|< C++ STL Container 8 | Main | C++ STL Container 10 >| Site Index | Download |

MODULE 29a

THE C++ STL CONTAINER PART 9

map, multimap, hash_map, hash_multimap, hash_set, hash_multiset

My Training Period: hours

Continue from the previous Module, compiled using VC++7.0 / .Net, win32 empty console mode application. Be careful with the source codes than span more than one line. Linux g++ compilation examples are given at the end of this Module. Source code is available in C++ STL Container source code.

C++ STL containers abilities that supposed to be acquired:

▪ Able to understand and use hash_map associative container.

▪ Able to understand and use hash_multimap associative container.

▪ Able to understand and use hash_set associative container.

What do we have in this page?

Hash Tables
hash_map
<hash_map> Header Members – operators, specialized template functions and classes
hash_map Template Class Members – typedefs, class member functions, class operators
hash_map Class
hash_map Constructor
hash_map, constructor program example
hash_multimap Members – typedefs & member functions
hash_multimap Class
hash_multimap Constructor
hash_multimap, constructor program example

29.5 Hash Tables

The hash table is a data structure for collections but it is not part of the C++ standard library. It is implementation dependant. Libraries typically provide four kinds of hash tables that are hash_map, hash_multimap, hash_set, and hash_multiset.

29.5.1 hash_map

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_map, 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.
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 good designed hash function produces a uniform distribution of hashed values and minimizes the number of collisions, where a collision is said to occur when distinct key values are mapped into the same hashed value. 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).
This type of structure is an ordered list of uniquely occurring keywords with associated string values. If, instead, the words had more than one correct definition, so that keys were not unique, then a hash_multimap would be the container of choice.
If, on the other hand, just the list of words were being stored, then a hash_set would be the correct container. If multiple occurrences of the words were allowed, then a hash_multiset would be the appropriate container structure.
The hash_map orders the sequence it controls by calling a stored hash Traits object of class value_compare. This stored object may be accessed by calling the member function key_comp(). Such a function object must behave the same as an object of class hash_compare<Key, less<Key> >. Specifically, for all values _Key of type Key, the call Traits(_Key) yields a distribution of values of type size_t.
The iterator provided by the hash_map class is a bidirectional iterator.

<hash_map> Header Members

Operators

Operator	Description
operator!=	Tests if the hash_map or hash_multimap object on the left side of the operator is not equal to the hash_map or hash_multimap object on the right side.
operator<	Tests if the hash_map or hash_multimap object on the left side of the operator is less than the hash_map or hash_multimap object on the right side.
operator<=	Tests if the hash_map or hash_multimap object on the left side of the operator is less than or equal to the hash_map or hash_multimap object on the right side.
operator==	Tests if the hash_map or hash_multimap object on the left side of the operator is equal to the hash_map or hash_multimap object on the right side.
operator>	Tests if the hash_map or hash_multimap object on the left side of the operator is greater than the hash_map or hash_multimap object on the right side.
operator>=	Tests if the hash_map or hash_multimap object on the left side of the operator is greater than or equal to the hash_map or hash_multimap object on the right side.
Table 29.12

Specialized Template Functions

Specialized template function

Description

swap()

Exchanges the elements of two hash_maps or hash_multimaps.

Table 29.13

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_map by comparing the values of their keys to determine their relative order in the hash_map.
hash_map	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 is unique and an associated data value.
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.
Table 29.14

hash_map Template Class Members

Typedefs

Typedef	Description
allocator_type	A type that represents the allocator class for the hash_map object.
const_iterator	A type that provides a bidirectional iterator that can read a const element in the hash_map.
const_pointer	A type that provides a pointer to a const element in a hash_map.
const_reference	A type that provides a reference to a const element stored in a hash_map 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_map.
difference_type	A signed integer type that can be used to represent the number of elements of a hash_map 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_map.
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_map.
key_type	A type describes the sort key object that constitutes each element of the hash_map.
mapped_type	A type that represents the data type stored in a hash_map.
pointer	A type that provides a pointer to an element in a hash_map.
reference	A type that provides a reference to an element stored in a hash_map.
reverse_iterator	A type that provides a bidirectional iterator that can read or modify an element in a reversed hash_map.
size_type	An unsigned integer type that can represent the number of elements in a hash_map.
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_map.
table 29.15

hash_map Template Class Member Functions

Member function	Description
begin()	Returns an iterator addressing the first element in the hash_map.
clear()	Erases all the elements of a hash_map.
count()	Returns the number of elements in a hash_map whose key matches a parameter-specified key.
empty()	Tests if a hash_map is empty.
end()	Returns an iterator that addresses the location succeeding the last element in a hash_map.
equal_range()	Returns a pair of iterators, respectively, to the first element in a hash_map with a key that is greater than a specified key and to the first element in the hash_map with a key that is equal to or greater than the key.
erase()	Removes an element or a range of elements in a hash_map from specified positions
find()	Returns an iterator addressing the location of an element in a hash_map that has a key equivalent to a specified key.
get_allocator()	Returns a copy of the allocator object used to construct the hash_map.
hash_map()	Constructs a hash_map that is empty or that is a copy of all or part of some other hash_map.
insert()	Inserts an element or a range of elements into a hash_map.
key_comp()	Returns an iterator to the first element in a hash_map with a key value that is equal to or greater than that of a specified key.
lower_bound()	Returns an iterator to the first element in a hash_map with a key value that is equal to or greater than that of a specified key.
max_size()	Returns the maximum length of the hash_map.
rbegin()	Returns an iterator addressing the first element in a reversed hash_map.
rend()	Returns an iterator that addresses the location succeeding the last element in a reversed hash_map.
size()	Specifies a new size for a hash_map.
swap()	Exchanges the elements of two hash_maps.
upper_bound()	Returns an iterator to the first element in a hash_map that with a key value that is greater than that of a specified key.
value_comp()	Retrieves a copy of the comparison object used to order element values in a hash_map.
Table 29.16

hash_map Template Class Operator

Operator

Description

operator[ ]

Inserts an element into a hash_map with a specified key value.

Table 29.17

hash_map Class

Stores and retrieves data quickly from a collection in which each element is a pair that has a sort key whose value is unique and an associated data value.

template < class Key, class Type, class Traits=hash_compare<Key, less<Key> >, class Allocator=allocator<pair <const Key, Type> > >

Parameters

Parameter	Description
Key	The element data type to be stored in the hash_map.
Type	The element data type to be stored in the hash_map.
Traits	The type which includes two function objects, one of class compare 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 hash_compare<Key, less<Key> > is the default value.
Allocator	The type that represents the stored allocator object that encapsulates details about the hash_map's allocation and de-allocation of memory. This argument is optional, and the default value is allocator<pair <const Key, Type> >.
Table 29.18

The hash_map is:

An associative container, which a variable size container that supports the efficient retrieval of element values based on an associated key value.

Reversible, because it provides a bidirectional iterator to access its elements.

Hashed, because its elements are grouped into buckets based on the value of a hash function applied to the key values of the elements.

Unique in the sense that each of its elements must have a unique key.

A pair associative container, because its element data values are distinct from its key values.

A template class, because the functionality it provides is generic and so independent of the specific type of data contained as elements or keys. The data types to be used for elements and keys are, instead, specified as parameters in the class template along with the comparison function and allocator.

hash_map Constructor

Constructs a hash_map that is empty or that is a copy of all or part of some other hash_map.
All constructors store a type of allocator object that manages memory storage for the hash_map and that can later be returned by calling get_allocator(). The allocator parameter is often omitted in the class declarations and preprocessing macros used to substitute alternative allocators.
All constructors initialize their hash_map.
All constructors store a function object of type Traits that is used to establish an order among the keys of the hash_map and that can later be returned by calling key_comp.
The first three constructors specify an empty initial hash_map, the second, in addition, specifying the type of comparison function (_Comp) to be used in establishing the order of the elements and the third explicitly specifying the allocator type (_Al) to be used.
The keyword explicit suppresses certain kinds of automatic type conversion.
The fourth constructor specifies a copy of the hash_map _Right.
The last three constructors copy the range [_First, _Last) of a hash_map with increasing explicitness in specifying the type of comparison function of class Traits and allocator.

// hash_map, constructor, compiled with visual C++ 7.0 or VC.Net, with some warnings

#include <hash_map>

#include <iostream>

using namespace std;

int main()

{

    typedef pair <int, int> Int_Pair;

    hash_map <int, int>::iterator hmp0_Iter, hmp1_Iter, hmp3_Iter, hmp4_Iter, hmp5_Iter, hmp6_Iter;

    hash_map <int, int, hash_compare<int, greater<int> > >::iterator hmp2_Iter;

    // create an empty hash_map hmp0 of key type integer

    hash_map <int, int> hmp0;

    // create an empty hash_map hmp1 with the key comparison function of less than, then insert 4 elements

    hash_map <int, int, hash_compare <int, less<int> > > hmp1;

    hmp1.insert(Int_Pair(1, 13));

    hmp1.insert(Int_Pair(3, 51));

    hmp1.insert(Int_Pair(7, 22));

    hmp1.insert(Int_Pair(2, 31));

    // create an empty hash_map hmp2 with the key comparison function of greater than, then insert 4 elements no duplicate key...

    hash_map <int, int, hash_compare <int, greater<int> > > hmp2;

    hmp2.insert(Int_Pair(1, 17));

    hmp2.insert(Int_Pair(2, 20));

    hmp2.insert(Int_Pair(4, 13));

    hmp2.insert(Int_Pair(3, 34));

    // create a hash_map hmp3 with the hash_map hmp1 allocator notice the duplicate key...

    hash_map <int, int>::allocator_type hmp1_Alloc;

    hmp1_Alloc = hmp1.get_allocator();

    hash_map <int, int> hmp3(less<int>(), hmp1_Alloc);

    hmp3.insert(Int_Pair(2, 17));

    hmp3.insert(Int_Pair(1, 12));

    hmp3.insert(Int_Pair(2, 15));

    hmp3.insert(Int_Pair(1, 22));

    // create a hash_map hm5 by copying the range hm1[_First, _Last)

    hash_map <int, int>::const_iterator hmp1_PIter, hmp1_QIter;

    hmp1_PIter = hmp1.begin( );

    hmp1_QIter = hmp1.begin( );

    hmp1_QIter++;

    hmp1_QIter++;

    hash_map <int, int> hmp5(hmp1_PIter, hmp1_QIter);

    // create a hash_map hm6 by copying the range hm2[_First, _Last) and with the allocator of hash_map hm2

    hash_map <int, int>::allocator_type hmp2_Alloc;

    hmp2_Alloc = hmp2.get_allocator();

    hash_map <int, int> hmp6(hmp2.begin(), ++hmp2.begin(), less<int>(), hmp2_Alloc);

    // ---------------------------------------------------------------

    cout<<"Operation: hash_map <int, int> hmp0\n";

    cout<<"hmp0 data: ";

    for(hmp0_Iter = hmp0.begin(); hmp0_Iter != hmp0.end(); hmp0_Iter++)

        cout<<hmp0_Iter->second<<" ";

    cout<<endl;

    cout<<"\nOperation1: hash_map<int, int, \nhash_compare<int, less<int> > > hmp1\n";

    cout<<"Operation2: hmp1.insert(Int_Pair(1, 13))...\n";

    cout<<"hmp1 data: ";

    for(hmp1_Iter = hmp1.begin(); hmp1_Iter != hmp1.end(); hmp1_Iter++)

        cout<<hmp1_Iter->second<<" ";

    cout<<endl;

    cout<<"\nOperation1: hash_map<int, int, \nhash_compare<int, greater<int> > > hmp2\n";

    cout<<"Operation2: hmp2.insert(Int_Pair(1, 17))...\n";

    cout<<"hmp2 data: ";

    for(hmp2_Iter = hmp2.begin(); hmp2_Iter != hmp2.end(); hmp2_Iter++)

        cout<<hmp2_Iter->second<<" ";

    cout<<endl;

    cout<<"\nOperation1: hash_map<int, int> hmp3(less<int>(), hmp1_Alloc)\n";

    cout<<"Operation2: hmp3.insert(Int_Pair(2, 17))...\n";

    cout<<"hmp3 data: ";

    for(hmp3_Iter = hmp3.begin(); hmp3_Iter != hmp3.end(); hmp3_Iter++)

        cout<<hmp3_Iter->second<<" ";

    cout<<endl;

    cout<<"\nOperation: hash_map<int, int> hmp5(hmp1_PIter, hmp1_QIter)\n";

    cout<<"hmp5 data: ";

    for(hmp5_Iter = hmp5.begin(); hmp5_Iter != hmp5.end(); hmp5_Iter++)

        cout<<hmp5_Iter->second<<" ";

    cout<<endl;

    cout<<"\nOperation: hash_map<int, int> hmp6(hmp2.begin(), \n++hmp2.begin(), less<int>(), hmp2_Alloc);\n";

    cout<<"hmp6 data: ";

    for(hmp6_Iter = hmp6.begin(); hmp6_Iter != hmp6.end(); hmp6_Iter++)

        cout<<hmp6_Iter->second <<" ";

    cout<<endl;

    return 0;

}

Output:

---------------------------------------------------------------------------------------------------------------------

C++ STL Associative container hash_map constructor

---------------------------------------------End of hash_map---------------------------------------

---www.tenouk.com---

29.5.2 hash_multimap 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.
Table 29.19

Member Functions

Member function	Description
begin()	Returns an iterator addressing the first element in the 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.
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.
max_size()	Returns the maximum length of the hash_multimap.
rbegin()

|< C++ STL Container 8 | Main | C++ STL Container 10 >| Site Index | Download |

MODULE 29a

THE C++ STL CONTAINER PART 9

map, multimap, hash_map, hash_multimap, hash_set, hash_multiset

My Training Period: hours

C++ STL containers abilities that supposed to be acquired:

▪ Able to understand and use hash_map associative container.

▪ Able to understand and use hash_multimap associative container.

▪ Able to understand and use hash_set associative container.

What do we have in this page?

Hash Tables

hash_map

<hash_map> Header Members – operators, specialized template functions and classes

hash_map Template Class Members – typedefs, class member functions, class operators

hash_map Class

hash_map Constructor

hash_map, constructor program example

hash_multimap Members – typedefs & member functions

hash_multimap Class

hash_multimap Constructor

hash_multimap, constructor program example

29.5 Hash Tables

29.5.1 hash_map

<hash_map> Header Members

Operators

Specialized Template Functions

Classes

hash_map Template Class Members

Typedefs

hash_map Template Class Member Functions

hash_map Template Class Operator

hash_map Class

Parameters

hash_map Constructor

Output:

29.5.2 hash_multimap Members

Typedefs

Member Functions