C++ Unordered_map Library - equal() Function


The C++ function std::unordered_map::equal() returns range of elements that matches specific key.

In unordered_map containers, where keys are unique, the range will include one element at most. If k does not match any key in the container, the range returned has end as both its lower and upper range bounds.


Following is the declaration for std::unordered_map::equal() function form std::unordered_map header.


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


k − Key to be searched.

Return value

If object is constant qualified then method returns a pair of constant iterator otherwise pair of non-constant iterator.

Time complexity

Constant i.e. O(1) in average case.

Linear i.e. O(n) in worst case.


The following example shows the usage of std::unordered_map::equal() function.

#include <iostream>
#include <unordered_map>

using namespace std;

int main(void) {
   unordered_map<char, int> um = {
            {'a', 1},
            {'b', 2},
            {'c', 3},
            {'d', 4},
            {'e', 5}

   auto ret = um.equal_range('b');

   cout << "Lower bound is " << ret.first->first 
       << " = "<< ret.first->second << endl;
   cout << "Upper bound is " << ret.second->first
       << " = " << ret.second->second << endl;

   return 0;

Let us compile and run the above program, this will produce the following result −

Lower bound is b = 2
Upper bound is c = 3