Unordered_map In C++

Create C++ STL unordered_map

An unordered_map in C++ is a container that stores key-value pairs in a hash table. It provides constant-time average complexity for insertions, deletions, and lookups, making it efficient for storing and accessing data.

Code:

• C++

C++

#include <iostream>
#include <unordered_map>

int main() {
    std::unordered_map<std::string, int> myMap;
    myMap["apple"] = 5;
    myMap["banana"] = 3;

    std::cout << "Value of apple: " << myMap["apple"] << std::endl;
    std::cout << "Value of banana: " << myMap["banana"] << std::endl;

    return 0;
}

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Output:

Value of apple: 5
Value of banana: 3

Explanation:

• We include the <unordered_map> header to use the unordered_map container.
• We declare an unordered_map named myMap with key type std::string and value type int.
• We insert key-value pairs into the map using the [] operator.
• We access the values using the same [] operator.

How to Initialize an Unordered_Map in C++?

An unordered_map in C++ can be initialized using various methods like assignment, initializer list, or pair-wise insertion. Initialization can be done during declaration or later in the code.

Code:

#include <iostream>
#include <unordered_map>

int main() {
    // Initialization during declaration
    std::unordered_map<std::string, int> myMap = {{"apple", 5}, {"banana", 3}};

    // Later initialization
    std::unordered_map<int, std::string> anotherMap;
    anotherMap[1] = "one";
    anotherMap[2] = "two";

    return 0;
}

Explanation:

• During declaration, we can initialize an unordered_map using brace-enclosed initializer lists.
• For later initialization, we declare an empty unordered_map and insert key-value pairs using the [] operator.

Methods on unordered_map

Internally, the C++11 library contains functions to see bucket count, bucket size and the used hash function and various hash policies. However, these are less useful in real-world applications.

Iterator can iterate over all elements of the unordered map.

The unordered_map has the following methods:

This concludes our topic of unordered_map in C++. Now let’s move on to the FAQ section.

Must Read Dynamic Binding in C++

Difference Between unordered_map and unordered_set

The major difference between an unordered map and an unordered set is that an unordered set does not support key values. In an unordered set, there is only one key. On the other hand, an unordered map has both a key and a value. If there is a situation where you have to count the individual frequencies of each tuple in a set, you will have to use the unordered map as the unordered set does not support it.

Frequently Asked Questions

In C++, which map is faster?

The unordered map is the fastest in C++ in most cases. In some circumstances, though, the map is speedier.

Why is it called Unordered_map?

It is called Unordered_map because the items in this map are kept in random order. The keys are not stored in any particular order, hence the name.

What does Unordered_map return in C++?

It returns a reference to the value mapped to a key that is equivalent to that key.

What does the count() method do in terms of unordered_map in C++?

In terms of unordered_map in C++, the count() method returns the number of elements with a specific key in the unordered_map container. It checks whether the container contains an element with the given key and returns either 0 (if not found) or 1 (if found).

What is the difference between map and unordered_map in C++?

The main difference between map and unordered_map in C++ is that map is a sorted associative container that stores key-value pairs, maintaining order based on the keys' comparison, while unordered_map is a hash table-based container that provides constant-time average complexity for insertions, deletions, and lookups.

Conclusion

In this article, we have extensively discussed the unordered map in C++ language and how we can use it in our program. We learned the syntax of unordered_map and its implementation with multiple examples. We learned the difference between unordered_map and unordered_set. We also learned the various methods of unordered_map.

Also, check out this article - 

• Pair in C++
• STL in C++

We hope that this blog has helped you enhance your knowledge regarding unordered maps in C++ STL(Standard Template Library), and if you would like to learn more, check out our articles in the Code360 library.