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

a[1] <-as.Date("2016-08-20")

Finding Next day (Here "Monday")

a[1] + match("Monday",weekdays(seq(a[1]+1, a[1]+6,"days")))

"2016-08-22"

Finding Last Day (Here "Friday")

a[1] + (match("Friday",weekdays(seq(a[1]+1, a[1]+6,"days")))-7)

"2016-08-19"