Bool Camparision

[CodeOpen]

Why d return true but e return false?

  int main() {
 bool d = .06 == .2*.3;
 bool e = .3  == .1 + .1 + .1;
    cout << d << endl << e;
}

[jlb]

Because floating point numbers are approximations. You may want to check this out: https://ece.uwaterloo.ca/~dwharder/NumericalAnalysis/02Numerics/Double/paper.pdf

[CodeOpen]

I know float is approximation so with that d should be false too?

[jlb]

Not necessarily, perhaps on your system .2 and .3 have opposite but equal errors. Try printing each of those constants (with a high precision) and see what they print.

For example:

#include <iostream>
#include <iomanip>

using namespace std;

int main()
{
    double f = .2;
    double e = .3 ;
    double g = .1 + .1 + .1;

    cout << fixed << setprecision(20);
    cout << e << " " << f << endl;
    cout << g << endl;
    cout << f + e << endl;

    return 0;
}

output:

0.29999999999999998890 0.20000000000000001110
0.30000000000000004441
0.50000000000000000000

This is why you should never compare floating point numbers for equality and your compiler should be able to warn you about these issues.

Jim