Reverse a Number in C++ [3 Methods]

Summary: In this example, we will learn different ways to reverse a number in the C++ programming language.

Example:

Input:  456
Output: 654

Input:  132
Output: 231

Method 1: Using While Loop

The idea is to extract the digits from the input number in reverse order and append them to form its reverse using the while loop.

#include <iostream>
using namespace std;
 
int main()
{
    int num;
    
    //Input a number
    cout << "Enter a Number: ";
    cin >> num;
    
    //Initialize rev with 0
    int rev=0;
    
    //Append digits in reverse order
    while(num>0){
        rev = rev*10 + num%10;
        num = num/10;
    }
    
    cout << "Reverse: " << rev;
    return 0;
}

Output:

Enter a Number: 243
Reverse: 342

To extract and append the digits from the original number to its reverse we use the following: rev =rev*10 + num%10.

After the operation on each digit, we remove it from the input number using the following: num=num/10.

Method 2: Using Recursion

What we have implemented in the above method can also be implemented using recursion.

To append the digits in reverse order in the recursive approach:

1. Extract the last digit i.e. num%10
2. Multiply the digit with pow(10, l-1), where l is the length of the number passed as an argument
3. Add the result to the next recursive call.

#include <iostream>
#include <cmath>
using namespace std;

//recursive_function(number, length_of_the_number)
int reverse(int num, int l){
    //base contition
    if(num==0)
        return 0;
        
    //extract the last digit    
    int digit = num%10;
    
    /*
        *Append them in reverse order
        *For e.g., to reverse 859
        *return 9*100 + 58 = 958
    */
    return digit*pow(10, l-1) + reverse(num/10, l-1);
}
 
int main()
{
    int num;
    
    //Input a number
    cout << "Enter a Number: ";
    cin >> num;
    
    //call the recursive funtion to reverse the number
    int length = to_string(num).length();
    int rev = reverse(num, length);
    
    //output the number
    cout << "Reverse: " << rev;
    return 0;
}

Output:

Enter a Number: 859
Reverse: 958

For each recursive call, we are eliminating the last digit from the number using num/10 and also decreasing the length of the number by 1 i.e. l-1.

Method 3: Converting int To string

We can transform the integer into a string using the inbuilt to_string() method and reverse it by using any string reversal methods in C++.

#include <iostream>
#include<algorithm>
using namespace std;
 
int main()
{
    int num;
    
    //Input a number
    cout << "Enter a Number: ";
    cin >> num;
    
    //convert the integer to string
    string str = to_string(num);
    
    /*
      *reverse the string using the reverse() method
      *defined in the algorithm header file
    */
    reverse(str.begin(), str.end());
    
    //output the string
    cout << "Reverse: " << str;
    return 0;
}

Output:

Enter a Number: 700
Reverse: 007

These were the three methods using which we can reverse any number in C++ programming language.