unordered_map at() in C++

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.

#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;
}

b

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

#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;
}

Key 30 with value GeeksforGeeks

Update the Value Associated with Given Key

#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;
}

Key 30 with value Geeks

Handle Exception if the Key Doesn’t Exist

#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;
}

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: