How to check if a "KEY" exist in a map for a given "VALUE" in C++

Question

Recently I had been learning maps in C++ and I came across various problems where I need to check if a "KEY" exists for a given value. In JAVA we can do so by following code snippet -

hash_map.containsValue(VALUE); // Returns true if there is a key for the value, as false

I am looking for similar stuff in C++. I have found resources that can check for existing of "KEY" like :

mp.count(k);  //will return 0 if a key "k" not exists, else it's count

But how to do it for "VALUE NOT KEY"?

Answer 1

In C++ there is no such direct method to check if a given map contains the value. One will have to iterate through the entire map and check if it contains the value or note. It can be achieved as follows

std::map<int,int> m;
//let's assume that map is already filled.
for(const auto& i:m){
    if(i.second==value)
        return true;
}
return false;

Hope it helps.

Answer 2

A clean way is to loop over the map with a structured binding (since c++17):

std::map<int, int> map;

for (const auto& [k, v] : map) {
    if (v == VALUE) {
        std::cout << "found value\n";
    }
}

---

I also recommend using std::find_if and checking if the returned found iterator is valid:

auto it = std::find_if(map.cbegin(), map.cend(), [&](const auto& entry) {
    return entry.second == VALUE;
});

if (it != map.cend()) {
    std::cout << "found value\n";
}

Answer 3

Late to the game, but with a full example (somewhat templated):

#include <iostream>
#include <map>
#include <string>

template<typename K, typename V>
bool containsValue(const std::map<K,V>& m, const V& v)
{
    for (const auto& x: m) {
        if (x.second == v) {
            return true;
        }
    }
    return false;
}

int main() 
{
    using namespace std::string_literals;
    
    std::map<std::string, std::string> mp = { 
        {"Jones", "James"},
        {"Smith", "Mary"},
        {"Garcia", "Robert"},
        {"Edwards", "Patricia"},
        {"Brown", "John"},
        {"Thompson", "Jennifer"},
        {"Sullivan", "Michael"},
        {"Rodriguez", "Linda"},
        {"Williams", "David"},
        {"Johnson", "Elizabeth"},
    };
    
    std::cout << std::boolalpha;
    
    std::cout << containsValue(mp, "Patricia"s) << '\n';
    std::cout << containsValue(mp, "Jack"s) << '\n';
        
    return 0;
}

This is not ideal, because it's really specific for std::map and it's sad that I cannot pass a string literal without the final s specifier. I'm sure someone knows how to improve the templated function.