BirthdayMonteCarlo.py

import datetime, random

def getBirthdays(numberOfBirthdays):
    """Returns a list of number random date objects for birthdays."""
    birthdays =[]
    for i in range (numberOfBirthdays):
        #The year is unimprtant for our simulation, as long as all
        #birthdays have the same year.
        startOfYear = datetime.date(2001, 1, 1)

        #Get a random day into the year:
        randomNumberOfDays = datetime.timedelta(random.randint(0, 364))
        birthday = startOfYear + randomNumberOfDays
        birthdays.append(birthday)
        return birthdays

def getMatch(birthdays):
    """Returns the date object of a birthday that occurs more than once in the birthdays list."""
    if len(birthdays) == len(set(birthdays)):
        return None #All birthdays are unique, so return None.

    #compare each birthday to every other birthday:
    for a, birthdayA in enumerate(birthdays):
        for b, birthdayB in enumerate(birthdays[a + 1:]):
            if birthdayA == birthdayB:
                return birthdayA #Return the matching birthday.


print ("This program will show the probabillity of more than one person having a birthday in a group. \nIt is using what is known as a Monte Carlo simulation. Look it up!")

#Set up a tuple of month names in order:
MONTHS = ('Jan', 'Feb', 'Mar', 'Apr', 'May', 'Jun',
         'Jul', 'Aug', 'Sep', 'Oct', 'Nov', 'Dec')

while True:  #Keep asking until the user enters a valid ammount.
    print('How many birthdays shall I generate? (Max 100)')
    response = input ('>')
    if response.isdecimal() and (0 < int(response) <= 100):
        numBDays = int(response)
        break # User has entered a valid amount.
print()

#Generate and display the birthdays:
print ('Here are', numBDays, 'birthdays:')
birthdays = getBirthdays(numBDays)
for i, birthday in enumerate(birthdays):
    if i !=0:
        #Display a comma for each birthday after the first birthday.
        print(',', end='')
    monthName= MONTHS[birthday.month - 1]
    dateText = '{} {}' .format(monthName, birthday.day)
    print(dateText, end='')
print()
print()

#Determine if there are two birthdays that match.
match = getMatch(birthdays)

#Display the results:
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, you special snowflake.')
print()

#Run through 100,000 simulations:
print('Generating',numBDays, 'random birthdays 100,000 times...')
input('Press Enter to begin...')

print('Run another 100,000 simulations')
simMatch = 0 # How many simulations had matching birthdays in them.
for i in range(100000):
    #Report on the progress every 10,000 simulations:
    if i % 10000 == 0:
        print(i, 'simulations runs...')
    birthdays = getBirthdays(numBDays)
    if getMatch(birthdays) != None:
        simMatch = simMatch +1
print('100,000 simulations run.')

# Display simulation results:
probability = round(simMatch / 100000 * 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!')