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unordered_map in C++

The C++ std::unordered_map is a container provided by the Standard Library that stores key-value pairs in an unordered manner. It is a type of associative container that provides fast access to values based on their keys. The primary characteristic of an std::unordered_map is that it uses a hash table data structure to achieve constant-time average complexity for search, insertion, and deletion operations.

Here’s how to use std::unordered_map:

#include <iostream>
#include <unordered_map>
#include <string>

int main() {
    // Declare an unordered_map with string keys and integer values
    std::unordered_map<std::string, int> myMap;

    // Insert key-value pairs into the unordered_map
    myMap["Alice"] = 25;
    myMap["Bob"] = 30;
    myMap["Charlie"] = 22;

    // Access values by key
    std::cout << "Bob's age is " << myMap["Bob"] << " years." << std::endl;

    // Check if a key exists in the unordered_map
    if (myMap.find("David") != myMap.end()) {
        std::cout << "David's age is " << myMap["David"] << " years." << std::endl;
    } else {
        std::cout << "David is not in the map." << std::endl;

    // Iterate through the unordered_map
    for (const auto& pair : myMap) {
        std::cout << pair.first << ": " << pair.second << " years" << std::endl;

    return 0;

In this example:

  • We include the <unordered_map> and <string> headers to work with std::unordered_map and strings.
  • We declare an std::unordered_map called myMap with string keys and integer values.
  • We insert key-value pairs into the unordered_map using the [] operator.
  • We access values by key using the [] operator.
  • We check if a key exists in the unordered_map using the find method.
  • We iterate through the unordered_map using a range-based for loop.

std::unordered_map is a versatile container suitable for various use cases where you need fast lookup times based on keys. Keep in mind that the order of elements in an std::unordered_map is not guaranteed to be in any particular order; it depends on the hash function and the underlying implementation.


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