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After wondering how human populations can increase in size and writing a simulator for the problem, a simpler question came to mind regarding a population without death. This creature population simulator is an old experiment that served its purpose well. The code here is committed for archival to be run under Python 2.5 or later versions.

Python, 93 lines
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def main():
    while True:
        creatures = get_creatures()
        years = get_years()
        summary = get_summary()
        target = get_target()
        simulate(creatures, years, summary, target)
        if get_exit():
            return

def get_creatures():
    while True:
        try:
            creatures = int(raw_input('How many creatures do you want to start with? '))
            if creatures > 0:
                return creatures
            print 'There must be at least one creature.'
        except:
            print 'You must enter a number.'

def get_years():
    while True:
        try:
            years = int(raw_input('How many years do you want simulated? '))
            if years > 0:
                return years
            print 'At least one year must be simulated.'
        except:
            print 'You must enter a number.'

def get_summary():
    while True:
        try:
            answer = raw_input('Do you want a summary of the simulation? ').lower()
            if answer == 'yes':
                return True
            if answer == 'no':
                return False
        except:
            pass
        print 'You must enter "yes" or "no".'

def get_target():
    while True:
        try:
            target = int(raw_input('What is the target population (0 for None)? '))
            if target > 0:
                return target
            return 0
        except:
            print 'You must enter a number.'

def simulate(creatures, years, summary, target):
    print
    divisions = [0 for division in range(21)]
    divisions[20] = creatures
    for year in range(years):
        if not summary:
            print 'It is year', year, 'and the divisions are as follows:'
            print str(divisions)[1:-1]
        children = divisions[20] / 2
        divisions[20] += divisions[19]
        for division in range(19):
            divisions[19 - division] = divisions[18 - division]
        divisions[0] = children
        if target:
            total = 0
            for division in range(21):
                total += divisions[division]
            if total >= target:
                break
    print 'It is year', year + 1, 'and the divisions are as follows:'
    print str(divisions)[1:-1]
    total = 0
    for division in range(21):
        total += divisions[division]
    print 'There are a total of', total, 'creatures.'
    print

def get_exit():
    while True:
        try:
            answer = raw_input('Do you wish to exit this program? ').lower()
            if answer == 'yes':
                return True
            if answer == 'no':
                return False
        except:
            pass
        print 'You must enter "yes" or "no".'

if __name__ == '__main__':
    main()
Created by Damian Esteban on Thu, 15 Aug 2013 (MIT)
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