Q10056: What is the Probability?

// Run time: 0.010
#include <stdio.h>
double calculate(int n, double p, int index);
int main()
{
   int num;  // 輸入組數
   int n; // 人數
   double prob; // Probability
   int index; // which player

   scanf("%d", &num);
   while (num--)
   {
       scanf("%d%lf%d", &n, &prob, &index);

       if (!prob)
           printf("0.0000\n");
       else
           printf("%.4lf\n", calculate(n, prob, index));
   }

   return 0;
}

double calculate(int n, double p, int index)
{
   int i;
   double arr[n];
   double non_win = 1-p;
   double weight = 1-p;
   double prod;

   arr[0] = p; // initial

   for ( i = 1; i < n; i++)
   {
       prod = non_win*p;  // 前面都沒發生的機率 * 事件發生的機率
       arr[i] = prod;
       non_win = non_win*weight;
   }

   double sum = 0;

   for ( i = 0; i < n; i++)
       sum += arr[i];

   return arr[index-1]/sum;
}