C++ STL MAP and multiMAP:

Description, use and examples of C++ STL "pair", "map" and "multimap" associative containers. The STL associative container class is a variable sized container which supports retrieval of an element value given a search key.

STL pair: A container which holds two values. The data types may or may not be different.
STL map: Associative key-value pair held in balanced binary tree structure. Each key is unique.
STL multimap: Same as an STL map except that duplicate keys are allowed.

Also see:

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Example: STL map:

Sparse array example: (why hold space for thousands of elements when all we have is five)

#include <string.h>
#include <iostream>
#include <map>
#include <utility>

using namespace std;

int main()
{
   map<int, string> Employees;

   // 1) Assignment using array index notation
   Employees[5234] = "Mike C.";
   Employees[3374] = "Charlie M.";
   Employees[1923] = "David D.";
   Employees[7582] = "John A.";
   Employees[5328] = "Peter Q.";

   cout << "Employees[3374]=" << Employees[3374] << endl << endl;

   cout << "Map size: " << Employees.size() << endl;

   for( map<int,string>::iterator ii=Employees.begin(); ii!=Employees.end(); ++ii)
   {
       cout << (*ii).first << ": " << (*ii).second << endl;
   }
}

Compile: g++ testMap.cpp
Run: ./a.out

Employees[3374]=Charlie M.

Map size: 5
1923: David D.
3374: Charlie M.
5234: Mike C.
5328: Peter Q.
7582: John A.

Example using a "string" as an array index:

#include <string.h>
#include <iostream>
#include <map>
#include <utility>

using namespace std;

int main()
{
   map<string, int> Employees;

   // Examples of assigning Map container contents

   // 1) Assignment using array index notation
   Employees["Mike C."] = 5234;
   Employees["Charlie M."] = 3374;

   // 2) Assignment using member function insert() and STL pair
   Employees.insert(std::pair<string,int>("David D.",1923));
 
   // 3) Assignment using member function insert() and "value_type()"
   Employees.insert(map<string,int>::value_type("John A.",7582));

   // 4) Assignment using member function insert() and "make_pair()"
   Employees.insert(std::make_pair("Peter Q.",5328));

   cout << "Map size: " << Employees.size() << endl;

   for( map<string, int>::iterator ii=Employees.begin(); ii!=Employees.end(); ++ii)
   {
       cout << (*ii).first << ": " << (*ii).second << endl;
   }
}

Compile: g++ testMap.cpp
Run: ./a.out

Map size: 5
Charlie M.: 3374
David D.: 1923
John A.: 7582
Mike C.: 5234
Peter Q.: 5328

Note: The fully defined STL map defines a comparison operator in the map declaration so that the indexes can be ordered. This is used to provide a default comparison operator for the data type. In the above example, the key type is an integer and the C++ "equals" (=) and "less than" operator (<) can operate on an integer so the example works. The use of the STL algorithm std::less<> can be specified explicitly as in the following declaration:

std::map<int, string, std::less< int > >

If defining your own class as the index (first value), C++ will not know how to perform the comparison, thus you will have to provide an operator to perform this function.

The first element in a map can be a class or even another STL container such as a pair. If using a class, the class must provide an overloaded "equals" (=) and "less than" operator (<).

Thus using "char" instead of "string" requires the use of a comparison function:

#include <string.h>
#include <iostream>
#include <map>
#include <utility>

using namespace std;

struct cmp_str 
{
   bool operator()(char const *a, char const *b) 
   {
      return std::strcmp(a, b) < 0;
   }
};

int main()
{
   map<char *, int, cmp_str> Employees;

   // Examples of assigning Map container contents

   // 1) Assignment using array index notation
   Employees["Mike C."] = 5234;
   Employees["Charlie M."] = 3374;

   // 2) Assignment using member function insert() and STL pair
   Employees.insert(std::pair<char *,int>("David D.",1923));
 
   // 3) Assignment using member function insert() and "value_type()"
   Employees.insert(map<char *,int>::value_type("John A.",7582));

   // 4) Assignment using member function insert() and "make_pair()"
   Employees.insert(std::make_pair((char *)"Peter Q.",5328));

   cout << "Map size: " << Employees.size() << endl;

   for( map<char *, int, cmp_str>::iterator ii=Employees.begin(); ii!=Employees.end(); ++ii)
   {
       cout << (*ii).first << ": " << (*ii).second << endl;
   }
}

Compile: g++ testMap.cpp
Run: ./a.out

Map size: 5
Charlie M.: 3374
David D.: 1923
John A.: 7582
Mike C.: 5234
Peter Q.: 5328

SGI Map Documentation:

Example: STL multimap:

The STL multipmap allows duplicate keys.

 
#include <string.h>
#include <iostream>
#include <map>
#include <utility>

using namespace std;

int main()
{
  // Compare (<) function not required since it is built into string class.
  // else declaration would comparison function in multimap definition.
  // i.e. multimap<string, int, compare> m;

  multimap<string, int> m;

  m.insert(pair<string, int>("a", 1));
  m.insert(pair<string, int>("c", 2));
  m.insert(pair<string, int>("b", 3));
  m.insert(pair<string, int>("b", 4));
  m.insert(pair<string, int>("a", 5));
  m.insert(pair<string, int>("b", 6));

  cout << "Number of elements with key a: " << m.count("a") << endl;
  cout << "Number of elements with key b: " << m.count("b") << endl;
  cout << "Number of elements with key c: " << m.count("c") << endl;

  cout << "Elements in m: " << endl;
  for (multimap<string, int>::iterator it = m.begin();
       it != m.end();
       ++it)
   {
       cout << "  [" << (*it).first << ", " << (*it).second << "]" << endl;
   }

   pair<multimap<string, int>::iterator, multimap<string, int>::iterator> ppp;

   // equal_range(b) returns pair<iterator,iterator> representing the range
   // of element with key b
   ppp = m.equal_range("b");

   // Loop through range of maps of key "b"
   cout << endl << "Range of \"b\" elements:" << endl;
   for (multimap<string, int>::iterator it2 = ppp.first;
       it2 != ppp.second;
       ++it2)
   {
       cout << "  [" << (*it2).first << ", " << (*it2).second << "]" << endl;
   }

// Can't do this (??)
//   cout << ppp.first << endl;
//   cout << ppp.second << endl;

   m.clear();
}

Compile: g++ testMultimap.cpp
Run: ./a.out

Number of elements with key a: 2
Number of elements with key b: 3
Number of elements with key c: 1
Elements in m:
  [a, 1]
  [a, 5]
  [b, 3]
  [b, 4]
  [b, 6]
  [c, 2]

Range of "b" elements:
  [b, 3]
  [b, 4]
  [b, 6]

SGI STL Documentation:

Books:

	The C++ Standard Library: A Tutorial Reference Nicolai M. Josuttis ISBN #0201379260, Addison Wesley Longman This book is the only book I have seen which covers string classes as implemented by current Linux distributions. It also offers a fairly complete coverage of the C++ Standard Template Library (STL). Good reference book.
	STL for C++ programmers Leen Ammeraal ISBN #0 471 97181 2, John Wiley & Sons Ltd. Short book which teaches C++ Standard Template Library (STL) by example. Not as great as a reference but is the best at introducing all the concepts necessary to grasp STL completely and good if you want to learn STL quickly. This book is easy to read and follow.
	Data Structures with C++ Using STL William Ford, Willaim Topp ISBN #0130858501, Prentice Hall
	STL Tutorial and Reference Guide: C++ Programming with the Standard Template Library David R. Musser, Gillmer J. Derge, Atul Saini ISBN #0201379236, Addison-Wesley Publications
	The C++ Templates: The complete guide. David Vandevoorde, Nicolai Josuttis ISBN #0201734842, Addison Wesley Pub Co. Covers complex use of C++ Templates.
	C++ How to Program by Harvey M. Deitel, Paul J. Deitel ISBN #0131857576, Prentice Hall Fifth edition. The first edition of this book (and Professor Sheely at UTA) taught me to program C++. It is complete and covers all the nuances of the C++ language. It also has good code examples. Good for both learning and reference.