C++ multiset erase()

C++ Multimultiset erase() function is used to remove either a single element associated with given key or a range of elements ([first, last)) from the multiset container. Hence, the size will be reduced by the number of elements removed.

Syntax

void erase (iterator position);                       	  //until C++ 11
size_type erase (const value_type& val);    		  //until C++ 11
void erase (iterator first, iterator last);  		  //until C++ 11
iterator  erase (const_iterator position);		  //since C++ 11
size_type erase (const value_type& val);		  //since C++ 11	
iterator  erase (const_iterator first, const_iterator last); //since C++ 11

Parameter

position: Iterator pointing to an element to be removed from the container.

val: Value to be removed from the multiset.

first: Beginning of the range to erase.

last: End of the range to erase.

Return value

The erase() function returns an iterator that point to the next element of the deleted element or returns the number of deleted elements.

Complexity

erase(position): Amortized constant.

erase (val): Logarithmic in container size.

erase(first,last): Linear in the distance between first and last of the container.

Iterator validity

Iterators, references and the pointers pointing to elements removed by the function are invalidated.All other iterators, pointers and references keep their validity.

Data Races

The container is modified.

The elements removed are modified. Iterating ranges in the container is not safe although concurrently accessing other elements is safe.

Exception Safety

This function does not throw exception.

If an invalid range or position is specified, it causes undefined behavior.

Example 1

Let's see the simple example to erase the element by the iterator.

#include <iostream>
#include <set>

using namespace std;

int main ()
{
  multiset<int> mymultiset;
  multiset<int>::iterator it;

  mymultiset = {10,20,30,20};
  
  cout<<"Before erasing the element: \n";
   for (it=mymultiset.begin(); it!=mymultiset.end(); ++it)
    cout << *it << '\n';

  it=mymultiset.find('b');
  mymultiset.erase (*it);                   // erasing by iterator

  cout<<"\nAfter erasing the element: \n";
  for (it=mymultiset.begin(); it!=mymultiset.end(); ++it)
    cout << *it << '\n';
    
  return 0;
}

Output:

Before erasing the element: 
10
20
20
30

After erasing the element: 
10
20
20
30

In the above example, element is erased by the iterator 'it'.

Example 2

Let's see a simple example to erase the element of the multiset with the given key value:

#include <iostream>
#include <set>

using namespace std;

int main ()
{
  multiset<int> mymultiset;
  multiset<int>::iterator it;

  mymultiset = {10, 20, 30, 40, 10, 30};
  
  cout<<"Before erasing the element: \n";
   for (it=mymultiset.begin(); it!=mymultiset.end(); ++it)
    cout << *it<< '\n';

   mymultiset.erase (30);                  // erasing by value

  cout<<"\nAfter erasing the element: \n";
  for (it=mymultiset.begin(); it!=mymultiset.end(); ++it)
    cout << *it<< '\n';
  return 0;
}

Output:

Before erasing the element: 
10
10
20
30
30
40

After erasing the element: 
10
10
20
40

In the above example, erase(value) function uses the value 30 from the multiset.

Example 3

Let's see a simple example to erase the element by the given range:

#include <iostream>
#include <set>

using namespace std;

int main ()
{
  multiset<int> mymultiset;
  multiset<int>::iterator it;

  mymultiset = {10, 20, 30, 20};
  
  cout<<"Before erasing the element are: \n";
   cout<<"Size is: "<<mymultiset.size()<<'\n';
   for (it=mymultiset.begin(); it!=mymultiset.end(); ++it)
   cout << *it << '\n';

   mymultiset.erase ( mymultiset.begin () ,  mymultiset.end () );   // erasing by range

  cout<<"\nAfter erasing the element are: \n";
  cout<<"Size is: "<<mymultiset.size();
  for (it=mymultiset.begin(); it!=mymultiset.end(); ++it)
  cout << *it << '\n';
  return 0;
}

Output:

Before erasing the element are: 
Size is: 3
10
20
30

After erasing the element are: 
Size is: 0

In the above example, erase(first, last) function is used to erase the element with the given range i.e. begin to end.

Example 4

Let's see a simple example to erase all the odd numbers from the multiset:

#include <set>
#include <iostream>

using namespace std;

int main()
{
    multiset<int> m = {1,2,3,4,5,6,7,8,9,10,11,12,11,10,4};
                          
    // erase all odd numbers from m
    cout<<"After erasing odd numbers,elements are:\n ";
    for(auto it = m.begin(); it != m.end(); )
        if(*it % 2 == 1)
            it = m.erase(it);
        else
            ++it;
    for(auto& p : m)
        cout << p << ", ";
}

Output:

After erasing odd numbers, elements are:
 2, 4, 4, 6, 8, 10, 10, 12,

In the above example, all the odd numbers has been erased and displaying even numbers.

Example 5

Let's see another example:

#include <set>  
#include <string>  
#include <iostream>  
#include <iterator> // next() and prev() helper functions  
  
using namespace std;  
  
using mymultiset = multiset<string>;  
  
void printmultiset(const mymultiset& s) {  
    for (const auto& iter : s) {  
        cout << " [" << iter << "]";  
    }  
    cout << endl << "size() == " << s.size() << endl << endl;  
}  
  
int main()  
{  
    mymultiset s1;  
  
    // Fill in some data to test with, one at a time  
    s1.insert("Bob");  
    s1.insert("Robert");  
    s1.insert("Rob");  
    s1.insert("Rob");  
    s1.insert("Bob");  
  
    cout << "Starting data of multiset s1 is:" << endl;  
    printmultiset(s1);  
    // The 1st member function removes an element at a given position  
    s1.erase(next(s1.begin()));  
    cout << "After the 2nd element is deleted, the multiset s1 is:" << endl;  
    printmultiset(s1);  
  
    // Fill in some data to test with, one at a time, using an intializer list  
    mymultiset s2{ "meow", "nikita", "nikita", "growl", "yellow" };  
  
    cout << "Starting data of multiset s2 is:" << endl;  
    printmultiset(s2);  
    // The 2nd member function removes elements  
    // in the range [First, Last)  
    s2.erase(next(s2.begin()), prev(s2.end()));  
    cout << "After the middle elements are deleted, the multiset s2 is:" << endl;  
    printmultiset(s2);  
  
    mymultiset s3;  
  
    // Fill in some data to test with, one at a time, using emplace  
    s3.emplace("C");  
    s3.emplace("C#");  
    s3.emplace("D");  
    s3.emplace("D#");  
    s3.emplace("E");  
    s3.emplace("E#");  
    s3.emplace("F");  
    s3.emplace("F#");  
    s3.emplace("G");  
    s3.emplace("G#");  
    s3.emplace("E");  
    s3.emplace("E#");  
    s3.emplace("B");  
  
    cout << "Starting data of multiset s3 is:" << endl;  
    printmultiset(s3);  
    // The 3rd member function removes elements with a given Key  
    mymultiset::size_type count = s3.erase("E#");  
    // The 3rd member function also returns the number of elements removed  
    cout << "The number of elements removed from s3 is: " << count << "." << endl;  
    cout << "After the element with a key of \"E#\" is deleted, the multiset s3 is:" << endl;  
    printmultiset(s3);  
}  

Output:

Starting data of multiset s1 is:
 [Bob] [Bob] [Rob] [Rob] [Robert]
size() == 5

After the 2nd element is deleted, the multiset s1 is:
 [Bob] [Rob] [Rob] [Robert]
size() == 4

Starting data of multiset s2 is:
 [growl] [meow] [nikita] [nikita] [yellow]
size() == 5

After the middle elements are deleted, the multiset s2 is:
 [growl] [yellow]
size() == 2

Starting data of multiset s3 is:
 [B] [C] [C#] [D] [D#] [E] [E] [E#] [E#] [F] [F#] [G] [G#]
size() == 13

The number of elements removed from s3 is: 2.
After the element with a key of "E#" is deleted, the multiset s3 is:
 [B] [C] [C#] [D] [D#] [E] [E] [F] [F#] [G] [G#]
size() == 11

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