C++ unordered_set - insert() Function
The C++ unordered_set::insert function is used to insert new elements in the container. This results into increasing the unordered_set size by the number of elements inserted. As the elements in a unordered_set are unique, therefore the insertion operation first checks if the inserted element is unique to the unordered_set then the element is inserted.
Syntax
//single element version pair<iterator,bool> insert (const value_type& val); pair<iterator,bool> insert (value_type&& val); //single element with hint version iterator insert (const_iterator position, const value_type& val); iterator insert (const_iterator position, value_type&& val); //range version template <class InputIterator> void insert (InputIterator first, InputIterator last); //initializer list version void insert (initializer_list<value_type> il);
Parameters
position |
Specify the hint for the position where the element can be inserted. |
val |
Specify the value to be copied (or moved) to the inserted elements. |
first |
Specify the starting position of InputIterator. Copies of the elements in the range [first,last) are inserted in the container. |
last |
Specify the last position of InputIterator. Copies of the elements in the range [first,last) are inserted in the container. |
il |
Specify the initializer_list object. |
Return Value
The single element version returns a pair, with pair::first unordered_set to an iterator pointing to either newly inserted element or to the equivalent element present in the unordered_set. The pair::second is unordered_set to true if new element is inserted in the unordered_set, false otherwise.
The single element with hint version returns an iterator pointing to either newly inserted element or to the equivalent element present in the unordered_set.
Time Complexity
- Single element insertion: Average case - constant i.e, Θ(1). Worst case - linear i.e, Θ(n).
- Multiple elements insertion: Average case - linear in number of elements inserted. Worst case - quadratic: number of elements inserted multiplied by (container size + 1).
Example:
In the example below, the unordered_set::insert function is used to insert elements in the given unordered_set.
#include <iostream>
#include <unordered_set>
using namespace std;
int main (){
unordered_set<int> uSet1 = {10, 20, 30};
unordered_set<int> uSet2 = {10, 20, 30};
unordered_set<int>::iterator it;
//single element version
uSet1.insert(55);
//single element with hint version
it = uSet2.begin();
uSet2.insert(++it, 15);
cout<<"uSet1 contains: ";
for(it = uSet1.begin(); it != uSet1.end(); ++it)
cout<<*it<<" ";
cout<<"\nuSet2 contains: ";
for(it = uSet2.begin(); it != uSet2.end(); ++it)
cout<<*it<<" ";
return 0;
}
The output of the above code will be:
uSet1 contains: 10 55 20 30 uSet2 contains: 15 10 20 30
Example:
A range of elements can also be inserted into a unordered_set. Consider the example below.
#include <iostream>
#include <unordered_set>
#include <vector>
using namespace std;
int main (){
unordered_set<int> uSet = {10, 20, 30};
vector<int> MyVec = {15, 30, 45, 60, 75};
unordered_set<int>::iterator set_it;
vector<int>::iterator vec_it;
//range version - insert a range of
//elements of MyVec into uSet
vec_it = MyVec.begin();
uSet.insert(vec_it, vec_it + 3);
cout<<"uSet contains: ";
for(set_it = uSet.begin(); set_it != uSet.end(); ++set_it)
cout<<*set_it<<" ";
return 0;
}
The output of the above code will be:
uSet contains: 15 45 10 20 30
Example:
Similarly, the initializer list version can be used to insert elements in the given unordered_set.
#include <iostream>
#include <unordered_set>
using namespace std;
int main (){
unordered_set<int> uSet = {10, 15};
unordered_set<int>::iterator it;
//initializer list version
initializer_list<int> ilist = {11, 12, 13, 14};
uSet.insert(ilist);
cout<<"uSet contains: ";
for(it = uSet.begin(); it != uSet.end(); ++it)
cout<<*it<<" ";
return 0;
}
The output of the above code will be:
uSet contains: 14 13 12 11 10 15
❮ C++ <unordered_set> Library
