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<const Key, T> 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
- map<string,int> 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;
-
- pair<map<char, 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));
-
- pair<map<char, 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;
-
