Warning compared between floats how to proceed

Question

In an exercise I did, you are asked to make a compound interest calculation with for for the values of 5%, 6%, 7%, 8%, 9%, and 10%.

As I could not use a check per counter being the counter a type float, nor use switch, then I chose to make a conditional test where the user inserts the value, but generates me the Warning:

Warning:Comparing floating point with == or != is unsafe [-Wfloat-Equal]

I understand the reason for Warning is by comparison between floating points be inaccurate, and by the flag -Wfloat-equal] be active, but when "running" the program is not generated me any error, if this way of performing the comparison generates a Warning, what would be the correct way to compare two values of the type float.

Follow the code for evaluation:

#include <iostream>
#include <iomanip>
#include <cmath>

using std::cout;
using std::cin;
using std::endl;
using std::fixed;
using std::setw;
using std::setprecision;
using std::pow;

int main()
{
    //quantia em depósito no fim de cada ano.
    double deposito;
    //quantia inicial antes dos juros.
    double principal = 1000.0;
    //taxa de juros.
    double taxa;
    cout<<"Insira a taxa de juros entre 0.05 á 0.10: ";
    cin>>taxa;
    //exibe cabeçalhos.
    cout << " Ano(s) " << setw(21)<<"Quantidade depositada"<<endl;
    //configura o formato de saída do valo de ponto flutuante.
    cout << fixed << setprecision(2);
    //calcula a quantia de depósito para cada um dos dez anos.
    for( int ano = 1; ano <= 10; ano++ )
    {
        if(taxa == .05)
        {
            //calcula a nova quantia durante o ano especificado
            deposito = principal * pow(1.0 + taxa, ano);
            //exibe o ano e a quantia
            cout << setw(4) << ano << setw(21) << deposito << endl;
        }
        else if(taxa == .06)
        {

            deposito = principal * pow(1.0 + taxa, ano);
            cout << setw(4) << ano << setw(21) << deposito << endl;
        }
        else if(taxa == .07)
        {

            deposito = principal * pow(1.0 + taxa, ano);
            cout << setw(4) << ano << setw(21) << deposito << endl;
        }
        else if(taxa == .08)
        {

            deposito = principal * pow(1.0 + taxa, ano);
            cout << setw(4) << ano << setw(21) << deposito << endl;
        }
        else if(taxa == .09)
        {

            deposito = principal * pow(1.0 + deposito, ano);
            cout << setw(4) << ano << setw(21) << deposito << endl;
        }
        else if(taxa == .10)
        {

            deposito = principal * pow(1.0 + taxa, ano);
            cout << setw(4) << ano << setw(21) << deposito << endl;
        }
        else
        {
            //para que não seja recebido um valor além do especificado.
            cout<<"O valor de taxa inserido é inválido. ";
            return 0;
        }
    }
    return 0;
}

Answer 1

You really can’t make an accurate comparison with floating point. If you need this accuracy use another type. But if you want to minimize the problem, do a function that performs an approximation. Example:

bool isEqual(double x, double y) {
    return std::abs(x - y) <= 1e-5 * std::abs(x);
}

Behold working in the ideone. And in the repl it.. Also put on the Github for future reference.

Source. Note that the approximation is arbitrary.