Java Program to Reverse a Number and find the Sum of its Digits Using do-while Loop

Problem Statement: The number is supposed to be entered by the user be it any random number lying within the primitive data-type holding the number. First, the number needs to be reversed. Secondary the sum of the number is to be calculated with the constraint to use a do-while loop.

do-while loop: Now the user sometimes does get confused between a while and a do-while loop. While loop checks the condition first and then executes the statement whereas the Do-while loop executes the statement then check the condition. An important point arising out from Do-while is even if the condition fails out still it will be executed once. 

Syntax of do-While Loop:

do 
{
 // statement to be executed
}
while(condition check)

Use of Do-while Loop:

Showing some menu to the user in other words the game is implemented where the goal is to show the user press 1 to do this, 2 to do that, and so on where there is an option press Q to quit this game. So do-while loop wants to show the menu at least once to the user. When the user has taken the action appropriate steps are done. So, while the condition should be if the user presses Q to quit and the menu is there inside the do-while loop.

Approach: Using do-while Loop, find the reverse of a number as well as the sum of its digits. Enter any number as an input and after that use modulus and division, operators to reverse that particular number and find the sum of its digits.

Algorithm:

1. Taking a number from the user
2. Creating two variables namely reverse_number and sum and initializing both of  them to 0
3. Reversing a number
4. Printing Reversed number
5. Printing the final sum of the digits as procured from smaller sums.

Reversal Algorithm as discussed as follows:

• Multiply rev by 10 and add remainder of number that is remainder to reverseNumber.  // rem = num%10 ; rev = rev*10 + rem;
• Add rem to the current sum to find the sum of digits and storing them in a variable that will be returned holding the final sum. Final_sum=current_sum where current_sum=current_sum + remainder
• Dividing number by 10 to access digit preceding the current digit of the number as follows. // num=num/10; while(num > 0);

Example 1: Java program to reverse and sum up digits of the number without making functions

Java

// Java program to reverse and and sum up digits of number

// Importing generic Classes/Files
import java.io.*;
// Importing Scanner Class
import java.util.Scanner;

class GFG {

    // Main driver method
    public static void main(String[] args)
    {
        // creating variables
        int num, rem;

        // Creating variables and initializing at same time
        int rev = 0, sum = 0;

        // Using scanner to take input from user
        // Scanner sc = new Scanner(System.in);

        // Remove comments from lineNo20
        // to take dynamic user input

        // Displaying message
        System.out.println("Enter the number: 25 ");

        // Taking input from user
        // num = sc.nextInt();

        // Hard coded input
        num = 25;

        // Do-while loop for iteration over digits of number
        do {

            // Step1: Modulo with 10 to get last digit
            rem = num % 10;

            // Step2: Reverse the number
            rev = rev * 10 + rem;

            // Sum of the digits of number
            sum = sum + rem;

            // Step3: Dividing number by 10 to lose last
            // digit
            num = num / 10;
        }

        // Condition check
        // Remember: By this time 1 iteration is over even
        // if conditions false
        while (num > 0);

        // Printing the reverse number
        System.out.println("Reverse of given number: "
                           + rev);

        // Summing up digits of number as shown in above
        // steps
        System.out.println("Sum of digits of given number: "
                           + sum);
    }
}

Output

Enter the number: 25 
Reverse of given number: 52
Sum of digits of given number: 7

Example 2: In this example separately do-while loops are shown as an illustration by creating functions of them and later on calling them in the main driver method.

Java

import java.io.*;

// Importing specific Scanner Class to show menu
import java.util.Scanner;

class GFG {

    // Iterative function to reverse digits of number
    // static int reverseDigits(int num)

    // Hardcoded input where input number equals 25
    static int reverseDigits(int num)
    {

        // Making input hard coded else
        num = 25;
        // else do not initialize num

        // creating and Initialising reverseNo with 0
        // Creating remainder variable
        int rev = 0, rem;

        // Statements to be executed in do loop
        do {
            // Reversal of a number as discussed in
            // algorithm
            rem = num % 10;
            rev = rev * 10 + rem;
            num = num / 10;

        }

        // Condition check
        while (num > 0);

        // Returning reverse of the user enter number
        return rev;
    }

    // Iterative function findingsum of the digits of number
    // static int sumDigits(int num)

    // Hardcoded input where input number equals 25
    static int sumDigits(int num)
    {
        // Making input hard coded
        num = 25;
        // else do not initialize num

        // creating and Initialising final_sum with 0
        // Creating remainder variable
        int sum = 0, rem;

        // Statements to be executed in do loop
        do {
            // Retrieving steps as discussed in above 3
            // steps
            rem = num % 10;
            sum = sum + rem;
            num = num / 10;

        }
        // condition check
        while (num > 0);

        // Returning Sum of digits of a reversed number
        return sum;
    }

    // Main driver method
    public static void main(String[] args)
    {
        // declaring variable to store user entered number
        // int num;

        // Scanner Class to read the entered number

        // Commented out to hard code the input
        // else remove comments
        // Scanner sc = new Scanner(System.in);

        // Input is hardcoded for display
        // else remove comments from line no 77

        // Considering hard coded input
        int num = 25;

        // Printing message
        System.out.println(num);

        // Taking input from user
        // Making input hard coded else
        // remove comments from line no 82
        // num = sc.nextInt();

        // Calling above functions to print reversed number
        System.out.println("Reverse of given number: "
                           + reversDigits(num));

        // Calling above functions to print reversed number
        System.out.println("Sum of digits of given number: "
                           + sumDigits(num));
    }
}

Output

25
Reverse of given number: 52
Sum of digits of given number: 7

Time complexity: O(logn) where n is  given input number.

Auxiliary space: O(1)