Path: csiph.com!usenet.pasdenom.info!news.albasani.net!rt.uk.eu.org!newsfeed.xs4all.nl!newsfeed3.news.xs4all.nl!xs4all!post.news.xs4all.nl!not-for-mail
Date: Tue, 13 Aug 2013 07:12:05 -0500
From: Anthony Papillion <papillion@gmail.com>
User-Agent: Mozilla/5.0 (X11; Linux i686; rv:17.0) Gecko/20130625 Thunderbird/17.0.7
MIME-Version: 1.0
To: python-list@python.org
Subject: Question about function failing with large number
Content-Type: text/plain; charset=ISO-8859-1
Content-Transfer-Encoding: 7bit
Precedence: list
Newsgroups: comp.lang.python
Message-ID: <mailman.532.1376395829.1251.python-list@python.org>
Lines: 84
NNTP-Posting-Host: 2001:888:2000:d::a6
Xref: csiph.com comp.lang.python:52454

-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA512

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:     17772098750@in.callcentric.com
XMPP:    cypherpunk@patts.us

www.cajuntechie.org
-----BEGIN PGP SIGNATURE-----

iQIcBAEBCgAGBQJSCiKNAAoJEKCKfnPhYIFFn7gP/A8CHOyTv71J/uVpMVYRcDRp
KRwbL9A1gkzUpycQibN3Q90FwY6nsGGPCeOkZByfYZnMKQH0o4Kd7QQf0hEOkhzO
BLiQjbkjzuUq7usE5TIqjTi0pJ6J7DcRm6U77yhAWrVt60MpfOucojLzX8ZolTD6
7Ha1gJ+9uEcwjosx1ynjVt7MQ/uGZwM7xS6WNfOpOwIYnoT5zBUzlKbw1HqSGYLu
6cWmAFTnnnXv6qymbGTxdZf0dxciODXy5xIMp5CzG6zIeHIOvjG03AbcY/+nI5FI
b9fKqjbblE/Npnh9GPXOLpI+I05VZMoO1b0AJSlU+Iq1liZAZOA4s2kf7XCrSb7Y
8Zn6qMPMTuNBPZpRJykTJSrA8s+4RxA0BWoq9rnTNXJPVR6imt6USOtwY4UssPxw
HIUrbSmfAEF9+/g08mcKHTVFstMyQCuAbUGx+LxoxkySnwZkfcwTRQ2vuoBDd7XP
92IJlkAdwepEa748P6NHNkSN4+OV3zAeTczHkzD0OL2KAeCuPY5tlqsI0MAngKOu
TIZrG+w1rvkz1gU3TBILLySuQTk/ioHNoVAH46bvp6ARRJiHJ4Ub61NoyooMCOgX
Lg+XtmcYz9VdUzdayg5uwNQAb/2/DboAyvmuuRDrA2Lzndv1JO3ye222/WCI9e7V
aB1ghvNnNMtrOWA0ZVQX
=BZUP
-----END PGP SIGNATURE-----