FactorialExample.java

/**
 * This program shows the different kinds of calls that can be made for functions like the factorial, and fibonacci 
 *
 */
public class FactorialExample {

	public static void main(String[] args) {
		System.out.println( factRecursive(5));
		System.out.println( factTailRecursive(5));
		System.out.println( factIterative(5));
	}


	/**
	 * This is a simple version of factorial function making recursive calls.
	 * It is not tail recursive, and so cannot be optimized in a functional compiler,
	 * or be made into an iterative function for compilers like Java & C#.
	 * @param n - the number to find the factorial for
	 * @return the factorial of n (assumes positive number, negatives are treated as the value 1)
	 */
	public static long factRecursive(long n) {
		if (n <= 1) {
			// base case - keep counting down until this case is reached (assuming positive numbers)
			return 1;
		}
		else {
			// recursive call - more work has to be done in the next call
			return n * factRecursive(n-1);
		}
	}
	
	
	/**
	 * This is the initial call for the fact2 function. The accumulator is set to 1, and other calls are made
	 * with the accumulator passed in.
	 * @param n - the number to find the factorial for
	 * @return the factorial of n (assumes positive number, negatives are treated as n = 1)
	 */
	public static long factTailRecursive(long n) {
		// set the default value for the accumulator if this is the first call
		long accumulator = 1;
		if (n <= 1) {
			return accumulator;
		}
		else {
			return fact2(n-1, accumulator * n);
		}
	}
	
	
	/**
	 * Iterative call that does not use recursion. It is possible to do this because there is a tail recursive version.
	 * You can always rewrite tail recursive calls as iterative, and vice versa.
	 * @param n - the number to find the factorial for
	 * @return the factorial of n (assumes positive number, negatives are treated as n = 1)
	 */
	public static long factIterative(long n) {
		long acc = 1;
		// iterative version
		while (n > 1) {
			acc = acc * n;
			n--;
		}
		
		return acc;
	}
}