[Tutor] prime numbers

[Gerrit Holl]

ahimsa schreef op maandag 24 februari om 22:14:07 +0000:
> Hello folks
> I am wanting to do an exercise that calls for a function to calculate prime 
> numbers. I know that primes can *only* be divided by themself or by 1, and 
> nothing else. My difficulty is translating this into parsimonious code 
> without labouriously working out all the possible prime numbers as conditions 
> for the if control.
> So far I have attempted % and /, and various combinations, but after a while I 
> am almost including all the primes anyway. Can someone give me a nudge in the 
> right direction please.
> 
> Much appreciated

The demo/scripts directory contains exactly this; maybe it can help you.

#! /usr/bin/env python

# Print prime numbers in a given range

def main():
        import sys
        min, max = 2, 0x7fffffff
        if sys.argv[1:]:
                min = int(eval(sys.argv[1]))
                if sys.argv[2:]:
                        max = int(eval(sys.argv[2]))
        primes(min, max)

def primes(min, max):
        if 2 >= min: print 2
        primes = [2]
        i = 3
        while i <= max:
                for p in primes:
                        if i%p == 0 or p*p > i: break
                if i%p <> 0:
                        primes.append(i)
                        if i >= min: print i
                i = i+2

main()

yours,
Gerrit.

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