Bigint *operator

Question

I'm doing a bigint project and I am stumped as to why my multiplication operator isn't working properly on a test case.

I'm excluding the .h file because its probably unnecessary.

bigint.cpp:

// Definition of multiplication operator
BigInt BigInt::operator*(BigInt addend2)
{
 BigInt product;
   short int first,                  // a block of 1st addend (this object)
             second,                 // a block of 2nd addend (addend2)
             result,                 // a block in their sum
             carry = 0;              // the carry in adding two blocks

   list<short int>::reverse_iterator // to iterate right to left
      it1 = myList.rbegin(),         //   through 1st list, and
      it2 = addend2.myList.rbegin(); //   through 2nd list

   while (it1 != myList.rend() || it2 != addend2.myList.rend())
   {
      if (it1 != myList.rend())
      {
         first = *it1;
         it1++ ;
      }
      else
         first = 0;
      if (it2 != addend2.myList.rend())
      {
         second = *it2;
         it2++ ;
      }
      else
         second = 0;

      short int temp = first * second;
      result = temp % 1000;
      carry = temp / 1000;
      product.myList.push_front(result);
   }

   if (carry > 0)
      product.myList.push_front(carry);

   return product;
}

Main.cpp (Test case):

int main()
{
    char response;
       do
       {
      cout << "\nMultiplication part:" << endl;
      cout << "The multiplication of\n\t"
           << number1 << " * " << number2
           << "\nis\n\t" << number1 * number2 << endl;

      cout << "\nAdd more integers (Y or N)? ";
      cin >> response;
}

When I run the code, the multiplication is wrong.

Below is a sample run: The multiplication of 123 * 423 is -507 which is obviously not correct.

I'm pretty sure I messed up on the definition of the multiplication but can anyone say where I messed up?

Edit: Just letting everyone know, my code does compile, but the product is sometimes wrong. I also change all my short int to long int.

For example:

978 * 878 = 858,684 Which is correct

But when I use bigger numbers then the problem occurs.

Example:

432,454 * 765,534 = 330,722,436 which is not correct. The correct answer is 3.32 * 10^11

Answer 1

Don't use short int for your intermediate values: 1000 * 1000 will likely overflow a short. Use int, and ideally somewhere static_assert(1000 * 1000 <= std::numeric_limits<int>::max()), "oops - int is too small!");.

123 * 423 = 52029. On a two's complement machine with 16 bit shorts, unsigned short(52029) = -13507. -13507 % 1000 = -507. I'm not sure what happened to the carry. though.