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C++ Unordered Set Insert

The C++ std::unordered_set::insert() function is used to insert the elements into the unordered_set and extend the container size by one. This function does not insert duplicate elements, and it only inserts an element if the unordered_set container does not contain equivalent element to the specified element.

This function has 4 polymorphic variants: with using the hint, value, range, and ilist (you can find the syntaxes of all the variants below).

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

pair<iterator,bool> insert (const value_type& val);
or
iterator insert (const_iterator hint, const value_type& val);
or
void insert (InputIterator first, InputIterator last);
or
void insert (initializer_list<value_type> il);

• hint − It specify the hint for the position to insert the element.
• val − It indicates the value that we want to insert in the unordered_set
• first, last − It specify the range to insert the elements from the iterator first to the last.
• ilist − It specify the initializer_list object.

The function returns an iterator pointing to the newly inserted element.

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

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

int main(void) {
   unordered_set<char> myUset = {'a', 'b'};
   auto ins = myUset.insert('d');
   cout<<"'d' was inserted: "<< boolalpha << ins.second << '\n';
   cout<<" After insertion unordered_set contains"<<endl;
   for (auto it: myUset)
      cout<<" "<< it<<endl;
   return 0;
}

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

'd' was inserted: true
 After insertion unordered_set contains
 d
 b
 a

Consider the following example, where we are going to insert a value at the beginning of the current unordered_set.

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

int main(void) {
   unordered_set<char> myUset = {'a', 'b'};
   auto it =  myUset.begin();
   myUset.insert(it, 'd');
   cout<<" After insertion unordered_set contains"<<endl;
   for (auto it: myUset)
      cout<<" "<< it<<endl;
   return 0;
}

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

After insertion unordered_set contains
 d
 b
 a

Inserting elements in the range −

Following is the another example of the usage of insert() function to insert the array element into the unordered_set.

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

int main(void) {
   unordered_set<int> myUset = {1, 2};
   array<int, 4> a  = {10, 11, 12, 13};
   myUset.insert (a.begin (), a.end ()); // insert range
   cout<<" After insertion unordered_set contains"<<endl;
   for (auto it: myUset)
      cout<< it<<" ";
   return 0;
}

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

After insertion unordered_set contains
13 12 11 10 2 1 

In the following example, we are going to consider the two unordered_set one is filled with elements and another one is empty and performing the insert() function.

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

int main(void) {
   unordered_set<int> uSet = {1, 2, 3, 5};
   //declare an empty unordered set
   unordered_set<int> myUset;
   //inserting element into the empty map from the first map
   myUset.insert(uSet.begin(), uSet.end());
   cout << "myUset contains following elements" << endl;
   for (auto it: myUset)
      cout <<it<<" ";
   return 0;
}

Following is the output of the above code −

myUset contains following elements
1 2 3 5

Following is the example, where we are going to consider the empty unordered-set and inserting the elements using the iterator.

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

int main(void) {
   //declare an empty unordered set
   unordered_set<int> myUset;
   myUset.insert ({20, 21, 22, 23}); // insert initializer_list
 
   cout << "myUset contains following elements after insertion: " << endl;
   for (auto it: myUset)
      cout <<it<<" ";
   return 0;
}

Output of the above code is as follows −

myUset contains following elements after insertion: 
23 22 21 20 

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