C++ set empty()

C++ empty() function is used to check whether the set container is empty or not. It returns true if the set container is empty (size is 0) otherwise, it returns false.

Syntax

bool empty() const;               // until C++ 11
bool empty const noexcept;    //since C++ 11

Parameter

None

Return value

It returns true if the set container is empty (size is 0) otherwise, it returns false.

Complexity

Constant.

Iterator validity

No changes.

Data Races

The container is accessed.

Concurrently accessing the elements of set is safe.

Exception Safety

This function never throws exception.

Example 1

Let's see the simple example to check if a set contains any element or not:

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

int main()
{
    set<int> numbers;
    cout << " Initially, numbers.empty(): " << numbers.empty() << "\n";
    numbers = {100, 200, 300};
    cout << "\n After adding elements, numbers.empty(): " << numbers.empty() << "\n";
}

Output:

 Initially, numbers.empty(): 1

 After adding elements, numbers.empty(): 0

In the above example, initially size of set is 0 hence, empty() function returns 1(true) and after adding elements it returns 0(false).

Example 2

Let's see a simple example to check whether set is empty or not:

#include <iostream>
#include <set>

using namespace std;

int main(void) {

   set<char> s;

   if (s.empty())
      cout << "Set is empty." << endl;

   s = {100};

   if (!s.empty())
      cout << "Set is not empty." << endl;

   return 0;
}

Output:

Set is empty
Set is not empty

In the above example, if condition statement is used. If set is empty, it will return set is empty after and adding elements, it will return set is not empty.

Example 3

Let's see a simple example:

#include <iostream>
#include <set>

using namespace std;

int main ()
{
  set<int> myset;

  myset = {100, 200, 300};

  while (!myset.empty())
  {
    cout << *myset.begin()<< '\n';
    myset.erase(*myset.begin());
  }

  return 0;
}

Output:

100
200
300

In the above example, It simply uses the empty() function in while loop and prints the elements of set until the set is not empty.

Example 4

Let's see a simple example:

#include <iostream>
#include <set>
#include <string>

using namespace std;

int main() {

  typedef set<int> phoneSet;
   
   int number;
   phoneSet phone;
   
   if (phone.empty())
      cout << "Set is empty. Please insert content! \n " << endl;
   
   cout<<"Enter three sets of number: \n";
   
   for(int i =0; i<3; i++)
   {
       cin>> number;    // Get value
       phone.insert(number);   // Put them in set
   }

   if (!phone.empty())
   {
      cout<<"\nList of telephone numbers: \n";
      phoneSet::iterator p;
      for(p = phone.begin(); p!=phone.end(); p++)
      {
          cout<<(*p)<<" \n ";
      }
   }
   return 0;
}

Output:


Set is empty. Please insert content! 
 
Enter three sets of number: 
1111
5555
3333

List of telephone numbers: 
1111 
3333 
5555

In the above example, the program first creates phone set interactively with three set of numbers, then it checks if the set is empty or not. If set is empty, it displays a message otherwise, it displays all the telephone numbers available in the set.

Next TopicSet size() Function

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