Ensuring Sorted Inputs for Set Algorithms

Ensuring that your sets are sorted correctly before using set algorithms is crucial for their correct operation. Most set algorithms in the C++ Standard Library, such as std::ranges::set_union(), std::ranges::set_intersection(), and std::ranges::set_difference(), require sorted inputs to function correctly.

Sorting the Inputs

You can use the std::sort() function to sort your sets before passing them to set algorithms. Here’s how to do it:

1. Include the necessary headers: Include <algorithm> for sorting and set operations.
2. Sort the inputs: Use std::sort() to sort your containers.
3. Perform the set operation: Call the desired set algorithm with the sorted inputs.

Here’s an example:

1#include <algorithm>
2#include <iostream>
3#include <vector>
4
5int main() {
6  std::vector<int> A{3, 1, 4, 1, 5};
7  std::vector<int> B{5, 9, 2, 6, 5};
8  std::vector<int> Results;
9  Results.resize(A.size() + B.size());
10
11  // Sort the inputs
12  std::sort(A.begin(), A.end());
13  std::sort(B.begin(), B.end());
14
15  // Perform the set union
16  auto [AEnd, BEnd, UnionEnd] =
17    std::ranges::set_union(A, B, Results.begin());
18
19  Results.erase(UnionEnd, Results.end());
20
21  for (auto x : Results) {
22    std::cout << x << ", ";
23  }
24}

11, 1, 2, 3, 4, 5, 5, 6, 9,

Sorting with Custom Comparators

If you need a custom order, you can use a custom comparator with std::sort():

1#include <algorithm>
2#include <iostream>
3#include <vector>
4
5int main() {
6  std::vector<int> A{3, 1, 4, 1, 5};
7  std::vector<int> B{5, 9, 2, 6, 5};
8  std::vector<int> Results;
9  Results.resize(A.size() + B.size());
10
11  // Custom comparator for descending order
12  auto Comparer = [](const int& a, const int& b) {
13    return a > b;
14  };
15
16  // Sort the inputs
17  std::sort(A.begin(), A.end(), Comparer);
18  std::sort(B.begin(), B.end(), Comparer);
19
20  // Perform the set union
21  auto [AEnd, BEnd, UnionEnd] =
22      std::ranges::set_union(
23        A, B, Results.begin(), Comparer);
24
25  Results.erase(UnionEnd, Results.end());
26
27  for (auto x : Results) {
28    std::cout << x << ", ";
29  }
30}

19, 6, 5, 5, 4, 3, 2, 1, 1,

Verification

After sorting, it’s good practice to verify that your sets are sorted correctly:

1#include <algorithm>
2#include <iostream>
3#include <vector>
4
5bool is_sorted(const std::vector<int>& vec) {
6  return std::is_sorted(vec.begin(), vec.end());
7}
8
9int main() {
10  std::vector<int> A{3, 1, 4, 1, 5};
11
12  std::sort(A.begin(), A.end());
13
14  if (is_sorted(A)) {
15    std::cout << "A is sorted";
16  } else {
17    std::cout << "A is not sorted";
18  }
19}

1A is sorted

Summary

• Use std::sort() to ensure your inputs are sorted.
• Custom comparators can be used for custom sorting orders.
• Verify sorting with std::is_sorted().

Correctly sorting your sets ensures that set algorithms work as expected and produce correct results.