Map (C++)

Not to be confused with Map (higher-order function).

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. 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. 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).

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

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.

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	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 elements
  • map::operator= - Assigns values to the map
  • map::get_allocator - Returns the allocator used to allocate memory for the elements
• Iterators
  • map::begin - Returns an iterator to the beginning of the map
  • map::end - Returns an iterator to the end of the map
  • map::rbegin - Returns a reverse iterator to the reverse beginning of the map
  • map::rend - Returns a reverse iterator to the reverse end of the map
• Capacity
  • map::empty - Checks whether the map is empty
  • map::size - Returns the number of elements in the map.
  • map::max_size - Returns the maximum possible number of elements in the map.
• Modifiers
  • map::clear - Clears the contents
  • map::insert - Inserts elements
  • map::emplace (C++11) - Constructs elements in-place
  • map::emplace_hint (C++11) - Constructs elements in-place using a hint
  • map::erase - Erases elements
  • map::swap - Swaps the contents with another map
• Lookup
  • map::count - Returns the number of elements matching specific key
  • map::find - Finds an element with specific key
  • map::equal_range - Returns a range of elements matching specific key
  • map::lower_bound - Returns an iterator to the first element not less than the given value
  • map::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

    
    //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:

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:

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 NaNs.

External links

• C++ reference for std::map

See also

• Standard Template Library containers
• unordered_map