#### Getting NaN value when raising double by a fractional exponent of 1/3

I’m new to C++ so if there is a quick solution to this question please let me know in the comments.

I’m working on a third-degree polynomial equation solver application, and for that I need to divide a certain double value by a fractional exponent, which in this case is `1/3`.

Here is the code so far:

``````#include <iostream>
#include <valarray>
#include <vector>
#include <iomanip>

using namespace std;

void solveEquation(double a, double b, double c, double d);

int main() {
double a, b, c, d;

cout << "Input a value for 'a': " << endl;
cin >> a;

cout << "Input a value for 'b': " << endl;
cin >> b;

cout << "Input a value for 'c': " << endl;
cin >> c;

cout << "Input a value for 'd': " << endl;
cin >> d;

solveEquation(a, b, c, d);
return 0;
}

void solveEquation(double a, double b, double c, double d) {
vector<double> frac_vector1{((pow(-b, 3)) / (27 * pow(a, 3))),
((b * c) / (6 * pow(a, 2))),
-(d / (2 * a))};
double frac_vector1_result = 0;
for (double frac : frac_vector1) {
frac_vector1_result += frac;
}

double frac_vector1_result_pow = pow(frac_vector1_result, 2);

vector<double> frac_vector2 {(c/(3 * a)),
-((pow(b, 2))/(9 * pow(2, a)))};

double frac_vector2_result = 0;
for (double frac : frac_vector2) {
frac_vector2_result += frac;
}

double frac_vector2_result_pow3 = pow(frac_vector2_result, 3);

double first_half = (frac_vector1_result + sqrt(frac_vector1_result_pow + frac_vector2_result_pow3));

cout << pow(first_half, 1.0/3.0); //isNan ?
}
``````

When I input `a = -0.71, b = -0.3, c = 2.2, and d = -1.46` I get a NaN value, which is not what I get in a calculator.

When I debug this I get the following value for `first_half`: When I raise this value to `1/3` in Desmos I get `-0.853432944643`.

It’s not like I am square rooting a negative number (it’s `1/3` not `1/2`) so why am I getting this problem?

Cheers,

Tom

Source: Windows Questions C++