Answer by DavidRR for How to find if a given key exists in a std::map

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

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