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Hash Multimap

Hash multimap is a hash map that can contain multiple for a same key
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Key Facts

Gyroscopic Couple: The rate of change of angular momentum (\tau) = I\omega\Omega (In the limit).
  • I = Moment of Inertia.
  • \omega = Angular velocity
  • \Omega = Angular velocity of precession.


Blaise Pascal (1623-1662) was a French mathematician, physicist, inventor, writer and Catholic philosopher.

Definition

Hash_multimap template class is defined in the header <hash_map>, and in the backward-compatibility header <hash_map.h>. This class is an SGI extension; it is not part of the C++ standard.

Interface

#include <hash_map>
namespace std{
    template < class Key, class Type,
               class Traits = hash_compare<Key, less<Key> >, 
               class Allocator = allocator<pair <const Key, Type> > >
    class hash_multimap;
}

Parameters

Parameter Description
Key The element data type to be stored in the hash_multimap
Type The element data type to be stored in the hash_multimap
Traits The type that includes two function objects, one of class Traits that is a binary predicate able to compare two element values as sort keys to determine their relative order and a hash function that is a unary predicate mapping key values of the elements to unsigned integers of type size_t. This argument is optional, and the hash_compare<Key, less<Key> > is the default value
Allocator The type that represents the stored allocator object that encapsulates details about the hash_multimap's allocation and de-allocation of memory. This argument is optional, and the default value is allocator<pair <const Key, Type> >

Description

Hash_multimap is an extension of the Standard Template Library and is used for the storage and fast retrieval of data from a collection in which each element is a pair that has a sort key whose value need NOT be unique and an associated data value.

Hash_multimap is a Hashed Associative Container because its elements are grouped into buckets based on the value of a hash function applied to the key values of the elements and is a Pair Associative Container, because its element data values are distinct from its key values.

Hash_multimap is also an Multiple Associative Container meaning that its elements do NOT need to have a unique keys, so that one key value may have many element data values associated with it and is reversible, because it provides a bidirectional iterator to access its elements.

Examples of declaring a hash_multimap:
// Create an empty hash_multimap hmp0 of key type integer
hash_multimap <int, int> hmp0;
 
// Create an empty hash_multimap hmp1 with the key comparison function of less than
hash_multimap <int, int, hash_compare <int, less<int> > > hmp1;

Performance

The main advantage of hashing over sorting is greater efficiency; a successful hashing performs insertions, deletions, and finds in constant average time as compared with a time proportional to the logarithm of the number of elements in the container for sorting techniques.

The value of an element in a hash_multimap, but not its associated key value, may be changed directly. Instead, key values associated with old elements must be deleted and new key values associated with new elements inserted.

A collision occurs when distinct key values are mapped into the same hashed value.
Hashed associative containers are optimized for the operations of lookup, insertion and removal. The member functions that explicitly support these operations are efficient when used with a well-designed hash function, performing them in a time that is on average constant and not dependent on the number of elements in the container. A well-designed hash function produces a uniform distribution of hashed values and minimizes the number of collisions. In the worst case, with the worst possible hash function, the number of operations is proportional to the number of elements in the sequence (linear time).

Hash Multimap Header Members

Operators
Operator Description
operator!= Tests if the hash_multimap object on the left side of the operator is not equal to the hash_multimap object on the right side
operator< Tests if the hash_multimap object on the left side of the operator is less than the hash_multimap object on the right side
operator<= Tests if the hash_multimap object on the left side of the operator is less than or equal to the hash_multimap or object on the right side
operator== Tests if the hash_multimap object on the left side of the operator is equal to the hash_multimap object on the right side
operator> Tests if the hash_multimap object on the left side of the operator is greater than the hash_multimap object on the right side
operator>= Tests if the hash_multimap object on the left side of the operator is greater than or equal to the hash_multimap object on the right side
operator[ ] Inserts an element into a hash_multimap with a specified key value

Classes

Class Description
hash_compare Describes an object that can be used by any of the hash associative containers: hash_map, hash_multimap, hash_set, or hash_multiset, as a default Traits parameter object to order and hash the elements they contain
value_compare Provides a function object that can compare the elements of a hash_multimap by comparing the values of their keys to determine their relative order in the hash_multimap
hash_multimap Used for the storage and fast retrieval of data from a collection in which each element is a pair that has a sort key whose value need NOT be unique and an associated data value

Hash Multimap Template Class Members

Typedefs
Typedef Description
allocator_type A type that represents the allocator class for the hash_multimap object
const_iterator A type that provides a bidirectional iterator that can read a const element in the hash_multimap
const_pointer A type that provides a pointer to a const element in a hash_multimap
const_reference A type that provides a reference to a const element stored in a hash_multimap for reading and performing const operations
const_reverse_iterator A type that provides a bidirectional iterator that can read any const element in the hash_multimap
difference_type A signed integer type that can be used to represent the number of elements of a hash_multimap in a range between elements pointed to by iterators
iterator A type that provides a bidirectional iterator that can read or modify any element in a hash_multimap
key_compare A type that provides a function object that can compare two sort keys to determine the relative order of two elements in the hash_multimap
key_type A type that describes the sort key object that constitutes each element of the hash_multimap
mapped_type A type that represents the data type stored in a hash_multimap
pointer A type that provides a pointer to an element in a hash_multimap
reference A type that provides a reference to an element stored in a hash_multimap
reverse_iterator A type that provides a bidirectional iterator that can read or modify an element in a reversed hash_multimap
size_type An unsigned integer type that can represent the number of elements in a hash_multimap
value_type A type that provides a function object that can compare two elements as sort keys to determine their relative order in the hash_multimap

Member Functions
Function Description
begin() Returns an iterator addressing the first element in the hash_multimap
rbegin() Returns an iterator addressing the first element in a reversed hash_multimap
clear() Erases all the elements of a hash_multimap
count() Returns the number of elements in a hash_multimap whose key matches a parameter-specified key
empty() Tests if a hash_multimap is empty
end() Returns an iterator that addresses the location succeeding the last element in a hash_multimap
rend() Returns an iterator that addresses the location succeeding the last element in a reversed hash_multimap
equal_range() Returns an iterator that addresses the location succeeding the last element in a hash_multimap
erase() Removes an element or a range of elements in a hash_multimap from specified positions
find() Returns an iterator addressing the location of an element in a hash_multimap that has a key equivalent to a specified key
get_allocator() Returns a copy of the allocator object used to construct the hash_multimap
hash_multimap() Hash_multimap constructor, constructs a list of a specific size or with elements of a specific value or with a specific allocator or as a copy of some other hash_multimap
insert() Inserts an element or a range of elements into the hash_multimap at a specified position
key_comp() Retrieves a copy of the comparison object used to order keys in a hash_multimap
lower_bound() Returns an iterator to the first element in a hash_multimap that with a key value that is equal to or greater than that of a specified key
upper_bound() Returns an iterator to the first element in a hash_multimap that with a key value that is greater than that of a specified key
size() Specifies a new size for a hash_multimap
max_size() Returns the maximum length of the hash_multimap
swap() Exchanges the elements of two hash_multimaps
value_comp() Retrieves a copy of the comparison object used to order element values in a hash_multimap
Example:

Example - hash_multimap methods
Problem
This program illustrates a hash_multimap including a default constructor and the insert(), size(), clear() member functions of the STL hash_multimap interface.
Workings
#define _DEFINE_DEPRECATED_HASH_CLASSES 0
#include <hash_map>
#include <iostream>
 
using namespace std;
using namespace stdext;
 
int main()
{
  typedef pair<int, int> Int_Pair;
  hash_multimap<int, int> hm1;
  hash_multimap<int, int>::size_type i;
 
  hm1.insert(Int_Pair(1, 1));
  hm1.insert(Int_Pair(2, 4));
 
  i = hm1.size();
  cout <<"The size of the hash_multimap is initially "<<i<<"."<<endl;
 
  hm1.clear();
  i = hm1.size();
  cout <<"The size of the hash_multimap after clearing is "<<i<<"."<<endl;
 
  return 0;
}
Solution
Output:

The size of the hash_multimap is initially 2.
The size of the hash_multimap after clearing is 0.