Factorial Program in Java Using while Loop

The factorial of a number N is the product of all positive descending integers (integers less than or equal to N).

N! = N * (N - 1) ... * 3 * 2 * 1

In this section, we will create Java programs to find the factorial of a number using a while loop and do-while loop.

How to find factorial?

If we say N = 5,

Then the factorial of 5 is represented as 5!

And, 5! = 5 * 4 * 3 * 2 * 1 = 120

Note: The value of 0! is 1, according to the convention for an empty product.

We can find the factorial of a number in Java using the following ways:

for loop
Recursion (preferred)
while loop
do-while loop

Factorial Program Using while Loop

In the while loop, the condition is checked at the beginning of the iteration, and the loop incrementation occurs inside the loop body.

Following are the steps to write a Java program to find factorial of a number using while loop:

Declare a variable (int fact) and initialize it with 1.
Read a number whose factorial is to be found. Store it in a variable (int num).
Set the while loop to the condition (i <= num) where initial value of i = 1.
Inside the while loop, multiply the variable fact and variable i, and store the result in variable fact. Increment the loop variable i by 1 (i++).
Print the factorial.

Example 1:

FactorialUsingWhileLoop.java

//importing Scanner class
import java.util.Scanner;

public class FactorialUsingWhileLoop {
	public static void main(String[] args) {
		
		//declaring and intializing variables 
		int fact = 1;
		int i = 1;

		//creating object of Scanner class
		Scanner sc = new Scanner(System.in);

		//accepting a number from the user
		System.out.println("Enter a number whose factorial is to be found: ");
		int num = sc.nextInt();
		
		//counting the factorial using while loop
		while( i <= num ){
			fact = fact * i; 
			i++; //increment i by 1 
		}	

		//printing the result
		System.out.println("\nFactorial of " + num + " is: " + fact);
	}
}

Output:

Factorial Program in Java Using while Loop

Factorial Using while loop and user-defined function

Let's use the while loop in user defined function.

Example 2:

FactorialUsingWhileLoop2.java

import java.util.Scanner;
public class FactorialUsingWhileLoop2{

	//user-defined function
	public static int fact_with_while(int n){
		int fact = 1;
		int i = 1;

		while( i <= n ) {
			fact = fact * i; 
			i++; 
		}

		//returning result to main()
		return fact;

	}
public static void main(String[] args) {
		
		Scanner sc = new Scanner(System.in);

		//accepting a number from the user
		System.out.println("Enter a number whose factorial is to be found: ");
int num = sc.nextInt();
		
		//printing the result
		System.out.println("\nFactorial of " + num + " is: " + fact_with_while(num));
	}
}

Output:

Factorial Program Using do-while Loop

The factorial program using the do-while loop slightly differs from the while loop. Unlike the while loop, the condition (i <= num) is checked at the end of each iteration.

Example 3:

FactorialUsingDoWhileLoop.java

//importing Scanner class
import java.util.Scanner;

public class FactorialUsingDoWhileLoop {
	public static void main(String[] args) {
		
		//declaring and intializing variables 
		int fact = 1;
		int i = 1;

		//creating object of Scanner class
		Scanner sc = new Scanner(System.in);

		//accepting a number from the user
		System.out.println("Enter a number whose factorial is to be found: ");
		int num = sc.nextInt();
		
		//counting the factorial using do-while loop
		do {
			fact = fact * i; 
			i++; 
		} while( i <= num );

		//printing the result
		System.out.println("\nFactorial of " + num + " is: " + fact);
	}
}

Output:

Factorial using do-while loop and user-defined function

Let's use the do-while loop in user defined function.

Example 4:

FactorialUsingDoWhileLoop2.java

import java.util.Scanner;

public class FactorialUsingDoWhileLoop2{

	//user-defined function
	public static int fact_with_do_while (int n){
		int fact = 1;
		int i = 1;

		do {
			fact = fact * i; 
			i++; 
		} while( i <= n );

		//returning result to main()
		return fact;

	}
	public static void main(String[] args) {
		
		Scanner sc = new Scanner(System.in);

		//accepting a number from the user
		System.out.println("Enter a number whose factorial is to be found: ");
		int num = sc.nextInt();
	
		//printing the result
		System.out.println("\nFactorial of " + num + " is: " + fact_with_do_while(num));
	}
}

Output: