[Tutor] large number question

[Kent Johnson]

The problem is that xrange requires an int argument; it is the call to 
xrange() that is generating the error:

 >>> import sys
 >>> sys.maxint
2147483647
 >>> for i in xrange(2,2147483648L):
...   pass
...
Traceback (most recent call last):
   File "<stdin>", line 1, in ?
OverflowError: long int too large to convert to int
 >>> for i in xrange(2,2147483647L):
...   pass
...
(wait a while and ^C)
Traceback (most recent call last):
   File "<stdin>", line 1, in ?
KeyboardInterrupt

One solution is to rewrite your loop to compute the index yourself:
         x = 2
         limit = int(n**.5 + 1)
         while x < limit
             if n % x == 0:
                 prime = False
                 break
             x += 1

or write your own generator function that does the same thing as xrange:

 >>> def longRange(beg,end):
...   val = beg
...   while val < end:
...     yield val
...     val += 1
...
 >>> list(longRange(2147483640, 2147483649L))
[2147483640, 2147483641, 2147483642, 2147483643, 2147483644, 2147483645, 
2147483646, 2147483647, 2147483648L]

The first solution is likely to be faster.

Kent

At 09:08 PM 9/25/2004 -0700, Dick Moores wrote:
>I've written a program that will take a range of integers and tell me 
>which are prime, and also tell me the prime factors of the non-primes 
>(reinventing the wheel). It works well and efficiently, I believe, but I'm 
>wondering if there's a way around the apparent limit on the size of 
>integers. If I enter anything larger than about 4,610,000,000,000,000,000 
>(4 quintillion, 610 quadrillion in American English), I get
>"OverflowError: long int too large to convert to int"
>
>I won't attach my program here, but an obviously important function is
>================================
>def isPrime(n):
>     """returns True if n is prime; False if not"""
>     prime = True
>     if n < 2:
>         prime = False
>     else:
>         for x in xrange(2,int(n**.5 + 1)):
>             if n % x == 0:
>                 prime = False
>                 break
>     return prime
>==============================
>
>If I use just isPrime alone, I get the same error message. Where does this 
>limit come from? From Python? From Windows XP? From Dell (which made my 
>computer)?
>
>Python v2.3.4, Windows XP.
>
>Thanks, tutors.
>
>Dick Moores
>rdm at rcblue.com
>
>
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