cardCheck.cpp

#include <iostream>

int getDigit(const int number);
int sumOddDigits(const std::string cardNumber);
int sumEvenDigits(const std::string cardNumber);

int main(){

    std::string cardNumber;
    int result = 0;

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

    result = sumEvenDigits(cardNumber) + sumOddDigits(cardNumber);
    if(result % 10 == 0){
        std::cout << cardNumber << " is valid";
    }
    else{
        std::cout << cardNumber << " is not valid";
    }
    

    return 0;
}

int getDigit(const int number) {
    int doubledDigit = number * 2;
    return doubledDigit > 9 ? doubledDigit - 9 : doubledDigit;
}

int sumOddDigits(const std::string cardNumber){
    int sum = 0;
    for(int i = cardNumber.size() - 1; i >= 0; i-=2){
        sum += cardNumber[i] - '0';
    }
    return sum;
}
int sumEvenDigits(const std::string cardNumber) {
    int sum = 0;
    for (int i = cardNumber.size() - 2; i >= 0; i -= 2) {
        sum += getDigit(cardNumber[i] - '0');
    }
    return sum;
}