The C++ set_union function is employed to determine the union of two sorted intervals [first1, last1) and [first2, last2), consisting of elements existing in either set or both.
Elements are compared utilizing the < operator in the initial version or through the specified binary evaluation function comp in the subsequent version.
Syntax
default (1) template <class InputIterator1, class InputIterator2, class OutputIterator>
OutputIterator set_union (InputIterator1 first1, InputIterator1 last1,
InputIterator2 first2, InputIterator2 last2, OutputIterator result);
custom (2) template <class InputIterator1, class InputIterator2,
class OutputIterator, class Compare>
OutputIterator set_union (InputIterator1 first1, InputIterator1 last1,
InputIterator2 first2, InputIterator2 last2, OutputIterator result, Compare comp);
Parameter
An input iterator pointing to the initial element in the first of two sorted origin sequences.
last1: Refers to an input iterator indicating the element after the last element in the initial sorted sequence of the two source sequences.
An initial iterator pointing to the first item in the subsequent sorted origin sequence.
An input iterator pointing to the element just before the final element in the second sorted origin sequence.
A custom binary predicate function named comp is defined by the user. This function takes two arguments and evaluates to true if the arguments are in a specified order, otherwise it returns false. It adheres to the principles of strict weak ordering for arranging the elements.
An output iterator pointing to the location of the initial element in the target range.
Return value
This function provides an iterator pointing to the conclusion of the created range.
Complexity
The time complexity is proportional to the interval's length between [first1, last1) and [first2, last2): executing a maximum of 2*(count1+count2)-1 comparisons. Here, count1 is calculated as last1 - first1, and count2 is determined as last2 - first2.
Data races
The elements within the range [first1, last1) and [first2, last2) are both accessed.
The items within the range from the result to the output value are altered.
Exceptions
This function will raise an exception if any error occurs during element comparison, element assignments, or iterator operation.
Note: The invalid parameters cause an undefined behavior.
Example 1
Let's examine a basic example to showcase the functionality of set_union:
#include <iostream>
#include <set>
#include <list>
#include <vector>
#include <algorithm>
#include <iterator>
using namespace std;
int main()
{
list<int> a = {1, 2, 3, 4};
multiset<int> b = {4, 5, 6, 2};
vector<int> result;
set_union(begin(a), end(a),
begin(b), end(b),
inserter(result, end(result)));
for_each(begin(result), end(result), [](int x) {
cout << x << endl;
});
return 0;
}
Output:
1
2
3
4
5
6
Example 2
Let's see another simple example:
#include <iostream> // std::cout
#include <algorithm> // std::set_union, std::sort
#include <vector> // std::vector
using namespace std;
int main()
{
int first[] = { 5, 10, 15, 20, 25 };
int second[] = { 50, 40, 30, 20, 10 };
int n = sizeof(first) / sizeof(first[0]);
// Print first array
cout << "First array contains:";
for (int i = 0; i < n; i++)
cout << " " << first[i];
cout << "\n";
// Print second array
cout << "Second array contains:";
for (int i = 0; i < n; i++)
cout << " " << second[i];
cout << "\n\n";
vector<int> v(10);
vector<int>::iterator it, st;
sort(first, first + n);
sort(second, second + n);
// Using default function
it = set_union(first, first + n, second, second + n, v.begin());
cout << "The union has " << (it - v.begin()) << " elements:\n";
for (st = v.begin(); st != it; ++st)
cout << ' ' << *st;
cout << '\n';
return 0;
}
Output:
First array contains: 5 10 15 20 25
Second array contains: 50 40 30 20 10
The union has 8 elements:
5 10 15 20 25 30 40 50
Example 3
Let's explore another straightforward example to identify the roster of students enrolled in both courses:
#include <iostream>
#include <algorithm>
#include <vector>
#include <string>
using namespace std;
// Driver code
int main()
{
string first[] = { "Nikita", "Divya", "Deep", "Kashish" };
string second[] = { "Aman", "Nikita", "Amita", "Deep" };
int n = sizeof(first) / sizeof(first[0]);
// Print students of first list
cout << "Students in first subject:";
for (int i = 0; i < n; i++)
cout << " " << first[i];
cout << "\n";
// Print students of second list
cout << "Students in second subject:";
for (int i = 0; i < n; i++)
cout << " " << second[i];
cout << "\n\n";
vector<string> v(10);
vector<string>::iterator it, st;
// Sorting both the list
sort(first, first + n);
sort(second, second + n);
// Using default operator<
it = set_union(first, first + n, second, second + n, v.begin());
cout << "Students attending both subjects are:\n";
for (st = v.begin(); st != it; ++st)
cout << ' ' << *st;
cout << '\n';
return 0;
}
Output:
Students in first subject: Nikita Divya Deep Kashish
Students in second subject: Aman Nikita Amita Deep
Students attending both subjects are:
Aman Amita Deep Divya Kashish Nikita
Example 4
Let's see a simple example:
#include <vector>
#include <iostream>
#include <algorithm>
#include <iterator>
using namespace std;
int main()
{
vector<char> v1 = {'A', 'B', 'C'};
vector<char> v2 = { 'C', 'D', 'E', 'F'};
vector<char> dest1;
set_union(v1.begin(), v1.end(),
v2.begin(), v2.end(),
back_inserter(dest1));
for (const auto &i : dest1) {
cout << i << ' ';
}
cout << '\n';
return 0;
}
Output:
A B C D E F
