C++ Unordered_set::size() Function

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The C++ std::unordered_set::size() function is used to return the number of elements in the unordered_set container. or we can say that this function calculates the distance between begin() and end(). If the unordered_set does not contain any elements, then this function returns 0.

Syntax

Following is the syntax of std::unordered_set::size.

size_type size() const noexcept;

Parameters

This function does not accepts any parameter.

Return Value

This function returns the number of elements in the unordered_set container.

Example 1

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

#include <iostream>
#include <string>
#include <unordered_set>

int main () {
   std::unordered_set<std::string> myset;
   std::cout << "0. size: " << myset.size() << std::endl;

   myset = {"sairamkrishna","mammahe"};
   std::cout << "1. size: " << myset.size() << std::endl;

   myset.insert ("kittuprasad");
   std::cout << "2. size: " << myset.size() << std::endl;

   myset.erase ("tutorialspoint");
   std::cout << "3. size: " << myset.size() << std::endl;

   return 0;
}

Output

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

0. size: 0
1. size: 2
2. size: 3
3. size: 3

Example 2

Let's look at the following example, where we are going to use the unordered_set::size() function to get the total size of the set and also displaying all the elements of myUset.

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

int main () {
   unordered_set<int> myUset = {1, 2, 3, 4, 5};
   int size = myUset.size();
   cout<<"Total size of unordered set: "<<size<<endl;
   cout<<"Each element of the unordered_set: ";
   for(auto it: myUset)
      cout<<it<<" ";
   return 0;
}

Output

Following is the output of the above code −

Total size of unordered set: 5
Each element of the unordered_set: 5 4 3 2 1

Example 3

In the following example, we are going to consider the empty unordered_set and applying the unordered_set::size() function to get the total size of the unordered_set before and after the insertion of elements.

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

int main () {
   unordered_set<int> myUset;
   int size = myUset.size();
   cout<<"Total size of unordered set: "<<size<<endl;
   
   myUset.insert({10, 20, 30});
   int tSize = myUset.size();
   cout<<"Total size of unordered set after insertion: "<<tSize<<endl;
   return 0;
}

Output

Output of the above code is as follows −

Total size of unordered set: 0
Total size of unordered set after insertion: 3