[Tutor] Help

[Nitin Pawar]

Adding to what Andre said,

another way of optimizing the problem would be
storing the prime number in the range you want to check an array and see if
the given number is divisible by any of those prime number

This improves the performance.

Thanks,
nitin

On Tue, Jul 13, 2010 at 3:52 PM, Andre Engels <andreengels at gmail.com> wrote:

> On Tue, Jul 13, 2010 at 11:50 AM, Dipo Elegbede <delegbede at dudupay.com>
> wrote:
> > I was trying to write a code that prints prime numbers between 1 and 20.
> >
> > I have by myself seen that something is wrong with my code and also my
> > brain.
> >
> > Could anyone be kind enough to tell me what to do....
> >
> > Where I am confused is how to test for other numbers without one and the
> > number itself. It turns out that all numbers pass the condition I set so
> > that would presuppose that all numbers are prime which is not.
> >
> > How exactly can I get it to run checks with other numbers such that it
> > doesn't include the number itself and 1.
> >
> > The code is as follows:
> >
> > for i in range(1,20):
> >
> >     if float(i) % 1 == 0 and float(i) % i == 0:
> >         print i, 'is a prime number'
>
> Your code only checks whether the number divides by 1 and itself. It
> should check the numbers in between, and if _any_ divides the number,
> decide it is not a prime number. This is best done in a separate
> function (note: I am writing it here for clarity of the underlying
> algorithm, there are various ways in which it could be made faster,
> shorter or more Pythonic):
>
> def isPrime(n):
>    divisorFound = False
>    for i in xrange(2, n):
>        if n % i == 0:
>            divisorFound = True
>    return not divisorFound     # divisorFound is true if and only if
> there is a number i (1<i<n) with n % i == 0
>
> for i in range(2,20):
>    if isPrime(i):
>         print i, 'is a prime number'
>
> By the way, do note that your cast to float is not a good idea. It
> probably won't hurt you in this case, but it definitely won't improve
> things. You'd much rather check exact equality with integers than with
> floats.
>
> --
> André Engels, andreengels at gmail.com
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-- 
Nitin Pawar
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