Weird problem using decimals in a subtracting loop.

[bndcntn]

#include <iostream>
using namespace std;

int main() {
  double a, b;

  while (1) {
    cout << "Enter 1st number (0 to quit): ";
    cin >> a;
      if (a == 0)
        break;

    cout << "Enter 2nd number (0 to quit): ";
    cin >> b;
      if (b == 0)
        break;

    while (a >= b)
      a -= b;

    cout << "Result: " << a << endl;
  }

  return 0;
}

Now if I enter, for example, 90 and 3, I get the result 0. But if I try 9 and 0.3 (which should give me an identical response), I get... 1.22125e-15 !

Can someone explain why this is happening?

thanks
--

[SlyMaelstrom]

It's a rounding error. In case you don't understand scientific notation, 1.22125e-15 is the equivilent of .00000000000000122 which as you can tell, is very, very close to zero.

[linucksrox]

check out the stuff in #include <iomanip> that should most likely help you out with the problem you're having

[SlyMaelstrom]

...and if you have real interest in programming, you can check out this link to understand floating-point numbers and why that rounding error occured.

http://www.petebecker.com/js200006.html