Java Glossary : factorial

Last updated 2004-06-28 by Roedy Green ©1996-2004 Canadian Mind Products

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factorial

Factorial is often written recursively just to show off. Iterative coding with a pedestrian for loop is actually faster.

/** calculate n factorial. Only good for 0 <= n <= 20 * @author Roedy Green email
  */
static long factorial (int n )
   {
   if ( n < 0 || n > 20 )
      {
      throw new IllegalArgumentException( "factorial can only handle 0 <= n <= 20" );
      }
   if ( n == 0 )
      {
      return 1;
      }
   long result = n;
   for ( int i=n-1; i>1; i-- )
      {
      result *= i;
      }
   return result;
   }

Since you can't fit n! into a long for n>20, you might as well use table lookup for a very fast factorial:

view

Unfortunately, you can only calculate factorial of integers 20 or smaller, even with long arithmetic. To handle bigger ones you must use Sterling's approximation or a Gamma function.

view

For hints on faster algorithms you might use to calculate large factorials see Peter's Math Pages.

recursion

List of Factorials

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