In C++, the Standard Template Library (STL) provides the ‘unordered_map’ container, which implements a hash table-based associative array. It allows efficient key-value lookup and provides several member functions for manipulating and accessing elements.

Creating an ‘unordered_map’:

Code:

#include

#include <unordered_map>

using namespace std;

int main() {

unordered_map<string, int> myMap; // Create an empty unordered_map

return 0;

}

1. Inserting elements: Using insert function or [] operator

- The ‘insert()’ function takes a pair of the key and value.

- The ‘[]’ operator can be used to access or insert elements. If the key doesn't exist, a new element is inserted.

Code:

unordered_map<string, int> myMap;

myMap.insert(make_pair("apple", 10));

myMap["banana"] = 20;

2. Accessing and modifying elements:

Code:

unordered_map<string, int> myMap = {{"apple", 10}, {"banana", 20}};

int count = myMap["apple"]; // Access the value associated with the key "apple"

myMap["banana"] = 30; // Modify the value associated with the key "banana"

3. Checking existence and retrieving size:

-Count() function to check if a key exists in the ‘unordered_map’. It returns 1 if the key exists, 0 otherwise.

- The size() function returns the number of elements in the ‘unordered_map’.

Code:

unordered_map< string, int> myMap = {{"apple", 10}, {"banana", 20}};

bool exists = myMap.count("apple"); // Check if the key "apple" exists

size_t numElements = myMap.size(); // Get the number of elements in the unordered_map

4. Iterating over elements:

- You can iterate over the ‘unordered_map’ using iterators.

- Each element in the ‘unordered_map’ is represented as a ‘pair’ containing the key-value pair.

Code:

unordered_map<string, int> myMap = {{"apple", 10}, {"banana", 20}};

for (const auto& pair : myMap) {

cout << pair.first << ": " << pair.second << endl;

}

5. Erasing elements:

- The ‘erase()’ function removes the element associated with the given key.

Code:

unordered_map<string, int> myMap = {{"apple", 10}, {"banana", 20}};

myMap.erase("apple"); // Remove the element with the key "apple"