Euler Problem 19 – Counting Sundays

Counting Sundays

Problem 19

You are given the following information, but you may prefer to do some research for yourself.

• 1 Jan 1900 was a Monday.
• Thirty days has September,
  April, June and November.
  All the rest have thirty-one,
  Saving February alone,
  Which has twenty-eight, rain or shine.
  And on leap years, twenty-nine.
• A leap year occurs on any year evenly divisible by 4, but not on a century unless it is divisible by 400.

How many Sundays fell on the first of the month during the twentieth century (1 Jan 1901 to 31 Dec 2000)?

This was a simple one where I just create a sequence of dates for 1^st of every month in the duration given using seq and then get the weekday of those dates and count the number of days that were Sunday.

get_number_of_days_in_timerange = function(start_date, end_date, day) {
sum(weekdays(seq(as.Date(start_date), as.Date(end_date), by = "1 month"))
                                                   == day)
}
get_number_of_days_in_timerange("1901-01-01", "2000-12-31", "Sunday")
#[1] 171

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