Set Algorithms

# Ensuring Sorted Inputs for Set Algorithms

## How do I ensure my sets are sorted correctly before using 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 functionÂ correctly.

### Sorting the Inputs

You can use the std::sort() function to sort your sets before passing them to set algorithms. Hereâ€™s how to doÂ 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.

Hereâ€™s anÂ 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, itâ€™s good practice to verify that your sets are sortedÂ 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 correctÂ results.

This Question is from the Lesson:

### Set Algorithms

An introduction to set algorithms, and how to implement them using the C++ standard library

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

This Question is from the Lesson:

### Set Algorithms

An introduction to set algorithms, and how to implement them using the C++ standard library

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