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C++ program to reverse a number

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".

Extras:

  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:

  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

Reverse an integer number in C++



#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!!!





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