Question about function failing with large number


A

Anthony Papillion

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So I'm using the function below to test a large (617 digit) number for
primality. For some reason, when I execute the code, I get an error
telling me:

OverflowError: long int too large to convert to float

The error is being thrown on this line:

for x in range(3, int(n**0.5)+1, 2):

The odd thing is that the error is not thrown every single time. I can
run the function a few times, it will generate a large number (always
the same length) and run it through the function. After I do this a
few times, it fails with the error. I might get the error on the next
few runs but then, all of a sudden, it functions again.

Any ideas? The entire program, including the method, is below.

#!/usr/bin/env python

from random import getrandbits

bits = 2048

# Test if the number is a prime
def isprime(n):

# make sure n is a positive integer
n = abs(int(n))
# 0 and 1 are not primes
if n < 2:
return False
# 2 is the only even prime number
if n == 2:
return True
# all other even numbers are not primes
if not n & 1:
return False
# range starts with 3 and only needs to go up the squareroot of n
# for all odd numbers
for x in range(3, int(n**0.5)+1, 2):
if n % x == 0:
return False
return True

a = getrandbits(bits)
print "\nGenerated Number: ", a, "\n"
print "Number of digits: ", len(str(a))
isNumberPrime = isprime(a)
if isNumberPrime == True:
print "\nThis number is a prime.\n"
else:
print "\nThis number is not a prime.\n"

Thanks!
Anthony


- --
Anthony Papillion
Phone: 1.918.533.9699
SIP: (e-mail address removed)
XMPP: (e-mail address removed)

www.cajuntechie.org
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