C++ multimap clear() function

The C++ multimap clear() function is used to remove all the elements of the multimap container. It clears the multimap and sets its size to 0.

Syntax

void clear();                 //until C++ 11
void clear() noexcept;      //since C++ 11

Parameter

None

Return value

None

Complexity

Linear in size.

Iterator validity

All iterator, references and pointers related to this container are invalidated.

Data Races

The container is modified.

All contained elements are modified.

Exception safety

This function never throws exception.

Example 1

Let's see the simple example to calculate the size of multimap before and after clear operation:

#include <iostream>
#include <map>

using namespace std;

int main() {

   multimap<char, int> mymultimap = {
            {'a', 1},
            {'b', 2},
            {'b', 3},
            {'d', 4},
            {'d', 5},
            };

   cout << "Initial size of multimap before clear operation = " << mymultimap.size() << endl;

   mymultimap.clear();

   cout << "Size of multimap after clear operation = " << mymultimap.size() << endl;

   return 0;
}

Output:

Initial size of multimap before clear operation = 5
Size of multimap after clear operation = 0

In the above example, a multimap is initialized with 5 elements therefore, the size is 5 but after the clear operation, size becomes 0.

Example 2

Let's see a simple example to clear the elements of the multimap:

#include <iostream>
#include <map>

using namespace std;

int main ()
{
  multimap<int,string> mymultimap;

  mymultimap = {
               {1, "Nikita"},
               {2, "Divya" },
               {1, "Ashish"} };

  cout << "mymultimap contains:\n";
  for (multimap<int,string>::iterator it=mymultimap.begin(); it!=mymultimap.end(); ++it)
    cout << it->first << " : " << it->second << '\n';

  mymultimap.clear();
  
  mymultimap = {
               {1, "Aman"},
               {2, "Deep"}
               };
               
  cout << "\nmymultimap contains:\n";
  for (multimap<int,string>::iterator it=mymultimap.begin(); it!=mymultimap.end(); ++it)
    cout << it->first << " : " << it->second << '\n';

  return 0;
}

Output:

mymultimap contains:
1 : Nikita
1 : Ashish
2 : Divya

mymultimap contains:
1 : Aman
2 : Deep

In the above example, after clear the multimap, we can add the new elements without initialization.

Example 3

Let's see a simple example to clear the elements of the multimap:

#include <iostream>
#include <map>

using namespace std;

int main ()
{
  int n;
  multimap<int,string> m1,m2,m3;

  m1 = { {1, "Nikita"},
         {2, "Deep"},
         {1, "Ashish"}  };
  
  m2 = { {1, "Nidhi"},
        {2, "Priya"},
        {2, "Gitanjali"} };
  
  m3 = { {1, "Manas"},
       {3, "Fruti"},
       {3, "Kamlesh"} };
  

  cout << "m1 group has following members:\n";
  for (multimap<int,string>::iterator it=m1.begin(); it!=m1.end(); ++it)
    cout << it->first << " : " << it->second << '\n';
  

  cout << "m2 group has following members:\n";
  for (multimap<int,string>::iterator it=m2.begin(); it!=m2.end(); ++it)
    cout << it->first << " : " << it->second << '\n';

  cout << "m3 group has following members:\n";
  for (multimap<int,string>::iterator it=m3.begin(); it!=m3.end(); ++it)
    cout << it->first << " : " << it->second << '\n';
  
  cout<<"\nWhich group do you want to delete?\n 1.m1\n 2.m2\n 3.m3\n Please enter your choice: ";
  cin>>n;
  
  if(n==1){
  m1.clear();
  cout<<"\nGroup m1 has been cleared.";
  }
  else if(n==2){
  m2.clear();
  cout<<"\nGroup m2 has been cleared.";
  }
  else if(n==3){
  m3.clear();
  cout<<"\nGroup m3 has been cleared.";
  }
  else
  cout<<"Invalid option!";
  
  return 0;
}

Output:

m1 group has following members:
1 : Nikita
1 : Ashish
2 : Deep

m2 group has following members:
1 : Nidhi
2 : Priya
2 : Gitanjali

m3 group has following members:
1 : Manas
3 : Fruti
3 : Kamlesh

Which group do you want to delete?
 1.m1
 2.m2
 3.m3
 Please enter your choice: 2

Group m2 has been cleared.

In the above example, there are three groups of multimap and according to the user?s choice one group has been deleted.

Example 4

Let's see a simple example:

#include <iostream>
#include <map>

using namespace std;

int main() {
    
   int n;
   
   multimap<string, int> fruit = {
            {"Banana", 40},
            {"Apple", 190},
            {"Orange", 120},
            };

   cout << "Fruit bucket has following fruits = \n";
   for (multimap<string,int>::iterator it=fruit.begin(); it!=fruit.end(); ++it)
    cout << it->first << " : " << it->second << '\n';

   cout<<"\nDo you want to clear your fruit bucket?\nPress 1 for Yes and 0 for No: ";
   cin>>n;
   
   if( n==1){
   fruit.clear();
   cout<<fruit.size()<<" fruits in bucket \n";  
   }
   else if(n==0)
   cout <<fruit.size() << " fruits in bucket \n" ;
    
   
   return 0;
}

Output:

1. 
Fruit bucket has following fruits = 
Apple : 190
Banana : 40
Orange : 120

Do you want to clear your fruit bucket?
Press 1 for Yes and 0 for No: 0
3 fruits in bucket

2. 
Fruit bucket has following fruits = 
Apple : 190
Banana : 40
Orange : 120

Do you want to clear your fruit bucket?
Press 1 for Yes and 0 for No: 1
0 fruits in bucket

In the above example, a fruit multimap is initialized with three fruits. Asking for clear the multimap if you enter 0 then fruit bucket has 3 elements or if you enter 1 then it will clear the fruit multimap and the size becomes 0.

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