[Project Euler] 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)?

 

 

這道題，一開始讀題誤解了意思，浪費了不少時間。注意要求解的是21世紀裡有多少個星期天發生在一個月的第一天。

 

開始自己沒用庫類，活生生的寫出了下面的代碼：

def yearbeginweek(year):    """ Function to calculate the week of the first day in a year"""    yearrange = range(1900, year)    days = 0    for x in yearrange:        if x in leapyears:            days = days + 366        else:            days = days + 365    return  1 + days % 7 week = {1: "Mon", 2: "Tue", 3: "Wen", 4: "Thr", 5: "Fri", 6: "Sat", 7: "Sun"}years = range(1900, 2001)leapyears = [x for x in years if (x % 4 == 0 and x % 100 != 0) or (x % 100 == 0 and x % 400 == 0)]nonleapyears = [x for x in years if not x in leapyears]leapdays = [31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30]nonleapdays = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30]suncount = 0for year in range(1901, 2001):    # Calculate the first week day in the year    begin = yearbeginweek(year)    if begin == 7:        suncount += 1        begin = 0    # If it is a leap year    if year in leapyears:        for days in leapdays:            if (begin + days % 7) % 7 == 0:                suncount += 1            begin = (begin + days % 7) % 7    else:        for days in nonleapdays:            if (begin + days % 7) % 7 == 0:                suncount += 1            begin = (begin + days % 7) % 7print suncount            

這個比較囧，後來看了有個Datetime module，一句話就可以解決這個問題了：

import datetimeprint len([datetime.date(year, month, 1) for year in range(1901, 2001) for month in range(1, 13) if datetime.date(year, month, 1).weekday() == 6])

這不得不佩服python的類庫實在強大。