Find most recent Monday for a dataframe

Question

I have a dataframe object, and among the fields in it, I have a dates:

df$dates

I need to add a column which is \'Week Starting\', i.e.

Answer 1

How about just subtracting from the dates the number of days required to get to the previous Monday? e.g if your data is

dates <- as.Date(c("2000-07-12", "2005-02-19", "2010-09-01"))
weekdays(dates)
# [1] "Wednesday" "Saturday"  "Wednesday"

then you can compare this to a vector

wdays <- setNames(0:6, c("Monday", "Tuesday", "Wednesday",
            "Thursday", "Friday", "Saturday", "Sunday"))

and subtract the required number of days from each date, ie

dates - wdays[weekdays(dates)]
#   Wednesday     Saturday    Wednesday 
#"2000-07-10" "2005-02-14" "2010-08-30" 

will give the dates of the Monday preceding each date in dates. To test:

weekdays(dates - wdays[weekdays(dates)])
#Wednesday  Saturday Wednesday 
# "Monday"  "Monday"  "Monday" 

Everything can be written also in one line as

dates - match(weekdays(dates), c("Monday", "Tuesday", "Wednesday", 
   "Thursday", "Friday", "Saturday", "Sunday")) + 1
#"2000-07-10" "2005-02-14" "2010-08-30"