Range is too large Python

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: Py

Answer 1

I would definitely stick with xrange since creating a list between 0 and what looks like a number rivaled by infinity would be taxing for memory. xrange will generate only the numbers when asked. For the number too large problem, you might want to try a "long". This can be achieved by writing a L on the end of the number. I made my own version to test it out. I put in a small sleep as to not destroy my computer into virtually a while(1) loop. I was also impatient to see the program come to a complete end, so I put in print statements

from time import sleep

x = 600851475143L
maxPrime = 0

for i in xrange(1,x):
    isItPrime = True
    if (x%i) == 0:
        for prime in xrange(2,i-1):
            if (i%prime) == 0:
                isItPrime = False
                break
        if isItPrime:
            maxPrime = i
            print "Found a prime: "+str(i)
    sleep(0.0000001)


print maxPrime

Hope this helps!

EDIT: I also did a few more edits to yield this version. It is fairly efficient and I checked quite a few numbers this program provides (it seems to check out so far):

from time import sleep

x = 600851475143L

primes = []

for i in xrange(2,x):
    isItPrime = True
    for prime in primes:
        if (i%prime) == 0:
            isItPrime = False
            break
    if isItPrime:
        primes.append(i)
        print "Found a prime: "+str(i)
    sleep(0.0000001)


print primes[-1]