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include documentation explaining this template's current structure
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{{infobox_begin}}{{Task/Icon}}'''{{Twin-Prime ConjecturePAGENAME}}'''<includeonly>{{#set:is task=true}}</includeonly><br/>
You are encouraged to [[Rosetta Code:Solve a Task|solve this task]] according to the task description, using any language you may know.<includeonly>[[Category:Programming Tasks]]{{#if:{{{1|}}}|[[Category:{{{1}}}]]|[[Category:Solutions by Programming Task]]}}</includeonly>{{infobox_end}}<noinclude>
{{template|Task identifier}}</noinclude>
 
<hr />
A twin prime is a pair of natural numbers P1 and P2 that satisfy the following:
 
This template identifies a page as a task page and adds it to [[:Category:Programming Tasks]] and either the category identified by the first argument to the template instance or (if none is supplied) [[:Category:Solutions by Programming Task]]</noinclude>
1. P1 and P2 are primes
 
2. P1 + 2 = P2
 
Write a program that displays the number of twin primes that can be found under a user-inputted number.
 
Examples below:
{{out}}
<pre>
> Search Size: 100
> 8 twin prime pairs.
</pre>
 
<pre>
> Search Size: 1000
> 35 twin prime pairs.
</pre>
 
=={{header|Java}}==
BigInteger implementation:
<lang Java>
import java.math.BigInteger;
import java.util.Scanner;
 
public class twinPrimes {
public static void main(String[] args) {
Scanner input = new Scanner(System.in);
System.out.println("Search Size: ");
BigInteger max = input.nextBigInteger();
int counter = 0;
for(BigInteger x = new BigInteger("3"); x.compareTo(max) <= 0; x = x.add(BigInteger.ONE)){
BigInteger sqrtNum = x.sqrt().add(BigInteger.ONE);
if(x.add(BigInteger.TWO).compareTo(max) <= 0) {
counter += findPrime(x.add(BigInteger.TWO), x.add(BigInteger.TWO).sqrt().add(BigInteger.ONE)) && findPrime(x, sqrtNum) ? 1 : 0;
}
}
System.out.println(counter + " twin prime pairs.");
}
public static boolean findPrime(BigInteger x, BigInteger sqrtNum){
for(BigInteger divisor = BigInteger.TWO; divisor.compareTo(sqrtNum) <= 0; divisor = divisor.add(BigInteger.ONE)){
if(x.remainder(divisor).compareTo(BigInteger.ZERO) == 0){
return false;
}
}
return true;
}
}
</lang>
{{out}}
<pre>
<pre>
> Search Size:
> 100
> 8 twin prime pairs.
</pre>
 
<pre>
> Search Size:
> 1000
> 35 twin prime pairs.
</pre>
Rdm
6,951

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