Map in C++ STL

Try it on GfG Practice

In C++, maps are associative containers that store data in the form of key value pairs sorted on the basis of keys. No two mapped values can have the same keys. By default, it stores data in ascending order of the keys, but this can be changes as per requirement.

Example:

#include <bits/stdc++.h>
using namespace std;

int main() {
    
    // Creating a map of integer keys
    // and string values
    map<int, string> m {{1, "Geeks"},
             {2,"For"}, {3,"Geeks"}};

    for (auto& p : m)
        cout << p.first << " " << p.second
        << "\n";
    return 0;
}

1 Geeks
2 For
3 Geeks

Explanation: In the above program, we created a map m of integer keys and string values. We inserted three key-value pairs into the map: {1, "Geeks"}, {2, "For"}, and {3, "Geeks"}. The map automatically sorts the keys in ascending order.

Syntax

The map container is defined as std::map class template inside the <map> header file.

map<key_type, value_type, comp> m;

where,

• key_type: Data type of key.
• value_type: Data type of value.
• comp: Custom comparator function that defines how to compare two keys for sorting. It is optional and if not provided, sorts data in increasing order of the keys.
• m: Name assigned to map.

Declaration and Initialization

We can declare and initialize a map in different ways as shown in the below example:

#include <bits/stdc++.h>
using namespace std;

int main() {
    
    // Creating an empty map
    map<int, string> m1;

    // Initialze map with list
    map<int, string> m2 = {{1, "Geeks"},
              {2, "For"}, {3, "Geeks"}};

    for (auto& p : m2)
        cout << p.first << " " <<
        p.second << endl;
    return 0;
}

1 Geeks
2 For
3 Geeks

Example: In the above program,

• Statement map<int, string> m1 is an empty map with no elements.
• Statement map<int, string> m2 = {{1, "Geeks"}, {2, "For"}, {3, "Geeks"}} initialized to three key-value pairs using initializer list.

To see more ways to declare and initialize map, refer to this article - Different Ways to Initialize a Map

Basic Operations

Basic operations on map containers are shown below:

1. Inserting Elements

Elements can be inserted into a map using either [] operator or insert() method. If the element with the given key already exists, the insert() method skips the insertion but [] operator updates the associated value to the new value.

Example:

#include <bits/stdc++.h>
using namespace std;

int main() {
    map<int, string> m = {{2, "For"}, {3, "Geeks"}};

    // Inserting a key value pair
    m.insert({1, "Geeks"});

    for (auto x: m)
        cout << x.first << " " << x.second
        << endl;
    return 0;
}

1 Geeks
2 For
3 Geeks

We cannot specify any particular position to insert element as map automatically sort the data according to the order. To know more ways to insert elements in a map, refer this article - Different Ways to Insert Elements in a Map

2. Accessing Elements

Map elements can be accessed by using the corresponding key inside operator []. If the key exists, it will return the associated value but if the key doesn't exist, it will create a new element with the given key and the default value. To avoid this, we can also use at() method for accessing elements without any modification.

Example:

#include <bits/stdc++.h>
using namespace std;

int main() {
    map<int, string> m = {{1, "Geeks"},
             {2, "For"}, {3, "Geeks"}};

    // Accessing elements
    cout << m[1] << endl;
    cout << m.at(2);

    return 0;
}

Geeks
For

To know more methods to access values in a map, refer to the article - Different Ways to Access a Value in a Map

3. Updating Elements

The key of an already present elements cannot be modified in the map. But the associated value can be changed by first accessing the element and then using assignment operator to change the value.

Example:

#include <bits/stdc++.h>
using namespace std;

int main() {
    map<int, string> m = {{1, "Geeks"},
             {2, "For"}, {3, "Geeks"}};

    // Updating value
    m[0] = "Tweaks";
    m.at(1) = "By";
    
    cout << m[0] << endl;
    cout << m.at(1);
    return 0;
}

Tweaks
By

Explanation: In this program, expression m[0] = "Tweaks" updates the value associated with the key 0. Similarly, expression m.at(1) = "By" updates the value of the key 1.

To see more methods to update values in a map, refer to the article - Different Ways to Update Value of a Pair in Map

4. Finding Elements

Map provides fast element search by key using the find() member function. This function returns iterator the element if found, otherwise returns end() iterator.

Example:

#include <bits/stdc++.h>
using namespace std;

int main() {
    map<int, string> m = {{1, "Geeks"},
             {2, "For"}, {3, "Geeks"}};

    // Finding element with key 2
    auto it = m.find(2);
    
    if (it != m.end())
        cout << it->first << " " << it->second;
    else cout << "Key not Found!";
    return 0;
}

2 For

To know more methods to search element in map, refer to this article - Check if Map Contains a Specific Key

5. Traversing

Maps can be easily traversed by using either range based for loop or using begin() and end() iterator with traditional loops.

Example:

#include <bits/stdc++.h>
using namespace std;

int main() {
    map<int, string> m = {{1, "Geeks"},
             {2, "For"}, {3, "Geeks"}};
    
    // Traversing using iterators
    for (auto it = m.begin(); it != m.end(); ++it) 
        cout << it->first << " " << it->second
        << endl;

    return 0;
}

1 Geeks
2 For
3 Geeks

To see more methods to traverse a map, refer to the article - Different Ways to Traverse a Map

6. Deleting Elements

Map elements can be deleted from a map using erase() method by passing the key or an iterator.

Example:

#include <bits/stdc++.h>
using namespace std;

int main() {
    map<int, string> m = {{1, "Geeks"},
             {2, "For"}, {3, "Geeks"}};

    // Deleting by key
    m.erase(2);
    
    // Deleting by iterator
    m.erase(m.begin());
    
    for(auto i : m)
        cout << i.first << " " << i.second
        << endl;
    return 0;
}

3 Geeks

To know more ways to delete elements in a map, refer this article - Different Ways to Delete Elements from Map

Time Complexity

The below table lists the time complexity of the above operations on map:

Other Common Operations

Following are some other commonly used operations on a map in C++:

• Check if a Map is Empty
• Find the Size of a Map
• Swap Two Maps
• Convert a Vector of Pairs to a Map
• Sort Map in Custom Order
• Create a Map for User Defined Datatype
• Merge Two Maps

Internal Working

In C++, map is an associative container that provides the built-in implementation of Red-Black Tree. It stores the elements in some sorted order on the basis of keys. Due do RB Trees, insertion, deletion, and search operations takes logarithmic O(log n) time.

All Member Functions

Here's the list of all member functions of std::map:

Map in C++ STL

Visit Course