Question

Possible Duplicate:
Floating point inaccuracy examples

Im having a problem… When i compile the src, the variable showed isn’t the same that i initialized, see it:

#include <iostream>

    using namespace std;

    int main()
    {
      long double mynum = 4.7;
      cout.setf(ios::fixed,ios::floatfield);
      cout.precision( 20 );
      cout << mynum << endl;
    }

And then:

[fpointbin@fedora ~]$ ./a.out 
4.70000000000000017764

How to fix it? I want to “cout” shows 4.700000…

Answer 1

Your variable is long double, but the default precision of the literal 4.7 is only double. Since you’re printing it as long double, the interpretation chooses to print it with enough significant digits to distinguish it from neighbouring long double values, even though those neighbouring values are not possible doubles.