastWeekInMonth == 1) %>% 
  ungroup()%>%
  select(day, weekday, LastDayInMonth,LastFriayInMonth)

Same excerpt as above (%>% filter(LastDayInMonth | LastFriayInMonth == TRUE):

Source: local data frame [8 x 4]

         day    weekday LastDayInMonth LastFriayInMonth
      (date)     (fctr)          (lgl)            (lgl)
1 2016-01-29    Freitag           TRUE             TRUE
2 2016-02-26    Freitag          FALSE             TRUE
3 2016-02-29     Montag           TRUE            FALSE
4 2016-03-25    Freitag          FALSE             TRUE
5 2016-03-31 Donnerstag           TRUE            FALSE
6 2016-04-29    Freitag           TRUE             TRUE
7 2016-05-06    Freitag          FALSE             TRUE
8 2016-05-10   Dienstag           TRUE            FALSE

Answer 3

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.

Answer 4

I'm using lubridate for date calculations, but since you have missing days in between, I use lead() function from dplyr to find the next day (row) in the data.

library(lubridate)
library(dplyr)
my.data <- tbl_df(data)
my.data <- my.data %>% 
    # First last day of month
    # get the next day in the data
    mutate(next.day = lead(day)) %>%
    # it's month is different
    mutate(LastDayInMonth = ifelse(month(day) != month(next.day), T, F)) %>%
    # Now Last Friday
    mutate(LastFridayInMonth = 
               ifelse( (wday(day) == 6) & # It's a Friday
                # Check the month of next Friday, different
                (month(lead(day, 5)) != month(day)
                 | is.na(lead(day, 5))), T, F))