Map

1、说明：系统根据C++ Reference学习下STL--> Map

2、Map：Maps are associative containers that store elements formed by a combination of a key value and a mapped value, following a specific order. 就是说一个key(关键)值映射一个mapped(映射)值，并且按照欧一定的顺序排列。

(1) key value标记一个一个元素，mapped value通常用来key value对应的内容

(2) Key value和Mapped Value的类型可以不同，一般由

typedef pair value_type来绑定。

3、Map的基本成员函数

(1) map::at  --> Returns a reference to the mapped value of the element identified with key k

 mapint> mymap = {
     {"alpha", 0},
     {"beta", 0},
     {"gamma", 0}};


 mymap.at("alpha") = 10;
 mymap.at("beta") = 20;
 mymap.at("gamma") = 30;

 for(auto &x: mymap){
     cout << x.first << ": " << x.second << endl;
 }

(2)map::begin  --> Returns an iterator referring to the first element in the map container.

 map<char, int> mymap;
 map<char, int>::iterator it;

 mymap['b'] = 100;
 mymap['a'] = 200;
 mymap['c'] = 300;

 for(map<char, int>::iterator it = mymap.begin(); it != mymap.end(); it ++)
 {
     cout << it->first << " => " << it->second << endl;
 }

(3) map ::cbegin  -->  Returns a  const_iterator  pointing to the first element in the container.

 map<char, int> mymap;

 mymap['b'] = 100;
 mymap['a'] = 200;
 mymap['c'] = 300;

 cout << "mymap contains:\n";
 for(auto it = mymap.cbegin(); it != mymap.cend(); it ++)
 {
     cout << "[" << (*it).first << ": " << (*it).second << "]\n";
 }

     (4)map::clear  --> Removes all elements from the map container (which are destroyed), leaving the container with a size of 0.

       (5)map::count  --> Searches the container for elements with a key equivalent to k and returns the number of matches.  Map中的所有element都是唯一的。

 map<char, int> mymap;
 char c;

 mymap['a'] = 101;
 mymap['b'] = 202;
 mymap['f'] = 303;

 for(c = 'a'; c < 'h'; c ++)
 {
     cout << c;
     if(mymap.count(c) > 0)
         cout << " is an element of mymap.\n";
     else cout << " is not an element of mymap.\n";
 }

      (6)map::crbegin  --> A const_reverse_iterator to the reverse beginning of the sequence.

和cbegin的调用是一致的。

     (7)map::emplace  --> Inserts a new element in the map if its key is unique. This new element is constructed in place using args as the arguments for the construction of a value_type (which is an object of a pair type).  如果键值是唯一的，则插入成功。如果插入成功，则会依照原有的顺序插入到map中

 map<char, int> mymap;
 mymap.emplace('x', 100);
 mymap.emplace('y', 200);
 mymap.emplace('z', 300);

 cout << "mymap contains:\n";
 for(auto &x: mymap){
     cout << "[" << x.first << ", " << x.second << "]\n";
 }

         (8)map::emplace_hint   --> Inserts a new element in the map if its key is unique, with a hint on the insertion position. This new element is constructed in place using args as the arguments for the construction of a value_type (which is an object of a pairtype).

 map<char, int> mymap;
 auto it = mymap.end();

 it = mymap.emplace_hint(it, 'b', 10);
 mymap.emplace_hint(it, 'a', 12);
 mymap.emplace_hint(mymap.end(), 'c', 14);

 cout << "mymap contains: ";
 for(auto &x: mymap)
     cout << "[" << x.first << ", " << x.second << endl;

       (9)map::empty  -->  Returns whether the  map  container is empty (i.e. whether its  size  is  0 ).

       (10)map::equal_range  --> Returns the bounds of a range that includes all the elements in the container which have a key equivalent to k.

 map<char, int> mymap;
 mymap['a'] = 10;
 mymap['b'] = 20;
 mymap['c'] = 30;

 pairchar, int>::iterator, map<char, int>::iterator> ret;
 ret = mymap.equal_range('a');

 cout << "lower bound points to: ";
 cout << ret.first->first << " => " << ret.first->second << endl;
 cout << "upper bound points to: ";
 cout << ret.second->first << " => " << ret.second->second << endl;

        (11)map::erase  --> Removes from the map container either a single element or a range of elements ([first,last)).

 map<char, int> mymap;
 map<char, int>::iterator it;

 mymap['a'] = 10;
 mymap['b'] = 20;
 mymap['c'] = 30;
 mymap['d'] = 40;
 mymap['e'] = 50;
 mymap['f'] = 60;

 it = mymap.find('b');
 mymap.erase(it);        //erasing by iterator, erase b

 mymap.erase('c') ;       //erasing by key, erase c

 it = mymap.find('e');
 mymap.erase(it, mymap.end()) ;  //erasing by range, erase e, f

 //输出a d
 for(it = mymap.begin(); it != mymap.end(); ++ it)
     cout << it->first << " => " << it->second << endl;

      (12)map::find  --> Searches the container for an element with a key equivalent to k and returns an iterator to it if found, otherwise it returns an iterator to map::end.

 std::map<char,int> mymap;
 std::map<char,int>::iterator it;

 mymap['a']=50;
 mymap['b']=100;
 mymap['c']=150;
 mymap['d']=200;

 it=mymap.find('b');
 mymap.erase (it);
 mymap.erase (mymap.find('d'));

 // print content:
 std::cout << "elements in mymap:" << '\n';
 std::cout << "a => " << mymap.find('a')->second << '\n';
 std::cout << "c => " << mymap.find('c')->second << '\n';

      (13)map::get_allocate  --> Returns a copy of the allocator object associated with the map.

 int psize;
 map<char, int> mymap;
 pair<const char, int> *p;

 //allocate an array of 5 elements using mymap's allocater.
 p = mymap.get_allocator().allocate(5);

 //assign some values to array
 psize = sizeof(map <char, int>::value_type) * 5;
 cout << "the allocated array has a size of " << psize << " bytes\n";
 mymap.get_allocator().deallocate(p, 5);

      (14)map::insert  --> Extends the container by inserting new elements, effectively increasing the container size by the number of elements inserted.  插入时会检查是否已经存在和当前键值一样的元素。

 map<char, int> mymap;

 //first insert function version(single parameter)
 mymap.insert(pair<char, int>('a', 200));
 mymap.insert(pair<char, int>('z', 200));

 pairchar, int>::iterator, bool> ret;

 ret = mymap.insert(pair<char, int>('z', 500));
 if(ret.second == false){
     cout << "element '" << ret.first->first << "' already exists";
     cout << " with a value of " << ret.first->second << endl;
 }

 //second insert function version(with hint position)
 map<char, int>::iterator it = mymap.begin();
 mymap.insert(it, pair<char, int>('b', 300));
 mymap.insert(it, pair<char, int>('c', 400));

 //third insert function version(range insertion);
 map<char, int> anothermap;
 anothermap.insert(mymap.begin(), mymap.find('c'));

 //showing contents
 cout << "mymap contains:\n";
 for(it = mymap.begin(); it != mymap.end(); ++it)
     cout << it->first << " => " << it->second << endl;

 cout << "anothermap contains:\n";
 for(it = anothermap.begin(); it != anothermap.end(); ++ it)
     cout << it->first << " => " << it->second << endl;

     (15)map::key_comp  --> Returns a copy of the comparison object used by the container to compare keys.

 map<char, int> mymap;

 map<char, int>::key_compare mycomp = mymap.key_comp();
 mymap['b'] = 400;
 mymap['e'] = 500;
 mymap['f'] = 300;
 mymap['a'] = 100;

 cout << "mymap contains:\n";
 //key value of last element
 char highest = mymap.rbegin()->first;

 map<char, int>::iterator it = mymap.begin();
 do{
     cout << it->first << " => " << it->second << endl;
 }while(mycomp((*it ++).first, highest));

 cout << endl;

     (16)map::lower_bound  -> Returns an iterator pointing to the first element in the container whose key is not considered to go before k (i.e., either it is equivalent or goes after).  he mapcontains an element with a key equivalent to k: In this case, lower_bound returns an iterator pointing to that element, whereas upper_bound returns an iterator pointing to the next element.

 map<char, int> mymap;
 map<char, int>::iterator itlow, itup;

 mymap['a'] = 20;
 mymap['b'] = 40;
 mymap['c'] = 60;
 mymap['d'] = 80;
 mymap['e'] = 100;

 //itlow points to b
 itlow = mymap.lower_bound('a');
 //itup points to e(not 'd')
 itup = mymap.upper_bound('d');

 //erase [itlow, itup)
 mymap.erase(itlow, itup);

 for(map<char, int>::iterator it = mymap.begin(); it != mymap.end(); ++ it)
 {
     cout << it->first << " => " << it->second << endl;
 }

      (17)map::max_size  --> Returns the maximum number of elements that the map container can hold.

 map<int, int> mymap;

 cout << mymap.max_size() << endl;
 if(mymap.max_size() > 1000){
     for(int i = 0; i < 1000; i ++)
         mymap[i] = 0;
     cout << "The map contains 1000 elements" << endl;;
 }
 else cout << "The map could not hold 1000 elements." << endl;

   

   (18)map::operator=  --> Assigns new contents to the container, replacing its current content.

 map<char, int> first;
 map<char, int> second;

 first['x'] = 8;
 first['y'] = 16;
 first['z'] = 32;

 second = first;
 second.insert(pair<char, int>('a', 100));
 first = second;

 cout << "Size of first: " << first.size() << endl;
 cout << "Size of second: " << second.size() << endl;

 first = map<char, int>();
 cout << "Size of first: " << first.size() << endl;
 cout << "Size of second: " << second.size() << endl;

    (19)map::operator[]  -->  A reference to the mapped value of the element with a key value equivalent to  k .

 map<char, string> mymap;

 mymap['a'] = "an element";
 mymap['b'] = "another elment";
 mymap['c'] = mymap['b'];

 cout << "mymap['a'] is " << mymap['a'] << endl;
 cout << "mymap['b'] is " << mymap['b'] << endl;
 cout << "mymap['c'] is " << mymap['c'] << endl;

 cout << "mymap now contains " << mymap.size() << endl;

    (20)map::rbegin  -->  Returns a  reverse iterator  pointing to the last element in the container (i.e., its  reverse beginning ).

 map<char, int> mymap;

 mymap['x'] = 100;
 mymap['y'] = 200;
 mymap['z'] = 300;

 map<char, int>::reverse_iterator rit;
 for(rit=mymap.rbegin(); rit != mymap.rend(); ++rit){
     cout << rit->first << " => " << rit->second << endl;
 }

    (21)map::size  ->  Returns the number of elements in the  map  container.

  

    (22)map::value_comp  --> Returns a comparison object that can be used to compare two elements to get whether the key of the first one goes before the second.

    (23)map::swap  --> Exchanges the content of the container by the content of x, which is another map of the same type. Sizes may differ.  需要是同种类型的

 map<char, int> foo, bar;

 foo['x'] = 100;
 foo['y'] = 200;

 bar['a'] = 11;
 bar['b'] = 22;
 bar['c'] = 33;

 foo.swap(bar);

 cout << "foo contains: \n";
 for(map<char, int>::iterator it = foo.begin(); it != foo.end(); ++it)
     cout << it->first << " => " << it->second << endl;

 cout << "bar contains: \n";
 for(map<char, int>::iterator it = bar.begin(); it != bar.end(); ++it)
     cout << it->first << " => " << it->second << endl;