wanted: simple c primality test
prime?() should return 0 if composite and 1 if prime -- I'm trying to plot this:
x_(n+1)=cos(y_(n) + 2( prime?(floor(128 * (sin ((e^(x_(n)) - (e^(y_n))))-1);
y_(n+1)=sin(x_(n) + 2( prime?(floor(128 * (sin ((e^(y_(n)) - (e^(x_n))))-1);
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Posted by: John (
 dolohov) Posted at: August 5th, 2001 06:57 am (UTC)
Prime Directive |
Well, since nobody else has responded, the simplest one I can think of is to set up a loop from 2 to N/2(rounded up), dividing N by the index every time. If you exhaust the loop without ever getting a perfect division, it's prime.
A little less simple (But faster) is to decrease the maximum number every time through the loop:
Test if 2 divides N evenly. If not, check and see if 3 > N/2. If so, N's prime. Otherwise,
Test if 3 divides N evenly. If not, check and see if 4 > N/3. If so, N's prime. Otherwise...
And so on, so forth. It's essentially the same as the first test, but you're not testing nearly as many numbers. (I'm pretty sure that's the complete method, but I'm a little groggy still so you might want to test it once or twice.)
My current plan is to test 0 to 128, drop that
in an array, and then just check if there's
a zero or one in it during the main loop.