Map in C++ STL

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Maps are associative containers that store data as sorted pairs of keys and values. It is an implementation of Self-Balancing Binary Search Tree, specifically a Red-Black Tree Which ensures,

• Maps allow searching, insertion, and deletion and take O(log n) time.
• Maps automatically avoid duplicate keys.
• Keys are stored in ascending order.
• Compared to unordered_map, the times taken for search, insert and delete are more in map, but we get items in sorted order and it supports additional functions like upper_bound() and lower_bound().

Let us take a look at an example to create a map and traverse it.

#include <iostream>
#include <map>
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

Syntax

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

map<key_type, value_type> m;

where,

• key_type: Data type of key.
• value_type: Data type of value.
• m: Name assigned to map.

Basic Operations

Basic operations on map containers are shown below:

Inserting Elements

• The insert() operation adds a new key-value pair to the map only if the key is not already present.
• If the key exists, insert() does not update the value and leaves the map unchanged.
• Time complexity to insert is O(log n).

#include <iostream>
#include <map>
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

Accessing Elements

• We can access elements with [] operator which returns the value for a given key and inserts the key with a default value if it doesn't exist.
• To check if a key exists without adding it by we can use find().
• Time complexity to access elements by key is O(log n).

#include <iostream>
#include <map>
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

Updating Elements

• To update a value, we can simply assign a new value to an existing key using map[key]= newValue; If the key already exists , the value gets updated.
• Time complexity to update element by key O(log n).

#include <iostream>
#include <map>
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

Finding Elements

• find() function is used to check if key exists in a map which looks for key and returns its position if found.
• If key is not present in the map, find() returns a special value called end(), meaning not found.
• Time complexity to find element by key is O(log n).

#include <iostream>
#include <map>
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

Traversing

• Loops can be used to traverse all key-value pairs in a map, which visits each pair in order sorted by the keys.
• Time complexity to traverse in a map is O(n).

#include <iostream>
#include <map>
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

Deleting Elements

• To delete a key and its value from map use erase(key), which deletes the pair if the key exists, else does nothing.
• Time complexity to delete an element by key O(log n).

#include <iostream>
#include <map>
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

Map in C++ STL

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