Twin primes: Difference between revisions

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===Extended Task===

<syntaxhighlight lang="java">

import java.util.BitSet;

public final class TwinPrimes {

public static void main(String[] args) {

final int limit = 1_000_000_000;

sievePrimes(limit);

int target = 10;

int count = 0;

boolean last = false;

boolean first = true;

for ( int index = 5; index <= limit; index += 2 ) {

last = first;

first = primes.get(index);

if ( last && first ) {

count += 1;

}

if ( index + 1 == target ) {

System.out.println(String.format("%8d%s%d", count, " twin primes below ", index + 1));

target *= 10;

}

}

}

private static void sievePrimes(int aLimit) {

primes = new BitSet(aLimit + 1);

primes.set(2, aLimit + 1);

for ( int i = 2; i <= Math.sqrt(aLimit); i = primes.nextSetBit(i + 1) ) {

for ( int j = i * i; j <= aLimit; j += i ) {

primes.clear(j);

}

}

}

private static BitSet primes;

}

</syntaxhighlight>

{{ out }}

<pre>

2 twin primes below 10

8 twin primes below 100

35 twin primes below 1000

205 twin primes below 10000

1224 twin primes below 100000

8169 twin primes below 1000000

58980 twin primes below 10000000

440312 twin primes below 100000000

3424506 twin primes below 1000000000