Factorial Program in Java using for loop Iterative method

In this tutorial, we are going to learn java program to find the factorial of the number given as input from the user using for loop.

Problem Statement

For any Input numbers given by the user, we have to calculate the factorial of that numbers.

For example

What is the Factorial of a Number?

The factorial of a number is the multiplication of each and every number till the given number except 0.

For example

Factorial of number 4 is the multiplication of every number till number 4, i.e. 1*2*3*4 = 24

Important Point

Our logic to Calculate Factorial of number using for loop

• Our program will create three variables i, num, factorial of long types
• Our program will take an integer input from the user which should be a whole number.
• If the number input is negative then print ‘The factorial can’t be calculated’.
• Then use for loop to calculate the factorial.
• After that, we will store the value of the factorial in a variable and finally print the value of the variable.

Algorithm to calculate Factorial of Number

Java Program to Calculate Factorial using For Loop

import java.util.*;
public class Main
{
    public static void main(String[] args) {
    long i,num,factorial=1;
    Scanner sc = new Scanner(System.in);
    System.out.println("Enter a number to calculate factorial: ");
    num = sc.nextInt();
    if(num<0){
        System.out.println("Factorial can't be calculated for negative number");
    }
    else if(num==0){
       System.out.println("Factorial of 0 is 1");
    }
    else{
        for(i=1;i<=num;i++)
          factorial=factorial*i;
        System.out.println("Factorial = "+factorial);
    }
    }
}

Output 1:

Enter a number to calculate factorial: 
5
Factorial = 120

Explanation

The input number from the user to find the factorial is 5. The calculated value of the factorial is 120.

Output 2:

Enter a number to calculate factorial: 
8
Factorial = 40320

The input number from the user to find the factorial is 8. The calculated value of the factorial is 40320.

Breaking Down the Program

Imports and Class Definition

import java.util.*;
public class Main

• import java.util.*;: This line tells Java to bring in all classes from the java.util package which includes the Scanner class used for input.
• public class Main: This defines a class named Main. In Java, everything runs inside classes, and this is the main container for our code.

Main Method

public static void main(String[] args)

This is the entry point of any Java application that runs as a standalone program. The main method is always called when the program starts.

Variable Declaration and Initialization

long i, num, factorial = 1;
Scanner sc = new Scanner(System.in);

• Variables: i, num, and factorial are declared. factorial is initialized to 1 because the factorial calculation involves multiplication, and starting with 1 ensures the correct calculation.
• Scanner: A Scanner object sc is created to read input from the user. This allows the program to take numbers inputted by the user.

Prompt for Input

System.out.println("Enter a number to calculate factorial: ");
num = sc.nextInt();

The program prints a message asking the user to enter a number. Then, it uses sc.nextInt() to read the integer input and stores it in num.

Conditionals and Loop for Calculation

if (num < 0) {
    System.out.println("Factorial can't be calculated for negative number");
} else if (num == 0) {
    System.out.println("Factorial of 0 is 1");
} else {
    for (i = 1; i <= num; i++)
        factorial = factorial * i;
    System.out.println("Factorial = " + factorial);
}

• Negative Check: If num is less than 0, it prints an error message because factorials of negative numbers are not defined.
• Zero Check: If num is 0, it prints that the factorial of 0 is 1 (by definition).
• Positive Numbers: If num is positive, it enters a loop from 1 up to num. Inside the loop, factorial is updated by multiplying it by each number i (this is how factorials are calculated).
• After the loop, it prints the calculated factorial.