How to Access a Value in a Map Using a Key in C++?

Last Updated : 23 Oct, 2024

In C++, std::map container store data in the form of key-value pairs where keys are associated with the given value. In this article, we will learn how to access a value in a std::map using key in C++.

Examples

Input: m = {{'a', 11}, {'b', 45}, {'c', 9}}, k = 'c'
Output: 9
Explanation: The key 'c' present in the map with value 9.
Input: m = {{'a', 11}, {'b', 45}, {'c', 9}}, k = 'a'
Output: 11
Explanation: The key a present in the map with value 11.

Following are the 3 different methods to access the value of key in std::map in C++:

Table of Content

Using Operator []

The simplest way is to use the [] operator to access the value associated with a key. We pass the key inside the [] brackets and it returns the associated value.

Example

C++

// C++ Program to access the value in a map // using a key with [] operator #include <bits/stdc++.h> using namespace std;  int main() {     map<char, int> m = {{'a', 11}, {'b', 45},                          {'c', 9}};     char k = 'c';      // Accessing the value of key in std::map     // using [] operator     cout << m[k];     return 0; }

Output

Time Complexity: O(log n), where n is the number of elements in the map.
Auxiliary Space: O(1)

Note: If the key doesn't exists in the map, then instead of returning an error, this operator will create a new element with the given key and default initialized value.

Using map::at() Method

The std::map::at() method provides a safer method to access to the value associated with a given key in std::map. Unlike [] operator, if the key does not exist, it throws an std::out_of_range exception.

Syntax

m.at(k);

where m is the map and k is the key to be searched.

Example

C++

// C++ Program to illustrate how to access // a value in std::map using a key with // std::map::at() method #include <bits/stdc++.h> using namespace std;  int main() {     map<char, int> m = {{'a', 11}, {'b', 45},                          {'c', 9}, {'d', 21}}; 	char k = 'c';      // Accessing the value of key in std::map     // using std::map::at()     cout << m.at(k);     return 0; }

Output

Time Complexity: O(log n), where n is the number of elements in the map.
Auxiliary Space: O(1)

Using map::find() Method

We can also access the value of key in std::map using std::map::find() method. This function will return an iterator to the key-value pair if the key present, else returns an iterator pointing to the std::map::end().

Syntax

m.find(k);

where m is the map and k is the key to be searched.

Example

C++

// C++ Program to illustrate how to access // a value in a map using a key with // std::map::find() method #include <bits/stdc++.h> using namespace std;  int main() {     map<char, int> m = {{'a', 11}, {'b', 45},                          {'c', 9}};     char k = 'c';     // Finding the key in std::map     auto it = m.find(k);      // Checking if the key is found     if (it != m.end())         cout << it->second;     else         cout << "Key " << k << " is NOT Present";     return 0; }

Output

Time Complexity: O(log n), where n is the number of elements in the map.
Auxiliary Space: O(1)