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HashTable .....

Posted on 2003-12-05
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Last Modified: 2013-12-14
Hi C++ Experts,

     I am trying to understand a HashTable class template and its implementation. But there are several places I couldn't figure out why. I would appreciate some helps.

Here is the class template of a  HashTable :
(from Data Structure with C++, Schaum's)
--------------------------------------
template <class K, class T, class H=Hash<K> >

class HashTable
{ protected :
    typedef pair<K,T> Pair ;
    typedef vector<Pair> Vector ;
    typedef Vector::iterator VIt ;
    typedef Vector::const_iterator CVIt ;
    static const float LOAD ; // line 01, Q1
    static const CAP = 109 ; // line 02, Q2

  public :
    HashTable(int cap=CAP, const H& h=H() ) : _(cap), _hash(h) {}
    T& operator[](K) ;
    bool insert(K,T) ;
    bool remove(K) ;
    int size() const ;
    int capacity() const { return _.size() ;}
    void dump const ;

  protected :
    Vector _ ;
    H _hash ;
    int hash(K) ;
    void rebuild() ;
} ;

template <class K, class T, class H>
const float HashTable<K,T,H>::LOAD = 0.75 ;  // line 03, Q1

template <class K, class T, class H>
T& HashTable<K,T,H>::operator[](K key)
{   const K ZERO_K = K() ;
    int k0 = hash(key), k = k0 ;
    while (_[k].first != key && _[k].first != ZERO_K)
      k = (k+1)% _.size() ;

    if( _[k].first == key) return _[k].second ;
    if( size()+1 < LOAD* _.size())        // line 04, Q1
    {  _[k] = Pair(key, T()) ;
       return _[k].second ;
    }
    else
    {  rebuild() ;
       return operator[](key) ;
    }
}

template <class K, class T, class H>
int HashTable<K,T,H>::size() const
{   int n = 0 ;
    const K ZERO_K = K() ;
    for(int i = 0 ; i<_.size(); i++)
      if (_[i].first != ZERO_K) ++n ;
    return n ;
}

template <class K>
struct Hash
{
   int operator()(K s)
   { int h = 0 ;
     for (int i = 0; i< s.length(); i++)
       h += s[i] ;
     return h ;
   }
} ;
blah ... blah ... blah ...
------------------------------------------
Here are my questions :
Q1 : line 01, 03, 04 : I have trouble understand the variable LOAD. I don't really know why we need it and what does it do ?

Q2 : line 02 : I thought a static variable of a class can NOT be defined inside the class ? It has to be defined outside the class like LOAD in line 03. Or did I miss understand anything ?

Q3 : Is there a built in HashTable class Template in STL ? or there is only <map> in STL ??

Thanks !!!

meow.
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Question by:meow00
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2 Comments
 
LVL 11

Accepted Solution

by:
bcladd earned 120 total points
ID: 9884898
(1) Load is a load factor for resizing the hash table. Hashes work best when they have some slack (empty space) so it looks like rebuild() expands the hash table when it is overloaded.

(2) static const members can be defined inside the class definition. Non-const cannot.

(3) hashmap is supposed to be in the next STL and is an extension in many compilers (and at Boost, too www.boost.org).

Hope this helps, -bcl
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LVL 30

Assisted Solution

by:Axter
Axter earned 30 total points
ID: 9888047
>>(3) hashmap is supposed to be in the next STL and is an extension in many compilers (and at Boost, too www.boost.org).

When the C++ standard adopts the new hash class, it will not be called hashmap.  They have decided to give them new names so as to avoid conflicts with existing libraries.
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