How do I find the second smallest or second largest element?

Finding the second smallest or second largest element in a collection using the standard library algorithms involves a few additional steps.

Since std::ranges::min() and std::ranges::max() only return the smallest or largest elements, you need to remove these elements and then find the next smallest or largest.

Finding the Second Smallest Element

Here’s how you can find the second smallest element in a vector:

1#include <algorithm>
2#include <iostream>
3#include <vector>
4
5int main() {
6  std::vector<int> numbers{5, 1, 4, 2, 3};
7
8  // Find the smallest element
9  auto minIt = std::ranges::min_element(numbers);
10  int minElement = *minIt;
11
12  // Remove the smallest element
13  numbers.erase(minIt);
14
15  // Find the second smallest element
16  int secondMinElement = std::ranges::min(numbers);
17
18  std::cout << "Smallest element: "
19    << minElement << "\n";
20  std::cout << "Second smallest element: "
21    << secondMinElement;
22}

1Smallest element: 1
2Second smallest element: 2

Finding the Second Largest Element

Similarly, to find the second largest element, you can follow these steps:

1#include <algorithm>
2#include <iostream>
3#include <vector>
4
5int main() {
6  std::vector<int> numbers{5, 1, 4, 2, 3};
7
8  // Find the largest element
9  auto maxIt = std::ranges::max_element(numbers);
10  int maxElement = *maxIt;
11
12  // Remove the largest element
13  numbers.erase(maxIt);
14
15  // Find the second largest element
16  int secondMaxElement = std::ranges::max(numbers);
17
18  std::cout << "Largest element: "
19    << maxElement << "\n";
20  std::cout << "Second largest element: "
21    << secondMaxElement;
22}

1Largest element: 5
2Second largest element: 4

Explanation

1. Find the Min/Max: Use std::ranges::min_element() or std::ranges::max_element() to find the smallest or largest element.
2. Remove the Element: Use erase() to remove the found element from the collection.
3. Find the Second Min/Max: Use std::ranges::min() or std::ranges::max() to find the next smallest or largest element.

Important Considerations

• Mutating the Collection: Removing elements mutates the original collection. If you need to preserve the collection, consider working on a copy.
• Performance: Removing elements can be costly in terms of performance for large collections. If performance is critical, consider alternative approaches like sorting.

By following these steps, you can effectively find the second smallest or second largest element in a collection using C++ standard library algorithms.