binary search.

/*binsearch: find x in v[0] <= v[1] ....<= v[n -1] */
int binsearch(int x, int v[],  int n)
{
   int low, high, mid;

   low = 0;
   high = n -1;
   while (low <= high){
     mid = (low + high) / 2;
     if (x < v[mid])
          high = mid - 1;
     else if (x > v[mid])
          low = mid + 1;
     else
         return mid;   /*found match */
   }
   return -1;   /* no match */
}