[Tutor] help with prime number program

[Kent Johnson]

Norman Silverstone wrote:
> On Mon, 2005-10-31 at 06:12 -0500, Kent Johnson wrote:
> 
>>Norman Silverstone wrote:
>>
>>>I am a beginner so, I hope what I give, makes sense. In it's simplest
>>>form what is wrong with :-
>>>
>>>n = input("Enter a number")
>>>if n % 2 != 0 and n % 3 != 0:
>>>   print n, " Is a prime number"
>>>
>>
>>This only gives the correct answer if n < 25. You can't test against a fixed list of divisors, there will always be a non-prime whose divisors are not in your list.
> 
> 
> Thank you for your comment which, if I understand you correctly, implies
> that prime numbers greater than 25 will not be recognised. Surely, that
> cannot be correct. However, if it is correct, could you please
> demonstrate what you mean for my education.

More precisely, it will give an incorrect answer for any non-prime that is not divisible by 2 or 3, for example 25. 

 >>> def test(n):
 ...   if n % 2 != 0 and n % 3 != 0:
 ...     print n, 'is prime'
 ...
 >>> test(2)
 >>> test(3)

Hmm, not looking so good for 2 and 3 either...
 >>> test(4)
 >>> test(5)
5 is prime
 >>> test(25)
25 is prime

Not so sure about that one either!

Kent

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