unordered_map at() in C++ (original) (raw)

Last Updated : 11 Jul, 2025

In C++, **unordered_map at() is a library function used to find the value associated with a given key in the unordered_map container.Let's look at an example to see how to use this function.

C++ `

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

int main() { unordered_map<int, string> um = {{1, "a"}, {2, "b"}, {3, "c"}};

// Finding the value associated with 2
cout << um.at(2);

return 0;

}

`

Syntax of unordered_map at()

unordered_map::at() is the member function of std::unordered_map class defined inside ****<unordered_map>** header file. So, we can use this function using (.) dot operator with any unordered_map object.

um.**at(key);

Parameters

• **key: Key whose associated value is to be found.

Return Value

• Returns a reference to the value associated with the given key if it is present.
• If the key is not present, throws **out_of_range exception.

Examples of unordered_map at()

The following examples demonstrates the use of **unordered_map at() function for different purposes and scenarios:

Find the Value Associated with Given Key

C++ `

#include <bits/stdc++.h> using namespace std; int main() { unordered_map<int, string> um = {{1, "One"}, {2, "Two"}, {3, "Three"}};

// Accessing value associated with 2
cout << um.at(2);

return 0;

}

`

Output

Key 30 with value GeeksforGeeks

Update the Value Associated with Given Key

C++ `

#include <bits/stdc++.h> using namespace std; int main() { unordered_map<int, string> um = {{1, "One"}, {2, "Two"}, {3, "Three"}};

// Updating value associated with key 2
um.at(2) = "Geeks";

cout << um.at(2);
return 0;

}

`

Output

Key 30 with value Geeks

Handle Exception if the Key Doesn't Exist

C++ `

#include <bits/stdc++.h> using namespace std; int main() { unordered_map<int, string> um = {{1, "One"}, {2, "Two"}, {3, "Three"}}; try {

    // Trying to access key that doesn't exist
    cout << um.at(10);
}
catch (const exception &e) {

    // Printing exception
    cout << "Exception Occurred: " << e.what();
}
return 0;

}

`

Output

Exception Occurred: _Map_base::at

**Difference between unordered_map at() and [] Operator

Both unordered_map at() and **unordered_map operator [] can be used to access or update the value of a key in an unordered_map but differs in the way how they work. The following table shows primary differences between these methods: