bool is Valid(const string& cardNumber)

Returns true/false indicating if the credit card number is potentially a valid number according to the luhn algorithm. This should use a call to luhnDigitSum and not reimplement the logic of that function.

Here's a copy of luhnDigitSum(const string& cardNumber) please only use luhnDigitSum(const string& cardNumber) in main.

 int luhnDigitSum(const string& cardNumber){

 int sum=0;

 for(int i=cardNumber.size()-2 ; i>=0; i=i-2){

 if( charToInt(cardNumber[i]) == -1 ){

 return 0;

 }

 sum = sum + doubleDigitValue(charToInt(cardNumber[i]));

 }

 for(int i=cardNumber.size()-1 ; i>=0; i=i-2){

 if( charToInt(cardNumber[i]) == -1 ){

 return 0;

 }

 sum = sum + charToInt(cardNumber[i]);

 }

 return sum;

 }