Birthday_Paradox_Simulation.py

import datetime, random

def getBirthdays(numberOfBirthdays):
    birthdays = []
    for i in range(numberOfBirthdays):
        startOfYear = datetime.date(2001,1,1)
        randomNumberOfDays = datetime.timedelta(random.randint(0,364))
        birthday = startOfYear + randomNumberOfDays
        birthdays.append(birthday)

    return birthdays

def getMatch(birthdays):
    if len(birthdays) == len(set(birthdays)):
        return None
    for a, birthdayA in enumerate(birthdays):
        for b, birthdayB  in enumerate(birthdays[a + 1 :]):
            if birthdayA == birthdayB:
                return birthdayA


print('''Birthday Paradox, by AI Sweigart al@inventwithpython.com
      The Birday Paradox show us that in a group of N people,the odds
      that two of them have matching birthdays is suprisingly large.
      This program does a Monte Carlo simulation (that is, repeated random simulations) to explore this concept.
      (It's not acctually a paradox, it's  just a suprising result.)
      ''')
MONTHS = ('Jan','Feb','Mar','Apr','May','Jun','Jul','Aug','Sep','Oct','Nov','Dec')
while True:
    print('How many birthdays shall I generate? (Max 100)')
    response = input('> ')
    if response.isdecimal() and (0 < int(response) <= 100):
        numBDays = int(response)
        break
print()
print('Here are',numBDays,'birthdays:')
birthdays = getBirthdays(numBDays)
for i , birthday in enumerate(birthdays):
    if i !=0:
        print(', ', end ='')
    monthName = MONTHS[birthday.month - 1]
    dateText = '{} {}'.format(monthName,birthday.day)
    print(dateText,end='')

print()
print()

match = getMatch(birthdays)
print('In this simulation,',end='')
if match != None:
    monthName = MONTHS[match.month - 1]
    dateText = '{} {}'.format(monthName,match.day)
    print('multiple people have a birthday on',dateText)

else:
    print('there are no matching birthdays.')
print()
print('Generating',numBDays,'random birthdays 100,000 times...')
input('Press Enter to begin....')
print('Let\'s run another 100,000 simulations.')
simMatch = 0
for i in range(100_000):
    if i % 10_000 == 0:
        print(i,'simulations run...')
    birthdays = getBirthdays(numBDays)
    if getMatch(birthdays) != None:
        simMatch = simMatch + 1

print('100,00 simulations run.')
probability = round(simMatch /100_000 * 100,2)
print('Out of 100,000 simulations of',numBDays,'people,there was a')
print('matching birthday in that group',simMatch,'times.This means')
print('that',numBDays,'people have a ',probability,'% chance of')
print('having a matching birthday in their group.')
print('That\'s probably more than you would think!')