C++ Unordered_set:: begin() Function

The C++ std::unordered_set::begin() function is used to return an iterator pointing to the first element of the unordered_set container. If the unordered_set is empty, the returned iterator will be equal to the end()

An iterator is an object (like a pointer) over elements that provides access to each individual element. Iterators can be used to modify pointed elements available in the unordered_set container.

Syntax

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

iterator begin() noexcept;
const_iterator begin() const noexcept;
or
local_iterator begin ( size_type n );
const_local_iterator begin ( size_type n ) const;

Parameters

  • n &minus It indicates the bucket number that must be less than bucket_count.

Return value

This function returns an iterator pointing to the first element in the unordered_set container or an iterator pointing to the first element of the specified bucket.

Example 1

Let's look at the following example, where we are going to demonstrate the usage of unordered_set::begin() function.

#include <iostream>
#include <string>
#include <unordered_set>
using namespace std;
int main () {
   unordered_set<string> myUset =
      {"100","200","300","400","500","600","700","800"};
   cout<<"Contents of the myUset are: "<<endl;
   for(auto it: myUset)
      cout<<it<<" ";
      cout<<"\nAn iterator of the first is: ";
      auto it = myUset.begin();
      cout<<*it;
   return 0;
}

Output

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

Contents of the myUset are: 
700 600 500 800 400 300 200 100 
An iterator of the first is: 700

Example 2

Consider the following example, where we are going to use the begin() function inside a for loop to display the elements of the container.

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

int main () {
   std::unordered_set<std::string> myUset = {"100","200","300","400","500"};
      
   std::cout << "myUset contains:";
   for ( auto it = myUset.begin(); it != myUset.end(); ++it )
      std::cout << " " << *it;
   std::cout << std::endl;
   
   return 0;
}

Output

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

myUset contains: 500 400 300 200 100

Example 3

In the following example, we are going to use the begin() function that accepts i as a parameter to return the element of each bucket.

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

int main () {
   std::unordered_set<std::string> myUset = {"100", "200", "300", "400", "500"};
      
   std::cout << "myUset's buckets contain:\n";
   for ( unsigned i = 0; i < myUset.bucket_count(); ++i) {
      std::cout << "bucket #" << i << " contains:";
      for ( auto local_it = myUset.begin(i); local_it!= myUset.end(i); ++local_it )
         std::cout << " " << *local_it;
      std::cout << std::endl;
   }
   return 0;
}

Output

Following is the output of the above code −

myUset's buckets contain:
bucket #0 contains:
bucket #1 contains: 400
bucket #2 contains: 500
bucket #3 contains:
bucket #4 contains: 100
bucket #5 contains:
bucket #6 contains:
bucket #7 contains:
bucket #8 contains:
bucket #9 contains:
bucket #10 contains: 300
bucket #11 contains: 200
bucket #12 contains:

Example 4

Following is the another example of the usage of begin() function to get the iterator pointing to the first element of the specified bucket.

#include <iostream>
#include <string>
#include <unordered_set>
using namespace std;
int main () {
   unordered_set<int> myUset = {10, 20, 30, 40, 50};
      
   cout << "Iterator pointing to the first element of the bucket 4 is: ";
   auto it = myUset.begin(4);
   cout<<*it<<endl;
   return 0;
}

Output

Output of the above code is as follows −

Iterator pointing to the first element of the bucket 4 is: 30
Advertisements