Question

Why does a1=72 instead of 73 in this (terrible) snippet of C++ code ?

#include <iostream>
#include <string>

using namespace std;

int main (int argc, char* argv[])
{   
    double       a  = 136.73;
    unsigned int a1 = (100*(a-(int)a));

    cout << (a-(int)a)     << endl; // 0.73
    cout << 100*(a-(int)a) << endl; // 73
    cout << a1             << endl; // 72 !
}

You can execute it at http://codepad.org/HhGwTFhw

Answer 1

If you increase the output precision, you’ll see that (a - (int) a) prints 0.7299999999999898.

Therefore, the truncation of this value (which you obtain when you cast it to an int) is indeed 72.

(Updated codepad here.)