How to find if a given key exists in a C++ std::map

Question

Use map::find

if ( m.find("f") == m.end() ) {
  // not found
} else {
  // found
}

To check if a particular key in the map exists, use the count member function in one of the following ways:

m.count(key) > 0
m.count(key) == 1
m.count(key) != 0

The documentation for map::find says: "Another member function, map::count, can be used to just check whether a particular key exists."

The documentation for map::count says: "Because all elements in a map container are unique, the function can only return 1 (if the element is found) or zero (otherwise)."

To retrieve a value from the map via a key that you know to exist, use map::at:

value = m.at(key)

Unlike map::operator[], map::at will not create a new key in the map if the specified key does not exist.

C++20 gives us std::map::contains to do that.

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

int main()
{
    std::map<int, std::string> example = {{1, "One"}, {2, "Two"}, 
                                     {3, "Three"}, {42, "Don\'t Panic!!!"}};

    if(example.contains(42)) {
        std::cout << "Found\n";
    } else {
        std::cout << "Not found\n";
    }
}

You can use .find():

map<string,string>::iterator i = m.find("f");

if (i == m.end()) { /* Not found */ }
else { /* Found, i->first is f, i->second is ++-- */ }

C++17 simplified this a bit more with an If statement with initializer. This way you can have your cake and eat it too.

if ( auto it{ m.find( "key" ) }; it != std::end( m ) ) 
{
    // Use `structured binding` to get the key
    // and value.
    const auto&[ key, value ] { *it };

    // Grab either the key or value stored in the pair.
    // The key is stored in the 'first' variable and
    // the 'value' is stored in the second.
    const auto& mkey{ it->first };
    const auto& mvalue{ it->second };

    // That or just grab the entire pair pointed
    // to by the iterator.
    const auto& pair{ *it };
} 
else 
{
   // Key was not found..
}

m.find == m.end() // not found 

If you want to use other API, then find go for m.count(c)>0

 if (m.count("f")>0)
      cout << " is an element of m.\n";
    else 
      cout << " is not an element of m.\n";