对排序向量进行二进制搜索

Question

template <class T, class C>
size_t idx(const std::vector< T >& elements, const C& val)
{
  if (val<elements[0]||val==elements[0])
    return 0;
  int s=elements.size();
  if (val>elements[s-1])
    return s;
  int min=0;
  int max=s-1;
  int mid;
  while (max >= min){
      mid=(max+min)/2;
      if (val<elements[min])
          return min;
      else if (val>elements[max])
          return max+1;
      else if((val==elements[mid])||((val>elements[mid-1])&&(val<elements[mid])))
          return mid;
      else if((val>elements[mid])&&(val<elements[mid+1]))
          return mid+1;
      else if (val>elements[mid+1])
          min=mid+1;
      else 
            max=mid-1;
    }
    return max;
}

This is related to btree . Element vector is sorted. I want to find the insertion point on this vector, if the value happens to be in the vector, return the index of it. Also, there is, for example, 8 insertion point on the vector of size: 7. I keep getting a segfault and I think this might be an: off by one error or similar, can some one help? Thanks.

Answer 1

If vector size is 2, then min == mid == 0 and max == 1 . And in the code you check element at mid - 1 , that is -1 , so your program exhibits undefined behaviour.