// https://godbolt.org/z/8ae83nPYs
#include <iostream> // cout
#include <iomanip> // setprecision, etc.
#include <cmath> // fabs
using std::cout; // cout as abbreviation for std::cout
int main() {
cout << std::fixed << std::setprecision(25); // for better readable output
// 0.1 and 0.01 cannot store double exactly
double x = 0.1 * 0.1;
cout << "0.1*0.1: " << x << "\n";
// output: 0.1*0.1: 0.0100000000000000019428903
if(x == 0.01) { // (ERR) never compare double with ==
cout << "Yes! x == 0.01" << "\n";
} else {
cout << "Uh-oh! x != 0.01" << "\n"; // you see this output
}
// Attention especially when comparing with 0.0
double null = x - 0.01;
cout << "null: " << null << "\n";
// Output: null: 0.0000000000000000017347235
if(std::fabs(null) < 0.00000001) { // compare with an "epsilon"
cout << "Yes! null is close to 0.0" << "\n"; // you see this output
} else {
cout << "Uh-oh! null not close to 0.0" << "\n";
}
// fractions of powers of 2 are less critical
double y = 0.5 * 0.5;
cout << "0.5*0.5: " << y << "\n";
// Output: 0.5*0.5: 0.2500000000000000000000000
if(y == 0.25) { // here the dangerous comparison works exceptionally
cout << "Yes! y == 0.25" << "\n"; // you see this output
} else {
cout << "Oh-oh! y != 0.25" << "\n";
}
//
return 0;
}