testing an isbn number to see if its valid - c++

Question

I am having trouble with a homework assignment where I need to check if the ISBN code is valid. This is a beginner C++ class, and I am new to all of this. Right now, no matter what ISBN I put in, it tells me they are all valid. If someone could help point me in the right direction to figure out if my formula is correct when in my for() statement.
I have no problem with the outside loops, it's just the conversion I believe I am doing wrong.
being this is an intro c++ class, we are still only using basic functions, so the professor suggested reading in the ISBN and storing it under a char variable.
**#include <iostream>
using namespace std;
int main()*
{
int Counter;
int WeightedSum;
char ISBN[11] = { 0 };
char Choice;
char Again{};
int Sum = 0;
const char X = 10;
int IterationCounter = 0;
do // begininng of do/while loop
{
cout << "Would you like to check if your ISBN is valid or not? ";
cout << "Press Y/y for yes or N/n for no and to end program: ";
cin >> Choice; //choice input
cout << endl << endl; // blank line
while (Choice != 'Y' && Choice != 'y' && Choice != 'N' && Choice != 'n') // beginning of while loop. checks user input to see if valid y/n choice
{
cout << "Invalid choice! Enter either Y/y or N/n"; // displays when anything other than y/n is entered
cout << "Press Y/y for yes or N/n for no and to end program: "; //gives user another chance to enter y/n
cin >> Choice;
cout << endl << endl;
}
if (Choice == 'Y' || Choice == 'y')
{
cout << "Enter the ISBN you wish to convert: " << endl;
cin >> ISBN;
cout << ISBN;
cout << endl << endl;
for (Counter = 0; Counter < 10; Counter++)
{
WeightedSum = ISBN[10] * (10 - Counter);
Sum = WeightedSum + Sum;
}
cout << Sum;
if (Sum % 11 == 0)
{
cout << Sum;
cout << " Is a valid ISBN " << endl;
}
else
{
cout << Sum;
cout << " Is invalid " << endl;
}*

Answer 1

The problem is this line
WeightedSum = ISBN[10] * (10 - Counter);
You're always performing math against the character at position 11. Because you multiply it by 10 - Counter, which is 0 through 9, you're going to basically be multiplying it by the sum of 1 through 10, or 55. Since 55 is divisible by 11, you'll always end up with if (Sum % 11 == 0) evaluating to true. Worst case, you may even hit an access violation since you're not checking the length of ISBN before accessing it.