P019.py

# -*- coding: utf-8 -*-
#==============================================================================
# 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)?
#==============================================================================


dow = 1 # Let Sunday = 0, Monday = 1... and dow = dow mod 7
yeardays = [31,28,31,30,31,30,31,31,30,31,30,31]
yeardaysleap = [31,29,31,30,31,30,31,31,30,31,30,31]

sun1st = 0

# Advance to 1 Jan 1901
for days in yeardays:
    dow = (dow + days) % 7
#print dow
# dow = 2, Thus 1 Jan 1901 is a Tuesday

year = 1901
while year<2001:
    #print year
    # If leap year
    if (year%4 == 0) and not (year%100==0 and not year%400==0):
        #print 'Im leap', year
        for days in yeardaysleap:
            dow = (dow + days) % 7
            if dow == 0:
                sun1st += 1
    else:
        for days in yeardays:
            dow = (dow + days) % 7
            if dow == 0:
                sun1st += 1
    year += 1

print sun1st