Function doesn't accept first return value, jumps back, calculates another time and returns a “false” value [C++]

Question

When I call calc(), he doesnt give me the first return value for x, intstead he jumps back to double x = v1*sqrt((-(2.0*log(s)))/s); and calculates the x for a second time and returns nan.

The debugger shows me, before he tries to return x for the first time, that the value of x is -0.525209. I thought that would be an acceptable return value (double).

Does anyone know how to solve this or why he jumps back? (sorry I'm still a beginner at c++)

#include <cmath>
#include <cstdlib>
#include <iostream>    

    double calc(){
    double u1 = static_cast <double> (rand()) / static_cast <double> (RAND_MAX);
    double u2 = static_cast <double> (rand()) / static_cast <double> (RAND_MAX);

    double v1 = (2.0*u1)-1.0;
    double v2 = (2.0*u2)-1.0;

    double s = pow(v1, 2.0)+pow(v2, 2.0);

    if (s >= 1.0){
        calc();
    }

    double x = v1*sqrt((-(2.0*log(s)))/s);

    std::cout << "before return " << x << std::endl;

    return x;
}

int main(){
   double hilfe = calc();

   std::cout << "my mumber "<<  hilfe << std::endl;
}

Answer 1

I guess you want this:

#include <cmath>
#include <cstdlib>
#include <iostream>    

double calc(){
    for (;;) {
    double u1 = static_cast <double> (rand()) / static_cast <double> (RAND_MAX);
    double u2 = static_cast <double> (rand()) / static_cast <double> (RAND_MAX);

    double v1 = (2.0*u1)-1.0;
    double v2 = (2.0*u2)-1.0;

    double s = pow(v1, 2.0)+pow(v2, 2.0);

    if (s >= 1.0)
        continue;

    double x = v1*sqrt((-(2.0*log(s)))/s);

    std::cout << "before return " << x << std::endl;

    return x;  
    }
}

int main(){
   double hilfe = calc();

   std::cout << "my mumber "<<  hilfe << std::endl;
}

This picks new random values until s < 1;