<unordered_map>

<unordered_map> is a hash table of key-value pairs with O(1) average insertion, deletion, and lookup. Keys are unordered, but you can iterate and access by key quickly.

Use unordered_map when you need fast lookup by key; use std::map if you need sorted order.

This example uses an unordered_map to count word frequencies, demonstrating fast insertion and lookup without order guarantees.

// compile: g++ -std=c++11 -o unorderedmapexample unorderedmapexample.cpp
// run: ./unorderedmapexample
// description: hash table map operations
 
#include <unordered_map>
#include <iostream>
#include <string>
 
int main() {
    std::unordered_map<std::string, int> count;
    count["apple"]++;
    count["banana"]++;
    count["apple"]++;
 
    for (const auto& [fruit, num] : count) {
        std::cout << fruit << ": " << num << "\n";
    }
 
    std::cout << "apple count: " << count["apple"] << "\n";
 
    return 0;
}