when using double, take note of its precision problem (float and double only store decimal estimates) if you find certain values does not match. A possible solution is to use a custom Round function.

double-precision-fix.cpp

#include <iostream>
#include <cmath>
 
using namespace std;
 
// rounds a double variable to nPlaces decimal places
double Round(double dbVal, int nPlaces /* = 0 */)
{
    const double dbShift = pow(10.0, nPlaces);
    return  floor(dbVal * dbShift + 0.5) / dbShift; 
}
 
int main()
{	
	double t = 0.1;
	double f = 0.2;
	t = t + f;
	double q = 0.3;
	cout << "t = " << t << endl;
	cout << "q = " << q << endl;
 
	/* uncomment to see round function at work */
	// t = Round(t, 1);
	if (t == q) {
		cout << "t and q are equal!" << endl;
	} else {
		cout << "t and q are NOT equal." << endl;
	}
 
    return 0;
}