luhn_algo.cpp

// C++ program to implement Luhn algorithm
#include <bits/stdc++.h>
using namespace std;

// Returns true if given card number is valid
bool checkLuhn(const string& cardNo)
{
	int nDigits = cardNo.length();

	int nSum = 0, isSecond = false;
	for (int i = nDigits - 1; i >= 0; i--) {

		int d = cardNo[i] - '0';

		if (isSecond == true)
			d = d * 2;

		// We add two digits to handle
		// cases that make two digits after
		// doubling
		nSum += d / 10;
		nSum += d % 10;

		isSecond = !isSecond;
	}
	return (nSum % 10 == 0);
}

// Driver code
int main()
{
	string cardNo = "79927398713";
	if (checkLuhn(cardNo))
		printf("This is a valid card");
	else
		printf("This is not a valid card");
	return 0;
}