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C++ Unordered Set Erase

The C++ std::unordered_set::erase() function is used to remove the specified element from the unordered_set container, either a single element or a range of elements.

This function has 3 polymorphic variants: with using the erase by position/iterator, erase by range and erase by key (you can find the syntaxes of all the variants below).

Following is the syntax of std::unordered_set::erase() function.

iterator erase ( const_iterator position );
or
size_type erase ( const key_type& k );
or
iterator erase ( const_iterator first, const_iterator last );

• position − It indicates iterator pointing to a single element to be removed.
• k − It indicates the value of element to be removed.
• (first, last) − It specify a range within the unordered_set.

The function returns the member type iterator is a forward iterator type.

Consider the following example, where we are going to demonstrate the usage of unordered_set::erase() function.

#include <iostream>
#include <string>
#include <unordered_set>
using namespace std;

int main () {
   unordered_set<string> uSet = {"USA","Canada","France","UK","Japan","Germany","Italy"};
   uSet.erase ( uSet.begin() );                  
   uSet.erase ( "France" );                       
   uSet.erase ( uSet.find("Japan"), uSet.end() ); 

   cout << "uSet contains:";
   for ( const string& x: uSet ) cout << " " << x;
      cout << endl;
   return 0;
}

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

uSet contains: Germany

Let's look at the following example, where we are going to use the erase()function to remove the odd element from the container.

#include <unordered_set>
#include <iostream>
using namespace std;

int main() {
   unordered_set<int> uSet = {10, 11, 12, 15, 17, 18};
 
   cout << "After erasing all odd numbers: ";
   for (auto it = uSet.begin(); it != uSet.end();) {
      if (*it % 2 != 0)
         it = uSet.erase(it);
      else
         ++it;
   }
    
   cout<<"{ ";
      for(auto elem: uSet){
         cout<<elem<<" ";
      }
   cout<<"}";
 
   return 0;
}

If we run the above code it will generate the following output −

After erasing all odd numbers: { 18 12 10 }

In the following example, we are going to erase the specified elements by using the erase() function.

#include <iostream>
#include <string>
#include <unordered_set>
using namespace std;

int main () {
   unordered_set<int> uSet = {10, 20, 30, 40, 50};
   uSet.erase ( 30 );   
   uSet.erase ( 40 ); 

   cout << "uSet contains:";
   for ( auto it: uSet )
      cout << " " << it;
   cout << endl;
   return 0;
}

Following is the output of the above code −

uSet contains: 50 20 10

Following is the example, where we are going to erase the elements in the range.

#include <iostream>
#include <string>
#include <unordered_set>
using namespace std;

int main () {
   unordered_set<char> uSet = {'a', 'b', 'c', 'd', 'e'};
   cout <<"uSet contains:";
   for ( auto it: uSet )
      cout <<" " <<it;
   cout <<endl;
   
   uSet.erase ( uSet.begin(), uSet.find('c') ); 
   
   cout <<"after erased uSet contains:";
   for ( auto it: uSet )
      cout <<" " <<it;
   cout <<endl;
   return 0;
}

Output of the above code is as follows −

uSet contains: e d c b a
after erased uSet contains: c b a

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