6.37 Credit Card Number Validation

#include <iostream>
#include <string>
#include <cmath>

using namespace std;

bool startsWith(const string& cardNumber, const string& substr)
{
//cout << "startsWith recieved cardNumber: " << cardNumber << " and substr: " << substr << endl;
        bool valid = true;
        int c = 0;

        while (c < int(substr.length()))
        {
//cout << "Loop where c = " << c << " < " << int(substr.length()) << " compares cardNumber.at(c) " << cardNumber.at(c) << " and substr.at(c) " << substr.at(c) << endl;
                if(cardNumber.at(c) != substr.at(c)){
                    valid = false;
                    break;
                }
                else
                    c++;
        }
//cout << "startsWith is returning (0|1 for false|true): " << valid << endl;
        return valid;
}

int getDigit(int number)
{
//cout << "getDigit recieved number: " << number  << endl;
        number = (number % 10) + (number / 10);

//cout << "getDigit is returning: " << number  << endl;
        return number;
}

int sumOfDoubleEvenPlace(const string& cardNumber)
{
//cout << "sumOfDoubleEvenPlace recieved cardNumber: " << cardNumber << endl;
    int sumOfEven = 0;
//cout << "sumOfEven is initialized as " << sumOfEven << endl;
//cout << "attempting to initialize for loop with first index of " << (cardNumber.length()-(cardNumber.length()%2))-1 << endl;
    for(int i=(cardNumber.length()-(cardNumber.length()%2))-2; i >= 0; i -= 2)
    {
        int doubledCardNumber = int(cardNumber.at(i)-'0');
        doubledCardNumber *= 2;
        sumOfEven += getDigit(doubledCardNumber);
//cout << "sumOfEven is " << sumOfEven << " += cardNumber.at(i) " << cardNumber.at(i) << " *2 = " << doubledCardNumber << " processed with getDigit()" << endl;
    }
//cout << "sumOfDoubleEvenPlace is returning: " << sumOfEven << endl;
        return sumOfEven;
}

int sumOfOddPlace(const string& cardNumber)
{
//cout << "sumOfOddPlace recieved cardNumber: " << cardNumber << endl;
    int sumOfOdd = 0;
    for(int i=cardNumber.length()+(-1+cardNumber.length()%2); i >= 0; i -= 2)
    {
//cout << "sumOfOdd is " << sumOfOdd << " plus cardNumber.at(i) " << cardNumber.at(i) << endl;
        sumOfOdd += int(cardNumber.at(i)-'0');
    }
//cout << "sumOfOddPlace is returning: " << sumOfOdd << endl;
        return sumOfOdd;
}

bool isValid(const string& cardNumber)
{
//cout << "isValid recieved cardNumber: " << cardNumber << endl;
    bool valid = false;
//cout << "The length of your input is " << cardNumber.length() << endl;
    int sum = (sumOfOddPlace(cardNumber) + sumOfDoubleEvenPlace(cardNumber)) %10;
//cout << "Sum % 10 is " << sum << endl;
    if((cardNumber.length() >= 13 && cardNumber.length() <= 16) &&
        sum == 0 &&
        (startsWith(cardNumber, "37") || startsWith(cardNumber, "4") ||
        startsWith(cardNumber, "5") || startsWith(cardNumber, "6")))
    {
        valid = true;
    }
//cout << "isValid is returning (0|1 for false|true): " << valid << endl;
        return valid;
}
// The main function that uses the other functions to determine whether the card is valid or not
void main()
{
        cout << "Enter a credit card number: ";
        string cardNumber;
        cin >> cardNumber;

        if(isValid(cardNumber))
                cout << "The credit card number is valid" << endl;
        else
                cout << "The credit card number is invalid" << endl;
}