Multimap store elements in key value pair. It can also have duplicate keys.

There can be many scenarios when we want to find all the values associated with a key in multimap.

For example, we have a multimap of char & int as key value pairÂ and initialzed with following values i.e.

{'a', 1}, {'b', 2}, {'c', 3}, {'a', 4}, {'c', 5}, {'c', 6}, {'d', 7},

There are 3 pairs withÂ duplicate key ‘c’. Values associated with key ‘c’ are 3,5 & 6. Now let’s see how to access all the values associated with a key in multimap.

### std::multimap::equal_range

We can find all values of a key in Multimap using is member function equal_range().

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pair<iterator,iterator> equal_range (const key_type& k);;

It accepts the key as an argument and returns a pair of multimap iterator. This returned pair has a range that represents the entries with given key.

Let’s see an example,

#include <iostream> #include <map> #include <iterator> #include <algorithm> int main() { //MultiMap of char and int std::multimap<char, int> mmapOfPos ={ {'a', 1}, {'b', 2}, {'c', 3}, {'a', 4}, {'c', 5}, {'c', 6}, {'d', 7}, }; typedef std::multimap<char, int>::iterator MMAPIterator; // It returns a pair representing the range of elements with key equal to 'c' std::pair<MMAPIterator, MMAPIterator> result = mmapOfPos.equal_range('c'); std::cout << "All values for key 'c' are," << std::endl; // Iterate over the range for (MMAPIterator it = result.first; it != result.second; it++) std::cout << it->second << std::endl; // Total Elements in the range int count = std::distance(result.first, result.second); std::cout << "Total values for key 'c' are : " << count << std::endl; return 0; }

**Output:**

All values for key 'c' are, 3 5 6 Total values for key 'c' are : 3

To compile the above code in Linux use following command,

**g++ –std=c++11 example.cpp**

### How to find if a key exists or not in multimap with equal_range()

As equal_range() returns a range representing all the elements with given key. So, to check if aÂ keyÂ exists or not, we will calculate the total number of elements in given range of iterators. It can be done using std::distance() algorithm i.e.

// Total Elements in the range int count = std::distance(result.first, result.second);

If the count is 0 i.e. no elements in range thenit means key don’t exists in multimap.

Checkout the example as follows,

#include <iostream> #include <map> #include <iterator> #include <algorithm> int main() { //MultiMap of char and int std::multimap<char, int> mmapOfPos ={ {'a', 1}, {'b', 2}, {'c', 3}, {'a', 4}, {'c', 5}, {'c', 6}, {'d', 7}, }; typedef std::multimap<char, int>::iterator MMAPIterator; // It returns a pair representing the range of elements with key equal to 'f' std::pair<MMAPIterator, MMAPIterator> result = mmapOfPos.equal_range('f'); // Total Elements in the range int count = std::distance(result.first, result.second); std::cout << "Total values for key 'f' are : " << count << std::endl; if(count == 0) std::cout<<"key 'f' don't exist in multimap"<<std::endl; return 0; }

**Output:**

Total values for key 'f' are : 0 key 'f' dont exist

To compile the above code in Linux use following command,

**g++ –std=c++11 example.cpp**

VishalThanks. The std::distance on the first and second iterator is something I hadn’t thought of. Will help me in a University assignment.