在Stl中查找键Hash_map

Question

I am beginner in c++ and have some problem with hash table. I need a Hash table structure for my program. first I use boost unordered_map. it have all things that I need, but it make my program so slow. then I want to test stl hash_map, but I can't do all thing that I need. this is my first code ( this is sample)

#include <hash_map>
using namespace std;

struct eqstr
{
  bool operator()(int s1, int s2) const
  {
    return s1==s2;
  }
};
typedef stdext::hash_map< int, int, stdext::hash_compare< int, eqstr > > HashTable;

int main()
{
  HashTable a;
  a.insert( std::pair<int,int>( 1, 1 ) );
  a.insert( std::pair<int,int>( 2, 2 ) );
  a.insert( std::pair<int,int>( 4, 4 ) );
//next i want to change value of key 2 to 20
  a[2] =  20;
//this code only insert pair<2,20> into a, buy when I use boost unordered_map this code                         modify previous key of 2  
//next I try this code for delete 2 and insert new one
  a.erase(2);//this code does work nothing !!!
//next I try to find 2 and delete it
  HashTable::iterator i;
  i = a.find(2);//this code return end, and does not work!!!
  a.erase(i);//cause error
//but when I write this code, it works!!!
  i=a.begin();
  a.erase(i);
//and finally i write this code
  for (i = a.begin(); i!=a.end(); ++i)
  {
    if (i->first == 2 )
      break;
  }
  if (i!= a.end())
    a.erase(i);
//and this code work 

but if i want to search over my data, i use array not hash_map, why I can't access, modity and delete from hash_map with o(1) what is my mistake, and which hash structure is fast for my program with many value modification in initializing phase. is google sparse_hash suitable for me, if it is, can give me some tutorial on it. thanks for any help

Answer 1

You may look at: http://msdn.microsoft.com/en-us/library/525kffzd(VS.71).aspx

I think stdext::hash_compare< int, eqstr > is causing the problems here. Try to erase it.

Another implementation of a hash map is std::tr1::unordered_map . But I think that performance of various hash map implementation would be similar. Could you elaborate more about how slow the boost::unordered_map was? How did you use it? What for?

Answer 2

There are so many different varieties of hash map and many developers still write their own because you can so much more often get a higher performance in your own one, written to your own specific use, than you can from a generic one, and people tend to use hash when they want a really high performance.

The first thing you need to consider is what you really need to do and how much performance you really need, then determine whether something ready-made can meet that or whether you need to write something of your own.

If you never delete elements, for example, but just write once then constantly look-up, then you can often rehash to reduce collisions for the actual set you obtain: longer at setup time but faster at lookup.

An issue in writing your own will occur if you delete elements because it is not enough to "null" the entry, as another one may have stepped over yours as part of its collision course and now if you look that one up it will give up as "not found" as soon as it hits your null.