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C++ program to reverse a number

Given a number, we have to reverse the given number using C++ program.
[Last updated : February 27, 2023]

Problem statement

In this program, we will learn how to reverse an integer number in C++?

Here, we will read an integer number through the keyboard and print its reverse number, let suppose input number is "16789", program will print "98761".

Steps to reverse a number

1. Declaring integer variable num (to store input number) and sum (to store reverse number) as long integer so that we can input and reverse a large value [Read long integer and its value range].
2. Checking that input number is positive or not, if it is negative program will be terminated.

Logic for reversing a number in C++

1. Run a loop till num is greater than zero
2. Get digit using modulus operator (digit = num%10)
3. Add digit with sum*10 to make number reverse (sum = (sum*10) +digit)
4. Divide num by to 10 so that we can get another digit
5. Run this program until num is not less than or equal to zero

C++ program to reverse a number

#include <iostream>
using namespace std;

int main()
{
    long int num, sum;
    int digit;

    //input integer number
    cout << "Enter an integer number: ";
    cin >> num;

    //check validation
    if (num < 0) {
        cout << "Input positive number!!!" << endl;
        return -1;
    }

    //Reverse number
    sum = 0;
    while (num > 0) {
        digit = num % 10; //get digit
        sum = (sum * 10) + digit;
        num = num / 10;
    }

    cout << "Reverse number is: " << sum << endl;

    return 0;
}

Output

First run:
Enter an integer number: 16789
Reverse number is: 98761

Second run:
Enter an integer number: -123
Input positive number!!!