Project Euler

Problem

This problem comes from Project Euler 19

Problem

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)?

Code

# Project Euler: Problem 19
# Counting Sundays

from datetime import date

count = 0
for year in range(1901, 2001):
    for month in range(1, 13):
        if date(year, month, 1).weekday() == 6:
            count += 1
print(count)

Solution

Thank goodness for Python, as it makes this problem trivial.

The question asks, “How many Sundays fell on the first of the month during the twentieth century?”

The trick to the question is this: date(year, month, 1).weekday() == 6.

This function has the form date(year, month, day), so the first of March, 1905 would be date(1905, 3, 1). But what day of the week is that? Easy, just add .weekday() and it will return a numeric day of the week for that date, which ranges from 0 (Monday) to 6 (Sunday).