C++ Program to Reverse a Number

In this article, you will learn and get code to reverse a number entered by user at run-time using a C++ program. Since there are many ways to create a C++ program to reverse a number, therefore we've used some famous method to do the task as given below:

Reverse a Number using while Loop
using for Loop
without using Loop
using Function
using Class and Object
using Array

These are the most common and popular ways to approach to reverse a given number in C++. Let's start with first way to find and print reverse of a number.

Reverse a Number using while Loop

To reverse a number in C++ programming, then you have to ask from user to enter a number. Now find the reverse of that number, and then print its reverse on output as shown in the program given below:

The question is, write a program in C++ to reverse a number. Here is its answer:

#include<iostream>
using namespace std;
int main()
{
    int num, rem, rev=0;
    cout<<"Enter the Number: ";
    cin>>num;
    while(num!=0)
    {
        rem = num%10;
        rev = rem + (rev*10);
        num = num/10;
    }
    cout<<"\nReverse = "<<rev;
    cout<<endl;
    return 0;
}

This program was build and run under Code::Blocks IDE. Here is its sample run:

Now supply the number say 123 as input, and press ENTER key to reverse it and print as shown in the snapshot given below:

The dry run of above program with user input 123 goes like:

Initial value, rev=0
When user enters the number 123, it gets stored in num
Now the condition num!=0 (of while loop) or 123!=0 evaluates to be true, therefore program flow goes inside the loop and all the three statements gets executed
That is, num%10 or 123%10 or 3 gets initialized to rem
Then rem + (rev*10) or 3 + (0*10) or 3 gets initialized to rev. So rev=3
And finally, num/10 or 123/10 or 12 gets initialized to num. So num=12
Now after executing all the three statements of while loop, its condition again gets evaluated
That is, the condition num!=0 or 12!=0 again evaluates to be true, therefore program flow again goes inside the loop
Inside the loop, again num%10 or 12%10 or 2 gets initialized to rem
Then rem + (rev*10) or 2 + (3*10) or 2+30 or 32 gets initialized to rev. So rev=32
num/10 or 12/10 or 1 gets initialized to num. So num=1
Again the condition of while loop gets evaluated. That is, num!=0 or 1!=0 evaluates to be true, therefore all the three statements gets executed
That is, num%10 or 1%10 or 1 gets initialized to rem. And rem + (rev*10) or 1 + (32*10) or 1+320 or 321 gets initialized to rev. Finally num/10 or 1/10 or 0 gets initialized to num. So after executing all the three statements, we've rev=321 and num=0
Now this time, the condition (of while loop) num!=0 or 0!=0 evaluates to be false. Therefore the evaluation of while loop gets ended
Now we've a variable rev, that holds the reverse (321) of given number (123)
So just print the value of rev as reverse of a number entered by user at run-time

Reverse a Number using for Loop

This program is created using for loop, instead of while loop. Rest of the things are same as of previous program:

#include<iostream>
using namespace std;
int main()
{
    int num, rem, rev;
    cout<<"Enter the Number: ";
    cin>>num;
    for(rev=0; num!=0; num=num/10)
    {
        rem = num%10;
        rev = rem + (rev*10);
    }
    cout<<"\nReverse = "<<rev;
    cout<<endl;
    return 0;
}

Here is its sample run with user input, 23056:

Reverse a Number without using Loop

This program doesn't use any type of loop, neither for nor while loop to reverse a number. Let's have a look at the program first, its explanation is given later on:

#include<iostream>
using namespace std;
int main()
{
    int num, rem, rev=0;
    cout<<"Enter the Number: ";
    cin>>num;
    CODESCRACKER:
        rem = num%10;
        rev = (rev*10) + rem;
        num = num/10;
        if(num!=0)
            goto CODESCRACKER;
    cout<<"\nReverse = "<<rev;
    cout<<endl;
    return 0;
}

This program produces the same output as of previous program.

Note - A label named CODESCRACKER is created, where all the three statements of reversing a number is written. After executing all the three statements, we've used if for the condition (to check) as done in while loop. That is, if the condition evaluates to be true, therefore using goto keyword followed by label name, program flow start from the label again.

Reverse a Number using Function

This program uses a user-defined function named rev() to reverse a number. It takes a number as its argument, and returns its reverse.

#include<iostream>
using namespace std;
int rev(int);
int main()
{
    int num, r;
    cout<<"Enter the Number: ";
    cin>>num;
    r = rev(num);
    cout<<"\nReverse = "<<r;
    cout<<endl;
    return 0;
}
int rev(int n)
{
    int rem, res=0;
    while(n!=0)
    {
        rem = n%10;
        res = (res*10) + rem;
        n = n/10;
    }
    return res;
}

Reverse a Number using Class

This program uses class and object, an object-oriented feature of C++ to reverse a number given by user:

#include<iostream>
using namespace std;
class CodesCracker
{
    public:
        int rev(int);
};
int CodesCracker::rev(int n)
{
    int rem, res=0;
    while(n!=0)
    {
        rem = n%10;
        res = (res*10) + rem;
        n = n/10;
    }
    return res;
}
int main()
{
    int num, r;
    CodesCracker c;
    cout<<"Enter the Number: ";
    cin>>num;
    r = c.rev(num);
    cout<<"\nReverse = "<<r;
    cout<<endl;
    return 0;
}

Note - An object c of type CodesCracker is created inside the main() function. Now using the object c we've called the member function of the class CodesCracker just like normal function calling using dot (.) operator. The function rev() also returns reverse of a number that is passed as its argument, just like the previous program of normal function.

Reverse a Number using Array

This is the last program on reversing of a number. This program is created using an array. That is, each digit of the number gets initialized as elements of array. And using array, we've printed the reverse of given number:

#include<iostream>
using namespace std;
int main()
{
    int num, rem, arr[10], i=0, tot=0;
    cout<<"Enter the Number: ";
    cin>>num;
    while(num!=0)
    {
        rem = num%10;
        arr[i] = rem;
        num = num/10;
        i++;
        tot++;
    }
    cout<<"\nReverse = ";
    for(i=0; i<tot; i++)
        cout<<arr[i];
    cout<<endl;
    return 0;
}