Factorial: Difference between revisions

===Parallelized (fastest and most efficient)===

<syntaxhighlight lang="java5">

package programas;

import java.math.BigInteger;

import java.util.InputMismatchException;

import java.util.Scanner;

import java.util.stream.Stream;

public class IterativeFactorial {

public BigInteger factorial(BigInteger n) {

if ( n == null ) {

throw new IllegalArgumentException("The argument cannot be null");

}

else if ( n.signum() == - 1 ) {

// negative

throw new IllegalArgumentException("Argument must be a non-negative integer");

}

else {

BigInteger result = Stream.iterate(BigInteger.TWO, bi -> bi.compareTo(n) <= 0, bi -> bi.add(BigInteger.ONE))

.parallel().reduce(BigInteger.ONE, BigInteger::multiply);

return result;

}

}

static long count(BigInteger k) { // Count all digits

double factor = Math.log(2) / Math.log(10);

int count = (int) (factor * k.bitLength() + 1);

if ( BigInteger.TEN.pow(count - 1).compareTo(k) > 0 ) { return count - 1; }

return count;

}

public static void main(String[] args) {

Scanner scanner = new Scanner(System.in);

BigInteger number, result;

boolean error = false;

System.out.println("FACTORIAL OF A NUMBER");

do {

System.out.println("Enter a positive integer:");

try {

number = scanner.nextBigInteger();

final long startTime = System.currentTimeMillis();

result = new IterativeFactorial().factorial(number);

final long endTime = System.currentTimeMillis();

error = false;

System.out.println("Factorial of " + number + ": " + result);

System.out.println("Total execution time: " + (endTime - startTime));

// System.out.println("Factorial of " + number + " has " + count(result) + "

// digits");

}

catch ( InputMismatchException | IllegalArgumentException e ) {

error = true;

scanner.nextLine();

}

}

while ( error );

scanner.close();

}

}

</syntaxhighlight>