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 base R approach with the function strftime.

df$Week.Start <- dates-abs(1-as.numeric(strftime(df$dates, "%u")))

This can be a one-liner but we'll create a few variables to see what's happening. The %u format pattern for dates returns the day of the week as a single decimal number. We can convert that number to numeric and subtract the distance from our dates. We can then subtract that vector from our date column.

day_of_week <- as.numeric(strftime(df$dates, "%u"))
day_diff <- abs(1-day_of_week)
df$Week.Start <- dates-day_diff
#        dates Week.Start
# 1 2042-10-22 2042-10-20
# 2 2026-08-14 2026-08-10
# 3 2018-11-23 2018-11-19
# 4 2017-08-21 2017-08-21
# 5 2022-05-26 2022-05-23
# 6 2037-05-27 2037-05-25

Data

set.seed(7)
all_dates <- seq(Sys.Date(), Sys.Date()+10000, by="days")
dates <- sample(all_dates, 20)
df <- data.frame(dates)