Question

I’m trying to find the largest prime factor of the number x, Python gives me the error that the range is too large. I’ve tried using x range but I get an OverflowError: Python int too large to convert to C long

x = 600851475143
maxPrime = 0


for i in range(x):
    isItPrime = True
    if (x%i == 0):
        for prime in range(2,i-1):
            if (i%prime == 0):
                isItPrime = False
        if (isItPrime == True):

            if (i > maxPrime):
                maxPrime = i;

print maxPrime

Answer 1

In old (2.x) versions of Python, xrange can only handle Python 2.x ints, which are bound by the native long integer size of your platform. Additionally, range allocates a list with all numbers beforehand on Python 2.x, and is therefore unsuitable for large arguments.

You can either switch to 3.x (recommended), or a platform where long int (in C) is 64 bit long, or use the following drop-in:

import itertools
range = lambda stop: iter(itertools.count().next, stop)

Equivalently, in a plain form:

def range(stop):
   i = 0
   while i < stop:
       yield i
       i += 1