Searching in std::map or in std::multimap

There are several ways to search a key in std::map or in std::multimap.

• To get the iterator of the first occurrence of a key, the find() function can be used. It returns end() if the key does not exist.

std::multimap< int , int > mmp{ {1, 2}, {3, 4}, {6, 5}, {8, 9}, {3, 4}, {6, 7} };
auto it = mmp.find(6);
if (it!=mmp.end())
    std::cout << it->first << ", " << it->second << std::endl; //prints: 6, 5
else
    std::cout << "Value does not exist!" << std::endl;

it = mmp.find(66);
if (it!=mmp.end())
    std::cout << it->first << ", " << it->second << std::endl; 
else
    std::cout << "Value does not exist!" << std::endl; // This line would be executed.

• Another way to find whether an entry exists in std::map or in std::multimap is using the count() function, which counts how many values are associated with a given key. Since std::map associates only one value with each key, its count() function can only return 0 (if the key is not present) or 1 (if it is). For std::multimap, count() can return values greater than 1 since there can be several values associated with the same key.

std::map< int , int > mp{ {1, 2}, {3, 4}, {6, 5}, {8, 9}, {3, 4}, {6, 7} };
if (mp.count(3) > 0) // 3 exists as a key in map
    std::cout << "The key exists!" << std::endl; // This line would be executed.
else
    std::cout << "The key does not exist!" << std::endl;

If you only care whether some element exists, find is strictly better: it documents your intent and, for multimaps, it can stop once the first matching element has been found.

• In the case of std::multimap, there could be several elements having the same key. To get this range, the equal_range() function is used which returns std::pair having iterator lower bound (inclusive) and upper bound (exclusive) respectively. If the key does not exist, both iterators would point to end().

auto eqr = mmp.equal_range(6);
auto st = eqr.first, en = eqr.second;
for (auto it = st; it != en; ++it){
    std::cout << it->first << ", " << it->second << std::endl; 
}
    // prints: 6, 5
    //         6, 7