collatz.py

# http://xkcd.com/710/                                                                                
# An iterator for showing the Collatz Conjecture
"""                                                                                                   
>>> [x for x in Collatz(6)]                                                                           
[3, 10, 5, 16, 8, 4, 2, 1]                                                                            
                                                                                                      
>>> [x for x in Collatz(11)]                                                                          
[34, 17, 52, 26, 13, 40, 20, 10, 5, 16, 8, 4, 2, 1]                                                   
                                                                                                      
>>> len([x for x in Collatz(27)])                                                                     
111                                                                                                   
"""                                                                                                   
                                                                                                      
class Collatz(object):                                                                                
    def __init__(self, start):                                                                        
        self.start = int(start)                                                                       
        self.n = self.start                                                                           
                                                                                                      
    def __iter__(self):                                                                               
        return self                                                                                   
                                                                                                      
    def next(self):                                                                                   
        if self.n == 1:                                                                               
            raise StopIteration                                                                       
        elif self.n % 2 == 0:                                                                         
            self.n = self.n / 2                                                                       
        else:                                                                                         
            self.n = (self.n*3) + 1                                                                   
        return self.n                                                                                 
                                                                                                      
                                                                                                      
if __name__ == '__main__':                                                                            
    import doctest                                                                                    
    doctest.testmod()