- map (C++)
-
Not to be confused with Map (higher-order function).
C++ Standard Library Standard Template Library C++11 array
forward_list
unordered_map
unordered_set
C Standard Library - Data types
- Character classification
- Strings
- Mathematics
- File input/output
- Date/time
- Localization
- Memory allocation
- Program control
- Miscellaneous headers:
The class template
std::map<Key, Data, Compare, Alloc>
is a standard C++ container. It is a sorted associative array of unique keys and associated data.[1] The types of key and data may differ, and the elements of the map are internally sorted from lowest to highest key value. Since each key value is unique, if an object is inserted with an already existing key, the object already present in the map does not change.[1] A variation on the map, called the multimap, allows duplicate keys.Iterators and references are not invalidated by insert and erase operations, except for iterators and references to erased elements. The usual implementation is a self-balancing binary search tree (but any other data structure that respects the complexity constraints can be used, like a skip list).
Contents
Design
Characteristics
- Each element has a unique key
- Each element is composed of a key and a mapped value
- Elements follow a strict weak ordering[1]
Performance
Maps are designed to be especially efficient in accessing its elements by their key, as opposed to sequence containers which are more efficient in accessing elements by their position.[1]
The asymptotic complexity of the operations that can be applied to maps are as follows:
Operation Complexity Searching for an element O(log n) Inserting a new element O(log n) Incrementing/decrementing an iterator O(log n) Removing a single map element O(log n) Copying an entire map [1] O(n) Iterating through all elements O(n) Overview of functions
-
map::map
(constructor) - Constructs the map from variety of sources.map::~map
(destructor) - Destructs the map and the contained elementsmap::operator=
- Assigns values to the mapmap::get_allocator
- Returns the allocator used to allocate memory for the elements
- Iterators
- Capacity
- Modifiers
- Lookup
map::count
- Returns the number of elements matching specific keymap::find
- Finds an element with specific keymap::equal_range
- Returns a range of elements matching specific keymap::lower_bound
- Returns an iterator to the first element not less than the given valuemap::upper_bound
- Returns an iterator to the first element greater than a certain value
- Observers
map::key_comp
- Returns key comparison function.map::value_comp
- Returns value comparison function.
Usage
The map is declared in this format:
map <key_type, value_type [, comparing_option [, memory_allocator] ] > map_name
The following code demonstrates how to use the
map<string, int>
to count occurrences of words. It uses the word as the key and the count as the value.#include <iostream> #include <string> #include <map> int main() { std::map<std::string, int> wordcounts; std::string s; while (std::cin >> s && s != "end") ++wordcounts[s]; while (std::cin >> s && s != "end") std::cout << s << ' ' << wordcounts[s] << '\n'; }
When executed, the user first types a series of words separated by spaces, and a word "end" to signify the end of input; then the user can input words to query how many times each word occurred in the previous series.
The above example also demonstrates that the operator [] inserts new objects (using the default constructor) in the map if there isn't one associated with the key. So integral types are zero-initialized, strings are initialized to empty strings, etc.
The following example illustrates inserting elements into a map using the insert function and searching for a key using a map iterator and the find function:
#include <iostream> #include <map> #include <utility> // make_pair int main() { typedef std::map<char, int> MapType; MapType my_map; // insert elements using insert function my_map.insert(std::pair<char, int>('a', 1)); my_map.insert(std::pair<char, int>('b', 2)); my_map.insert(std::pair<char, int>('c', 3)); my_map.insert(MapType::value_type('d', 4)); // all standard containers provide this typedef my_map.insert(std::make_pair('e', 5)); // can also use the utility function make_pair my_map.insert({'f', 6}); // using C++11 initializer list //map keys are sorted automatically from lower to higher. //So, my_map.begin() points to the lowest key value not the key which was inserted first. MapType::iterator iter = my_map.begin(); // erase the first element using the erase function my_map.erase(iter); // output the size of the map std::cout << "Size of my_map: " << my_map.size() << '\n'; std::cout << "Enter a key to search for: "; char c; std::cin >> c; // find will return an iterator to the matching element if it is found // or to the end of the map if the key is not found iter = my_map.find(c); if (iter != my_map.end() ) std::cout << "Value is: " << iter->second << '\n'; else std::cout << "Key is not in my_map" << '\n'; // clear the entries in the map my_map.clear(); }
In the above example, five elements are entered using the insertion function, and then the first element is deleted. Then, the size of the map is output. Next, the user is prompted for a key to search for. Using the iterator, the find function searches for an element with the given key. If it finds the key, the program prints the element's value. If it does not find it, an iterator to the end of the map is returned and it outputs that the key could not be found. Finally all the elements in the tree are erased.
Iterators
Maps may use iterators to point to specific elements in the container. An iterator can access both the key and the mapped value of an element:[1]
map<Key,T>::iterator it; // declares a map iterator it->first; // the key value it->second; // the mapped value (*it); // the "element value", which is of type: pair<const Key,T>
Below is an example of looping through a map to display all keys and values using iterators:
#include <iostream> #include <string> #include <map> int main() { typedef std::map <std::string, int> MapType; MapType data; // let's declare some initial values to this map data["Bobs score"] = 10; data["Martys score"] = 15; data["Mehmets score"] = 34; data["Rockys score"] = 22; data["Rockys score"] = 23; /*overwrites the 22 as keys are unique */ // Iterate over the map and print out all key/value pairs. // Using a const_iterator since we are not going to change the values. MapType::const_iterator end = data.end(); for (MapType::const_iterator it = data.begin(); it != end; ++it) { std::cout << "Who(key = first): " << it->first; std::cout << " Score(value = second): " << it->second << '\n'; } return 0; }
This will output the keys and values of the entire map, sorted by keys.
Member types
What follows is a list of the member types of the
map
container:[1]Member type Definition key_type
Key mapped_type
T
value_type
pair<const Key,T>
key_compare
Compare value_compare
Nested class to compare elements allocator_type
Allocator
reference
Allocator::reference
const_reference
Allocator::const_reference
iterator
Bidirectional iterator const_iterator
Constant bidirectional iterator size_type
Unsigned integral type (usually same as size_t
)difference_type
Signed integral type (usually same as ptrdiff_t
)pointer
Allocator::pointer
const_pointer
Allocator::const_pointer
reverse_iterator
reverse_iterator<iterator>
const_reverse_iterator
reverse_iterator<const_iterator>
Caveats
Because map requires a strict weak ordering, a map keyed on floating-point numbers can produce undefined behavior if any of the keys are not a numbers (NaN), since any comparison operation involving a NaN yields to false.
See also
- Standard Template Library containers
- unordered_map
References
- ^ a b c d e f g ISO/IEC 14882:2011 draft specification. p. 797, § 23.4.4. http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2011/n3242.pdf.
External links
Categories:- C++ Standard Library
- Articles with example C++ code
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