This is a very inefficient algorithm. What you are doing is counting all the divisors of a number (with the exception of divisors divisible by 5) and call it a prime if there are just 2 divisors (special casing a few numbers).

It's of course a lot more efficient to decide a number is prime as soon as you have found a divisor > 1 and smaller or equal to sqrt (number). No need to do millions of divisions to decide 100000000 is prime after finding out it's divisible by 2.

Abigail

In reply to Re: YAPNC: Yet another prime number checker? by Abigail-II
in thread YAPNC: Yet another prime number checker? by snafu

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