Map in C++ Standard Template Library (STL)

Maps are associative containers that store elements in a mapped fashion. Each element has a key value and a mapped value. No two mapped values can have the same key values.

Some basic functions associated with Map: 

• begin() – Returns an iterator to the first element in the map.
• end() – Returns an iterator to the theoretical element that follows the last element in the map.
• size() – Returns the number of elements in the map.
• max_size() – Returns the maximum number of elements that the map can hold.
• empty() – Returns whether the map is empty.
• pair insert(keyvalue, mapvalue) – Adds a new element to the map.
• erase(iterator position) – Removes the element at the position pointed by the iterator.
• erase(const g)– Removes the key-value ‘g’ from the map.
• clear() – Removes all the elements from the map.

Begin() function :

C++

#include <iostream>
#include <map>

int main()
{
  // Create a map of strings to integers
  std::map<std::string, int> map;

  // Insert some values into the map
  map["one"] = 1;
  map["two"] = 2;
  map["three"] = 3;

  // Get an iterator pointing to the first element in the map
  std::map<std::string, int>::iterator it = map.begin();

  // Iterate through the map and print the elements
  while (it != map.end())
  {
    std::cout << "Key: " << it->first << ", Value: " << it->second << std::endl;
    ++it;
  }

  return 0;
}

Output

Key: one, Value: 1
Key: three, Value: 3
Key: two, Value: 2

Time complexity: O(n) //where n is the size of map.

Auxiliary Space: O(n)

end ()function:

C++

#include <iostream>
#include <map>

int main()
{
  // Create a map of strings to integers
  std::map<std::string, int> map;

  // Insert some values into the map
  map["one"] = 1;
  map["two"] = 2;
  map["three"] = 3;

  // Get an iterator pointing to the first element in the map
  std::map<std::string, int>::iterator it = map.begin();

  // Iterate through the map and print the elements
  while (it != map.end())
  {
    std::cout << "Key: " << it->first << ", Value: " << it->second << std::endl;
    ++it;
  }

  return 0;
}

Output

Key: one, Value: 1
Key: three, Value: 3
Key: two, Value: 2

Time complexity: O(n) //where n is the size of map.

Auxiliary Space: O(n)

Size function:

C++

#include <iostream>
#include <map>

int main()
{
  // Create a map of strings to integers
  std::map<std::string, int> map;

  // Insert some values into the map
  map["one"] = 1;
  map["two"] = 2;
  map["three"] = 3;

  // Print the size of the map
  std::cout << "Size of map: " << map.size() << std::endl;

  return 0;
}

Output

Size of map: 3

Time complexity: O(1).

Implementation:

CPP

// CPP Program to demonstrate the implementation in Map
// divyansh mishra --> divyanshmishra101010
#include <iostream>
#include <iterator>
#include <map>
using namespace std;

int main()
{

    // empty map container
    map<int, int> gquiz1;

    // insert elements in random order
    gquiz1.insert(pair<int, int>(1, 40));
    gquiz1.insert(pair<int, int>(2, 30));
    gquiz1.insert(pair<int, int>(3, 60));
    gquiz1.insert(pair<int, int>(4, 20));
    gquiz1.insert(pair<int, int>(5, 50));
    gquiz1.insert(pair<int, int>(6, 50));

      gquiz1[7]=10;     // another way of inserting a value in a map


    // printing map gquiz1
    map<int, int>::iterator itr;
    cout << "\nThe map gquiz1 is : \n";
    cout << "\tKEY\tELEMENT\n";
    for (itr = gquiz1.begin(); itr != gquiz1.end(); ++itr) {
        cout << '\t' << itr->first << '\t' << itr->second
             << '\n';
    }
    cout << endl;

    // assigning the elements from gquiz1 to gquiz2
    map<int, int> gquiz2(gquiz1.begin(), gquiz1.end());

    // print all elements of the map gquiz2
    cout << "\nThe map gquiz2 after"
         << " assign from gquiz1 is : \n";
    cout << "\tKEY\tELEMENT\n";
    for (itr = gquiz2.begin(); itr != gquiz2.end(); ++itr) {
        cout << '\t' << itr->first << '\t' << itr->second
             << '\n';
    }
    cout << endl;

    // remove all elements up to
    // element with key=3 in gquiz2
    cout << "\ngquiz2 after removal of"
            " elements less than key=3 : \n";
    cout << "\tKEY\tELEMENT\n";
    gquiz2.erase(gquiz2.begin(), gquiz2.find(3));
    for (itr = gquiz2.begin(); itr != gquiz2.end(); ++itr) {
        cout << '\t' << itr->first << '\t' << itr->second
             << '\n';
    }

    // remove all elements with key = 4
    int num;
    num = gquiz2.erase(4);
    cout << "\ngquiz2.erase(4) : ";
    cout << num << " removed \n";
    cout << "\tKEY\tELEMENT\n";
    for (itr = gquiz2.begin(); itr != gquiz2.end(); ++itr) {
        cout << '\t' << itr->first << '\t' << itr->second
             << '\n';
    }

    cout << endl;

    // lower bound and upper bound for map gquiz1 key = 5
    cout << "gquiz1.lower_bound(5) : "
         << "\tKEY = ";
    cout << gquiz1.lower_bound(5)->first << '\t';
    cout << "\tELEMENT = " << gquiz1.lower_bound(5)->second
         << endl;
    cout << "gquiz1.upper_bound(5) : "
         << "\tKEY = ";
    cout << gquiz1.upper_bound(5)->first << '\t';
    cout << "\tELEMENT = " << gquiz1.upper_bound(5)->second
         << endl;

    return 0;
}

Output

The map gquiz1 is : 
    KEY    ELEMENT
    1    40
    2    30
    3    60
    4    20
    5    50
    6    50
    7    10


The map gquiz2 after assign from gquiz1 is : 
    KEY    ELEMENT
    1    40
    2    30
    3    60
    4    20
    5    50
    6    50
    7    10


gquiz2 after removal of elements less than key=3 : 
    KEY    ELEMENT
    3    60
    4    20
    5    50
    6    50
    7    10

gquiz2.erase(4) : 1 removed 
    KEY    ELEMENT
    3    60
    5    50
    6    50
    7    10

gquiz1.lower_bound(5) :     KEY = 5        ELEMENT = 50
gquiz1.upper_bound(5) :     KEY = 6        ELEMENT = 50

Time complexity: O(n) // n is size of the map

Auxiliary space: O(n)

Example: 

C++

#include <iostream>
#include <map>

int main()
{
  // Create a map of strings to integers
  std::map<std::string, int> map;

  // Insert some values into the map
  map["one"] = 1;
  map["two"] = 2;
  map["three"] = 3;

  // Print the values in the map
  std::cout << "Key: one, Value: " << map["one"] << std::endl;
  std::cout << "Key: two, Value: " << map["two"] << std::endl;
  std::cout << "Key: three, Value: " << map["three"] << std::endl;

  // Check if a key is in the map
  if (map.count("four") > 0)
  {
    std::cout << "Key 'four' is in the map" << std::endl;
  }
  else
  {
    std::cout << "Key 'four' is not in the map" << std::endl;
  }

  return 0;
}

Output

Key: one, Value: 1
Key: two, Value: 2
Key: three, Value: 3
Key 'four' is not in the map

List of all Functions of Map 

Function	Definition
map::insert()	Insert elements with a particular key in the map container –> O(log n)
map:: count()	Returns the number of matches to element with key-value ‘g’ in the map. –> O(log n)
map equal_range()	Returns an iterator of pairs. The pair refers to the bounds of a range that includes all the elements in the container which have a key equivalent to k.
map erase()	Used to erase elements from the container –> O(log n)
map rend()	Returns a reverse iterator pointing to the theoretical element right before the first key-value pair in the map(which is considered its reverse end).
map rbegin()	Returns a reverse iterator which points to the last element of the map.
map find()	Returns an iterator to the element with key-value ‘g’ in the map if found, else returns the iterator to end.
map crbegin() and crend()	crbegin() returns a constant reverse iterator referring to the last element in the map container. crend() returns a constant reverse iterator pointing to the theoretical element before the first element in the map.
map cbegin() and cend()	cbegin() returns a constant iterator referring to the first element in the map container. cend() returns a constant iterator pointing to the theoretical element that follows the last element in the multimap.
map emplace()	Inserts the key and its element in the map container.
map max_size()	Returns the maximum number of elements a map container can hold –> O(1)
map upper_bound()	Returns an iterator to the first element that is equivalent to mapped value with key-value ‘g’ or definitely will go after the element with key-value ‘g’ in the map
map operator=	Assigns contents of a container to a different container, replacing its current content.
map lower_bound()	Returns an iterator to the first element that is equivalent to the mapped value with key-value ‘g’ or definitely will not go before the element with key-value ‘g’ in the map –> O(log n)
map emplace_hint()	Inserts the key and its element in the map container with a given hint.
map value_comp()	Returns the object that determines how the elements in the map are ordered (‘<‘ by default).
map key_comp()	Returns the object that determines how the elements in the map are ordered (‘<‘ by default).
map::size()	Returns the number of elements in the map.
map::empty()	Returns whether the map is empty
map::begin() and end()	begin() returns an iterator to the first element in the map. end() returns an iterator to the theoretical element that follows the last element in the map
map::operator[]	This operator is used to reference the element present at the position given inside the operator.
map::clear()	Removes all the elements from the map.
map::at() and map::swap()	at() function is used to return the reference to the element associated with the key k. swap() function is used to exchange the contents of two maps but the maps must be of the same type, although sizes may differ.

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