pair map multimap说明

川长思鸟来 2022-08-06 05:16 158阅读 0赞

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:

*  [map][]

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

### Using STL MAP to store class objects: ###

This example uses a string as the MAP index and an object as the value pair. The overloaded = and < operators are required to satisfy MAP container use.

#include <iostream>
    #include <map>
    using namespace std;
    
    class AAA
    {
       friend ostream &operator<<(ostream &, const AAA &);
    
       public:
          int x;
          int y;
          float z;
    
          AAA();
          AAA(const AAA &);
          ~AAA(){};
          AAA &operator=(const AAA &rhs);
          int operator==(const AAA &rhs) const;
          int operator<(const AAA &rhs) const;
    };
    
    AAA::AAA()   // Constructor
    {
       x = 0;
       y = 0;
       z = 0;
    }
    
    AAA::AAA(const AAA &copyin)   // Copy constructor to handle pass by value.
    {                             
       x = copyin.x;
       y = copyin.y;
       z = copyin.z;
    }
    
    ostream &operator<<(ostream &output, const AAA &aaa)
    {
       output << aaa.x << ' ' << aaa.y << ' ' << aaa.z << endl;
       return output;
    }
    
    AAA& AAA::operator=(const AAA &rhs)
    {
       this->x = rhs.x;
       this->y = rhs.y;
       this->z = rhs.z;
       return *this;
    }
    
    int AAA::operator==(const AAA &rhs) const
    {
       if( this->x != rhs.x) return 0;
       if( this->y != rhs.y) return 0;
       if( this->z != rhs.z) return 0;
       return 1;
    }
    
    int AAA::operator<(const AAA &rhs) const
    {
       if( this->x == rhs.x && this->y == rhs.y && this->z < rhs.z) return 1;
       if( this->x == rhs.x && this->y < rhs.y) return 1;
       if( this->x < rhs.x ) return 1;
       return 0;
    }
    
    main()
    {
       map<string, AAA> M;
       AAA Ablob ;
    
       Ablob.x=7;
       Ablob.y=2;
       Ablob.z=4.2355;
       M["A"] = Ablob;
    
       Ablob.x=5;
       M["B"] = Ablob;
    
       Ablob.z=3.2355;
       M["C"] = Ablob;
    
       Ablob.x=3;
       Ablob.y=7;
       Ablob.z=7.2355;
       M["D"] = Ablob;
    
       for( map<string, AAA>::iterator ii=M.begin(); ii!=M.end(); ++ii)
       {
           cout << (*ii).first << ": " << (*ii).second << endl;
       }
    
       return 0;
    }

Compile: g++ testMap.cpp

Run:  ./a.out    
Output:

A: 7 2 4.2355
    
    B: 5 2 4.2355
    
    C: 5 2 3.2355
    
    D: 3 7 7.2355

\[Potential Pitfall\] : The following error occurs when the < operator is omitted from the class AAA.

/tmp/cca5a9G9.o: In function \`main':   
testMap.cpp:(.text+0x3c6): undefined reference to \`operator<<(std::basic\_ostream<char, std::char\_traits<char> >&, AAA const&)'   
collect2: ld returned 1 exit status

\[Potential Pitfall\] : The following error occurs when the = operator is omitted from the class AAA.

/tmp/cciGBZgS.o: In function `main':
    testMap.cpp:(.text+0x263): undefined reference to `AAA::operator=(AAA const&)'
    testMap.cpp:(.text+0x2c8): undefined reference to `AAA::operator=(AAA const&)'
    testMap.cpp:(.text+0x32e): undefined reference to `AAA::operator=(AAA const&)'
    testMap.cpp:(.text+0x3a2): undefined reference to `AAA::operator=(AAA const&)'
    collect2: ld returned 1 exit status

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

<table style="color:rgb(0,0,0); font-family:helvetica,arial,sans-serif"> 
 <tbody> 
  <tr> 
   <td style="font-family:helvetica,arial,sans-serif"><strong>Example: STL multimap:</strong></td> 
  </tr> 
 </tbody> 
</table>

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]

原文：

http://www.yolinux.com/TUTORIALS/CppStlMultiMap.html

[map]: http://www.sgi.com/tech/stl/Map.html