Classes arrays of objects value "nan" - c++

Question

I have a problem when I execute the program,the results that I have got is "nan" for the values of vector.I do not exactly where is the mistake. the method distancias generates a correct value but the method variogram does not generate the expected value instead of that generates a value "nan".Sorry for my english.
#include <iostream>
#include <math.h>
//this program is to calculate the kriging puntual
using namespace std;
////
class Points{
private:
float x;
float y;
public:
Points(float a,float b);
Points();
float distancia(float x_1,float y_1);
float variogram(float h);
float valor_1();
float valor_2();
void show(void);
};
Points::Points(){
}
Points::Points(float a,float b){
x=a;
y=b;
}
float Points::distancia(float x_1,float y_1){
float d;
d=pow(pow((x-x_1),2)+pow((y-y_1),2),0.5);
return d;
}
float Points::variogram(float h){
float v,c_0,c_1,a_1;
v=c_0+c_1*(1.5*(h/a_1)-0.5*pow((h/a_1),3));
return v;
}
void Points::show(void){
printf("%.2f,%.2f\n",x,y);
}
float Points::valor_1(){
return x;
}
float Points::valor_2(){
return y;
}
///////
int main(int argc, char** argv) {
float a_1,c_0,c_1; //parameters of variogram
float c,d; // position of point to determinate
float a,b; //positions of all points except the point to determinate
int i=0,n;
int j,k;
Points final; //point to determinate
//this part is to enter the values of function sphere variogram
printf("Enter the paramters of sphere variogram\n");
printf("Enter the value of c_0: ");
scanf("%f",&c_0);
printf("Enter the value of c_1: ");
scanf("%f",&c_1);
printf("Enter the value of a: ");
scanf("%f",&a_1);
//determinating the postion of final point
printf("Enter the position of the point to determinate: ");
scanf("%f,%f",&c,&d);
final=Points(c,d);
final.show();
printf("Enter the name of points for the krigeage: ");
scanf("%i",&n);
Points punto[n];
float vector[n];
do{
printf("Enter the position x,y of the point %i: ",i+1);
scanf("%f,%f",&a,&b);
punto[i]=Points(a,b);
punto[i].show();
vector[i]=punto[i].variogram(punto[i].distancia(c,d));
cout<<vector[i]<<endl;
i=i+1;
}while(i<n);
return 0;
}

Answer 1

The problem is in the following function:
float Points::variogram(float h){
float v,c_0,c_1,a_1;
v=c_0+c_1*(1.5*(h/a_1)-0.5*pow((h/a_1),3));
return v;
}
Here, you are declaring the local variables v, c_0, c_1 and a_1. These values are not initialized, they can contain anything. However, it is not unlikely they actually are equal to zero. So when you calculate h/a_1, the result of that is probably plus or minus infinity. When you subtract two infinities with the same sign from each other, the result will be NaN.
What you should do is pass the values for c_0, c_1 and a_1 from main() to the function:
float Points::variogram(float h, float c_0, float c_1, float a_1){
return c_0+c_1*(1.5*(h/a_1)-0.5*pow((h/a_1),3));
}
...
vector[i]=punto[i].variogram(punto[i].distancia(c,d), c_0, c_1, a_1);
Please compile your code with warnings enabled (if you use GCC, then use the -Wall command line option). Your compiler should then warn you about these uninitialized variables.