NaN in Program

This is supposed to be capable of solving quadratic equations, but it sometimes returns NaN. Such as when you would enter 3x^2 + 4x + 7. It returns NaN. I am not understanding why it returns NaN to me as the answer for the equation. If someone could please explain why it returns this to me and if there is a solution, please try and state it simply for me.

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  cout << "\n+----Quadratic Equation Solution----+\n"
       << "|    Enter the Coefficient on x^2   |\n"
       << "                  ";
  double avalue, bvalue, cvalue, b2, ac4, b2ac4, negbplus, negbminus, sqrb2ac4, twoa, plusover2a, minusover2a;
  cin >> avalue;
  cout << "|     Enter the Coefficient on x    |\n"
       << "                  ";
  cin >> bvalue;
  cout << "|        Enter the Third Term       |\n"
       << "                  ";
  cin >> cvalue;
  ac4 = avalue * cvalue * 4;
  b2 = pow(bvalue, 2);
  b2ac4 = b2 - ac4;
  sqrb2ac4 = sqrt(b2ac4);
  negbplus = -bvalue + sqrb2ac4;
  negbminus = -bvalue - sqrb2ac4;
  twoa = 2 * avalue;
  plusover2a = negbplus / twoa;
  minusover2a = negbminus / twoa;
  cout << "The Values of X are " << setprecision(4) << plusover2a << " and " << setprecision(4) << minusover2a << endl;


Also, the reason my variables are difficult to understand is because it is part of a larger code in a calculator I am creating.
The example you gave has complex roots. ie. b2a4c < 0.
This causes a problem when you try to take a square root.

Maybe introduce a bool rootsAreComplex; variable to watch for this?
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bool rootsAreComplex = false;
if( b2a4c < 0.0 )
{
    rootsAreComplex = true;
    b2a4c *= -1.0;// make it positive so you can take a square root.
}

rootAreComplex will affect how you display the roots.
I do not yet know the command bool. Could you please explain its usem
bool is not a command it is a type for boolean
you can use int instead and instead of false and 1 instead of true
there is another way of resolving the problem:
you can use the <complex> library http://www.cplusplus.com/reference/complex/

or you can create your own complex class
I have attempted to use the complex class, but I am unsure if I am using it correctly. I was setting an if statement so if b2ac4 is greater than 0, then it will use real, and if it is less than 0, then it will use imag. Every time i try to compile it, it fails to compile at the if statement saying that > is not an operator.
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 b2ac4 = b2 - ac4;

                if(b2ac4 > 0) //refuses to compile past this point
                {
                    sqrb2ac4real = sqrt(b2ac4.real());
                    negbplusrl = -bvalue + sqrb2ac4real;
                    negbminusrl = -bvalue - sqrb2ac4real;
                    twoa = 2 * avalue;
                    plusover2a = negbplusrl / twoa;
                    minusover2a = negbminusrl / twoa;
                    cout << " The Values of X are " << setprecision(4) << plusover2a
                         << " and " << setprecision(4) << minusover2a << endl;
                }
Last edited on
I was capable of finding a way of solving the quadratic problems without using the header <complex>. I would like to thank you anyways for your attempt of helping me with my problem. Also, I was not fully understanding of bool so i did not even attempt to place it into my code. This is what I am now using:

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if (algselection==4)
            {
                cout << "\n+----Quadratic Equation Solution----+\n"
                     << "|    Enter the Coefficient on x^2   |\n"
                     << "                  ";
                double avalue, bvalue, cvalue, b2, ac4, b2ac4, b2ac4topos, bover2a, b2ac4over2a, negbplus, negbminus, sqrb2ac4, twoa, plusover2a, minusover2a;
                cin >> avalue;
                cout << "|     Enter the Coefficient on x    |\n"
                     << "                  ";
                cin >> bvalue;
                cout << "|        Enter the Third Term       |\n"
                     << "                  ";
                cin >> cvalue;
                ac4 = avalue * cvalue * 4;
                b2 = pow(bvalue, 2);
                b2ac4 = b2 - ac4;

                if(b2ac4 >= 0)
                {
                    sqrb2ac4 = sqrt(b2ac4);
                    negbplus = -bvalue + sqrb2ac4;
                    negbminus = -bvalue - sqrb2ac4;
                    twoa = 2 * avalue;
                    plusover2a = negbplus / twoa;
                    minusover2a = negbminus / twoa;
                    cout << "|        The Values of X are        |\n"
                         << "                 " << setprecision(4) << plusover2a
                         << "\n                 " << setprecision(4) << minusover2a
                         << "\n+-----------------------------------+" << endl;
                }

                if(b2ac4 < 0)
                {
                    b2ac4topos = b2ac4 * -1;
                    sqrb2ac4 = sqrt(b2ac4topos);
                    twoa = 2 * avalue;
                    bover2a = -bvalue / twoa;
                    b2ac4over2a = sqrb2ac4 / twoa;
                    cout << "|        The Values of X are        |\n         " << setprecision(4) << bover2a
                         << " +/- " << setprecision(4) << b2ac4over2a << "i\n"
                         << "+-----------------------------------+" << endl;
                }
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