Accessing elements

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An std::map takes (key, value) pairs as input.

Consider the following example of std::map initialization:

std::map < std::string, int > ranking { std::make_pair("stackoverflow", 2), 
                                        std::make_pair("docs-beta", 1) };

In an std::map , elements can be inserted as follows:

ranking["stackoverflow"]=2;
ranking["docs-beta"]=1;

In the above example, if the key stackoverflow is already present, its value will be updated to 2. If it isn’t already present, a new entry will be created.

In an std::map, elements can be accessed directly by giving the key as an index:

std::cout << ranking[ "stackoverflow" ] << std::endl;

Note that using the operator[] on the map will actually insert a new value with the queried key into the map. This means that you cannot use it on a const std::map, even if the key is already stored in the map. To prevent this insertion, check if the element exists (for example by using find()) or use at() as described below.

Elements of a std::map can be accessed with at():

std::cout << ranking.at("stackoverflow") << std::endl;

Note that at() will throw an std::out_of_range exception if the container does not contain the requested element.

In both containers std::map and std::multimap, elements can be accessed using iterators:

// example using begin()
std::multimap < int, std::string > mmp { std::make_pair(2, "stackoverflow"),
                                         std::make_pair(1, "docs-beta"),
                                         std::make_pair(2, "stackexchange")  };
auto it = mmp.begin();
std::cout << it->first << " : " << it->second << std::endl; // Output: "1 : docs-beta"
it++;
std::cout << it->first << " : " << it->second << std::endl; // Output: "2 : stackoverflow"
it++;
std::cout << it->first << " : " << it->second << std::endl; // Output: "2 : stackexchange"

// Example using rbegin()
std::map < int, std::string > mp {  std::make_pair(2, "stackoverflow"),
                                    std::make_pair(1, "docs-beta"),
                                    std::make_pair(2, "stackexchange")  };
auto it2 = mp.rbegin();
std::cout << it2->first << " : " << it2->second << std::endl; // Output: "2 : stackoverflow"
it2++;
std::cout << it2->first << " : " << it2->second << std::endl; // Output: "1 : docs-beta"

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