std::unordered_map::count(3)

💖 std::unordered_map::count(3) 💖

okay bestie, you’re on the std::unordered_map::count page from the C++ Standard Libary manual, and it’s literally the “how many of this key do we actually have?” girlie. No cap, she’s concise, she’s useful, and she does not allow duplicates — periodt. 💅

💖 NAME 💖

std::unordered_map::count — std::unordered_map::count

✨ Synopsis ✨

size_type count( const Key& key ) const; (1) (since C++11)
template< class K >                      (2) (since C++20)
size_type count( const K& x ) const;

1. Returns the number of elements with key that compares equal to the specified argument key, which is either 0 or 1 since this container does not allow duplicates.
2. Returns the number of elements with key that compares equivalent to the specified argument x. This overload participates in overload resolution only if Hash::is_transparent and KeyEqual::is_transparent are valid and each denotes a type. This assumes that such Hash is callable with both K and Key type, and that the KeyEqual is transparent, which, together, allows calling this function without constructing an instance of Key.

🎀 Parameters 🎀

• key — key value of the elements to count
• x — a value of any type that can be transparently compared with a key

🌸 Return value 🌸

1. Number of elements with key key, that is either 1 or 0.
2. Number of elements with key that compares equivalent to x.

🦋 Complexity 🦋

Constant on average, worst case linear in the size of the container. So: average is chill, worst case is giving “whole container walk,” iykyk.

💅 Notes 💅

Feature-test macro tea ☕:

• Feature-test macro: __cpp_lib_generic_unordered_lookup
• Value: 201811L
• Std: (C++20)
• Feature: Heterogeneous comparison lookup in unordered associative containers, overload (2)

🍓 Example 🍓

Run this code 👇

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

int main()
{
    std::unordered_map<int, std::string> dict = {
        {1, "one"}, {6, "six"}, {3, "three"}
    };
    dict.insert({4, "four"});
    dict.insert({5, "five"});
    dict.insert({6, "six"});

    std::cout << "dict: { ";
    for (auto const& [key, value] : dict)
        std::cout << '[' << key << "]=" << value << ' ';
    std::cout << "}\n\n";

    for (int i{1}; i != 8; ++i)
        std::cout << "dict.count(" << i << ") = " << dict.count(i) <<
       '\n';
}

Possible output:

dict: { [5]=five [4]=four [1]=one [6]=six [3]=three }

dict.count(1) = 1
dict.count(2) = 0
dict.count(3) = 1
dict.count(4) = 1
dict.count(5) = 1
dict.count(6) = 1
dict.count(7) = 0

👑 See also 👑

• find — finds element with specific key (public member function)
• contains — checks if the container contains element with specific key (C++20) (public member function)
• equal_range — returns range of elements matching a specific key (public member function)

that’s the whole page, bestie 💖 source: http://cppreference.com · version/date: 2024.06.10 · manual page: std::unordered_map::count(3) 🫶