Set Algorithms

Ensuring Sorted Inputs for Set Algorithms

How do I ensure my sets are sorted correctly before using set algorithms?

Abstract art representing computer programming

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:

#include <algorithm>
#include <iostream>
#include <vector>

int main() {
  std::vector<int> A{3, 1, 4, 1, 5};
  std::vector<int> B{5, 9, 2, 6, 5};
  std::vector<int> Results;
  Results.resize(A.size() + B.size());

  // Sort the inputs
  std::sort(A.begin(), A.end());
  std::sort(B.begin(), B.end());

  // Perform the set union
  auto [AEnd, BEnd, UnionEnd] =
    std::ranges::set_union(A, B, Results.begin());

  Results.erase(UnionEnd, Results.end());

  for (auto x : Results) {
    std::cout << x << ", ";
  }
}
1, 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():

#include <algorithm>
#include <iostream>
#include <vector>

int main() {
  std::vector<int> A{3, 1, 4, 1, 5};
  std::vector<int> B{5, 9, 2, 6, 5};
  std::vector<int> Results;
  Results.resize(A.size() + B.size());

  // Custom comparator for descending order
  auto Comparer = [](const int& a, const int& b) {
    return a > b;
  };

  // Sort the inputs
  std::sort(A.begin(), A.end(), Comparer);
  std::sort(B.begin(), B.end(), Comparer);

  // Perform the set union
  auto [AEnd, BEnd, UnionEnd] =
      std::ranges::set_union(
        A, B, Results.begin(), Comparer);

  Results.erase(UnionEnd, Results.end());

  for (auto x : Results) {
    std::cout << x << ", ";
  }
}
9, 6, 5, 5, 4, 3, 2, 1, 1,

Verification

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

#include <algorithm>
#include <iostream>
#include <vector>

bool is_sorted(const std::vector<int>& vec) {
  return std::is_sorted(vec.begin(), vec.end());
}

int main() {
  std::vector<int> A{3, 1, 4, 1, 5};

  std::sort(A.begin(), A.end());

  if (is_sorted(A)) {
    std::cout << "A is sorted";
  } else {
    std::cout << "A is not sorted";
  }
}
A 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.

Answers to questions are automatically generated and may not have been reviewed.

A computer programmer
Part of the course:

Professional C++

Comprehensive course covering advanced concepts, and how to use them on large-scale projects.

Free, unlimited access

This course includes:

  • 125 Lessons
  • 550+ Code Samples
  • 96% Positive Reviews
  • Regularly Updated
  • Help and FAQ
Free, Unlimited Access

Professional C++

Comprehensive course covering advanced concepts, and how to use them on large-scale projects.

Screenshot from Warhammer: Total War
Screenshot from Tomb Raider
Screenshot from Jedi: Fallen Order
Contact|Privacy Policy|Terms of Use
Copyright © 2024 - All Rights Reserved