Why does my polynomial method work correctly in small arrays but wrong in bigger ones?

Question

double my_poly(double a[], double x, int degree){
   long int i;
   double result = a[0];
   double xpwr= 1/x;
   for(i=1; i<degree;i+=2){
      xpwr = x * x * xpwr;
      result = result + ((a[i] + (a[i+1] * x)) * xpwr); 
   }    
   return result;
}

The code above yields correct values for small-sized arrays. But when I use an array with a size 1000 the result comes wrong. Please, can you help me?

Edit: I have examined as suggested;

double b[4] = {1,2,4,5};
printf("%f\n", poly(b,2,3));
printf("%f\n", my_poly(b,2,3));

Output:

61.000000
21.000000

According to the @Ian Abbott 's comment I have added this code block before the return statement, to satisfy both even and odd degrees:

for(; i<=degree;i++){
    xpwr = x * x * xpwr;
    result += a[i] * xpwr;   
 }

So the test outputs are consistent.

printf("%f\n", poly(a,2,999));
printf("%f\n", my_poly(a,2,999));

Yields:

61.000000
61.000000

Answer 1

Adding that code block before the return statement solved my problem.

for(; i<=degree;i++){
    xpwr = x * x * xpwr;
    result += a[i] * xpwr;   
 }