std::ranges::set_union() vs std::ranges::merge()

std::ranges::set_union() and std::ranges::merge() are both used to combine two sorted ranges into a single sorted range, but they serve different purposes and have unique benefits.

std::ranges::merge()

The std::ranges::merge() algorithm combines two sorted ranges into one, including all elements from both ranges. If there are duplicate elements, they will appear in the output:

1#include <algorithm>
2#include <iostream>
3#include <ranges>
4#include <vector>
5
6int main() {
7  std::vector<int> vec1{1, 3, 5};
8  std::vector<int> vec2{2, 3, 6};
9  std::vector<int> result;
10  result.resize(vec1.size() + vec2.size());
11
12  std::ranges::merge(vec1, vec2, result.begin());
13
14  for (int n : result) {
15    std::cout << n << ", ";
16  }
17}

11, 2, 3, 3, 5, 6,

std::ranges::set_union()

The std::ranges::set_union() algorithm also combines two sorted ranges into one but removes duplicates. This is useful when you need a union of two sets without repeated elements:

1#include <algorithm>
2#include <iostream>
3#include <ranges>
4#include <vector>
5
6int main() {
7  std::vector<int> vec1{1, 3, 5};
8  std::vector<int> vec2{2, 3, 6};
9  std::vector<int> result;
10  result.resize(vec1.size() + vec2.size());
11
12  auto it = std::ranges::set_union(
13    vec1, vec2, result.begin());
14  result.erase(it.out, result.end());  
15
16  for (int n : result) {
17    std::cout << n << ", ";
18  }
19}

11, 2, 3, 5, 6,

Benefits of std::ranges::set_union()

1. No Duplicates: Automatically removes duplicates from the combined range.
2. Set Operations: Ideal for set operations where uniqueness is required.
3. Simpler Code: Eliminates the need for post-processing to remove duplicates, resulting in cleaner code.

Use Cases

Use std::ranges::merge():

• When you need to preserve all elements from both ranges, including duplicates.
• When combining data where duplicates are meaningful and necessary for the application logic.
• Suitable for scenarios like merging log files or combining data sets where all entries are needed.

Use std::ranges::set_union():

• When you need a union of two sets with no duplicates.
• Useful in mathematical set operations, ensuring the uniqueness of elements.
• Ideal for merging collections where each element should appear only once, such as combining lists of unique IDs or names.

Performance Considerations

Both algorithms are efficient, but their performance can vary based on the context:

• std::ranges::merge() might be faster when handling large data sets with many duplicates since it does not need to check for uniqueness.
• std::ranges::set_union() can save time and memory by avoiding duplicate elements, making it more efficient in scenarios where uniqueness is required.

In summary, std::ranges::set_union() is beneficial for combining ranges while ensuring uniqueness, making it ideal for set operations.

In contrast, std::ranges::merge() is better suited for scenarios where all elements, including duplicates, need to be preserved.

Understanding the specific requirements of your application will help you choose the appropriate algorithm.