Unusual Speed Difference between Python and C++

Question

I recently wrote a short algorithm to calculate happy numbers in python. The program allows you to pick an upper bound and it will determine all the happy numbers below it.

Answer 1

#!/usr/bin/env python

import timeit

upperBound = 0

def calcMain():
    known = set()
    for i in xrange(0,upperBound+1):
        next = False
        current = i
        history = set()
        while not next:
            squaresum=0
            while current > 0:
                current, digit = divmod(current, 10)
                squaresum += digit * digit
            current = squaresum
            if current in history:
                next = True
                if current == 1:
                    known.add(i)
            history.add(current)

while True:
    upperBound = input("Pick an upper bound: ")
    result = timeit.Timer(calcMain).timeit(1)
    print result, "seconds.\n"

I made a couple of minor changes to your original python code example that make a better than 16x improvement to the performance of the code. The changes I made took the 100,000 case from about 9.64 seconds to about 3.38 seconds.

The major change was to make the mod 10 and accumulator changes to run in a while loop. I made a couple of other changes that improved execution time in only fractions of hundredths of seconds. The first minor change was changing the main for loop from a range list comprehension to an xrange iterator. The second minor change was substituting the set class for the list class for both the known and history variables. I also experimented with iterator comprehensions and precalculating the squares but they both had negative effects on the efficiency. I seem to be running a slower version of python or on a slower processor than some of the other contributers. I would be interest in the results of someone else's timing comparison of my python code against one of the optimized C++ versions of the same algorithm. I also tried using the python -O and -OO optimizations but they had the reverse of the intended effect.