Credit Card Validator in C++

To create a simple credit card validator in C++, you can use the Luhn algorithm (also known as the “modulus 10” or “mod 10” algorithm). This algorithm is commonly used to validate a variety of identification numbers, including credit card numbers. Here’s a basic implementation:

#include <iostream>
#include <string>

using namespace std;

bool isNumeric(const string& str) {
    return !str.empty() && str.find_first_not_of("0123456789") == string::npos;
}

bool isValidCreditCard(const string& cardNumber) {
    if (!isNumeric(cardNumber)) {
        cout << "Error: Credit card number must contain only digits." << endl;
        return false;
    }

    int sum = 0;
    int len = cardNumber.length();
    bool alternate = false;

    for (int i = len - 1; i >= 0; --i) {
        int digit = cardNumber[i] - '0';

        if (alternate) {
            digit *= 2;
            if (digit > 9) {
                digit = (digit % 10) + 1;
            }
        }

        sum += digit;
        alternate = !alternate;
    }

    return (sum % 10 == 0);
}

int main() {
    string cardNumber;

    cout << "Enter credit card number: ";
    cin >> cardNumber;

    if (isValidCreditCard(cardNumber)) {
        cout << "Valid credit card number." << endl;
    } else {
        cout << "Invalid credit card number." << endl;
    }

    return 0;
}

Here’s a quick rundown of how this code works:

1. We define a function isNumeric to check if a string contains only digits.
2. The isValidCreditCard function uses the Luhn algorithm to validate the credit card number.
3. It first checks if the input contains only digits. If not, it prints an error message.
4. The algorithm processes the digits in reverse order, starting from the rightmost digit.
5. It doubles every second digit and adjusts the result if it’s greater than 9.
6. It accumulates the sum of the digits.
7. Finally, it checks if the sum is divisible by 10.

Keep in mind that this is a basic implementation and does not perform a thorough validation of all aspects of credit card numbers (e.g., it doesn’t check for specific card issuer patterns). For a production-level application, you would want to use a more comprehensive validation library or API provided by payment gateways.