CODEWITHSUNDEEP
javascript
Kevin Banas
Kevin Banas

Reputation: 321

Oscillate counter increment?

I want to reduce the computing load in a test for primes. Currently my loop just tests odds, like so: 

for (k=3; k<=end; k+=2) {

I read that every prime other than 2 and 3 is a function of k= 6 +/- 1 though. By only testing 2/3rds of the odds I can reduce computing load by 33%. The only way I can think to do this is to oscillate the counter to increment by 2, then 4, then 2, then 4 each iteration, for example testing 5, 7, 11, 13, etc.

Is there a way to tell the loop to do this? Is there another way to do this I'm not considering?

P.S. I'm aware of the sieve method of testing

Upvotes: 1

Views: 196

Answers (3)

Emissary
Emissary

Reputation: 10148

Just for the fun of it...

for (var k=3,l=0; k<=end; k+=2*(l%2+1),l++){ }

Upvotes: 2

Codeman
Codeman

Reputation: 12375

Something like this?

oscillate = false;
k = 3;
while(k <= end)
{
    testForPrimes(k);
    k += oscillate ? 2 : 4;
    oscillate != oscillate;
}

Upvotes: 2

jeffjenx
jeffjenx

Reputation: 17487

Use a while loop, instead:

k = 3;
odd = false;
while( k<=end ) {
    k += odd ? 2 : 4;
    odd = !odd;
}

Upvotes: 2

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