Tugger the SLUGger!SLUG Mailing List Archives

Re: [SLUG] c++... a bit OT


On Wed, Nov 15, 2000 at 11:37:18AM +1100, James Wilkinson wrote:
> This one time, at band camp, alex060@xxxxxxxxxxxxxxxx said:
> 
> >all primes end in either 1 3 7 or 9 except  2 and 5
> 
> How about 39?  That's not prime :)  You need a better heuristic ;)

No, he didn't say everything ending in 1 3 7 or 9 _is_ prime, just
that to be prime you have to end in 1 3 7 or 9 (except 2 and 5)

> 
> > so it is pointless to test it if it dosent ie ends in a 5, so how can i
> >test to see if the last number is 1 3 7 9  before i start the curnum % x
> >!=0
> 
> Well, you could convert it to a string, and look at the last character.
> I think you're going to spend more time in the conversion than you think
> you'd save by using it.

number % 10 will give you the last digit. There are much better methods
of finding prime numbers, though. sci.crypt has a lot of info about them.

	Stephen
-- 
Stephen Norris	  srn@xxxxxxxxx
Farrow Norris Pty Ltd	+61 2 417 243 239

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