中文Is the outcome of private provision of public good efficient? Why or why not? What is the efficiency condition for a public good? Measuring costs and benefits of public goods can be complicated. Why?

私人提供公共物品的结果是否有效率？为什么？公共物品的效率条件是什么？衡量公共物品的成本和效益可能会很复杂。为什么？

私人提供公共物品的结果可能不总是有效率的。这是因为私人提供者在提供公共物品时有盈利动机，他们可能不会按照最大化社会福利的方式分配资源。如果私人提供者无法捕捉到其提供所产生的全部利益，他们可能会低估公共物品的产量，导致从社会福利角度来看供应水平不够。

公共物品的效率条件是其边际社会效益应该等于其边际社会成本。这意味着公共物品所产生的总效益应该等于其总成本，考虑到与其供应相关的所有正面和负面外部性。

衡量公共物品的成本和效益可能会很复杂，因为它们通常具有外部性，这意味着与其提供相关的利益或成本并未完全被市场捕捉。例如，公园建设可能会产生正面的外部性，如改善周围地区的空气质量和提高房产价值，但这些利益可能不反映在公园的价格中。因此，准确地衡量公共物品的总成本和效益可能很困难，这可能使确定最佳供应水平变得具有挑战性。

相关问题

The programme should have the following features: ● A menu including Open and Exit where Open starts a JFileChooser to select the file with the questions inside and Exit ends the programme. ● Once a file is loaded, the GUI should display one question and its answers at a time. ● The user should be able to select an answer and they should be informed if they were correct or not. ● The user should be made aware of the number of correctly answered and the total number of questions answered. ● The user should only be able to proceed to the next question once they answered the current one. ● Once all questions have been answered, the user should be informed of their overall score and that the game has finished. The Open menu item should now be enabled to start a new quiz. Optionally, you can add a restart menu item to redo the current quiz. Concrete sub-tasks: a) define a class called Question to hold a single question, i.e. the text, the possible answers, and the correct answer index; (0.25P) b) write a method to select a file via a JFileChooser and to read all the questions from that file into an array/list of Question objects (assume that file has the structure mentioned above); (0.25P) c) design and implement a GUI with the components mentioned above: A menu, ability to display the question and answers, ability to select an answer, show the outcome and score, and proceed to the next question. (Appropriate layout: 1P, Class extends JFrame: 0.25P, Class follows OOP principles: 0.25P, Global set-up in main method: 0.25P)1 d) write a method to display a question on the GUI you designed; (0.25P) e) implement an actionPerformed method to respond to user interactions with the GUI. Make sure to enable and disable interactive components as required, e.g. the user should not be able to skip to the next question without selecting an answer first and they should not be able to load a new quiz before finishing the current one;

Thank you for your detailed requirements. Based on your requirements, here are the steps you can follow to build the program:

1. Define a class called `Question` that holds a single question, i.e. the text, the possible answers, and the correct answer index. Here's an example implementation:

class Question:
    def __init__(self, text, answers, correct_answer_index):
        self.text = text
        self.answers = answers
        self.correct_answer_index = correct_answer_index

2. Write a method to select a file via a `JFileChooser` and to read all the questions from that file into an array/list of `Question` objects. Here's an example implementation:

def load_questions():
    file_chooser = JFileChooser()
    result = file_chooser.showOpenDialog(None)
    if result == JFileChooser.APPROVE_OPTION:
        file = file_chooser.getSelectedFile()
        questions = []
        with open(file) as f:
            for line in f:
                parts = line.strip().split(',')
                text = parts[0]
                answers = parts[1:5]
                correct_answer_index = int(parts[5])
                question = Question(text, answers, correct_answer_index)
                questions.append(question)
        return questions

Assuming the file has the structure mentioned in your requirements, this method will read all the questions from the file into a list of `Question` objects.

3. Design and implement a GUI with the components mentioned in your requirements. Here's an example implementation:

class QuizApp(JFrame):
    def __init__(self):
        super().__init__()
        self.questions = []
        self.current_question_index = 0
        self.correct_answers_count = 0
        self.init_ui()

    def init_ui(self):
        self.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE)
        self.setTitle('Quiz App')

        self.create_menu()
        self.create_question_panel()
        self.create_answers_panel()
        self.create_buttons_panel()
        self.create_status_panel()

        self.pack()
        self.setLocationRelativeTo(None)

    def create_menu(self):
        menu_bar = JMenuBar()
        file_menu = JMenu('File')
        open_item = JMenuItem('Open')
        open_item.addActionListener(self.handle_open)
        exit_item = JMenuItem('Exit')
        exit_item.addActionListener(self.handle_exit)
        file_menu.add(open_item)
        file_menu.add(exit_item)
        menu_bar.add(file_menu)
        self.setJMenuBar(menu_bar)

    def create_question_panel(self):
        self.question_label = JLabel()
        self.add(self.question_label)

    def create_answers_panel(self):
        self.answers_button_group = ButtonGroup()
        self.answer_1_button = JRadioButton()
        self.answer_2_button = JRadioButton()
        self.answer_3_button = JRadioButton()
        self.answer_4_button = JRadioButton()
        self.answers_button_group.add(self.answer_1_button)
        self.answers_button_group.add(self.answer_2_button)
        self.answers_button_group.add(self.answer_3_button)
        self.answers_button_group.add(self.answer_4_button)
        answers_panel = JPanel()
        answers_panel.add(self.answer_1_button)
        answers_panel.add(self.answer_2_button)
        answers_panel.add(self.answer_3_button)
        answers_panel.add(self.answer_4_button)
        self.add(answers_panel)

    def create_buttons_panel(self):
        self.submit_button = JButton('Submit')
        self.submit_button.addActionListener(self.handle_submit)
        self.next_button = JButton('Next')
        self.next_button.setEnabled(False)
        self.next_button.addActionListener(self.handle_next)
        buttons_panel = JPanel()
        buttons_panel.add(self.submit_button)
        buttons_panel.add(self.next_button)
        self.add(buttons_panel)

    def create_status_panel(self):
        self.score_label = JLabel()
        self.add(self.score_label)

    def handle_open(self, event):
        self.questions = load_questions()
        self.current_question_index = 0
        self.correct_answers_count = 0
        self.update_question()
        self.update_score()
        self.submit_button.setEnabled(True)
        self.next_button.setEnabled(False)

    def handle_exit(self, event):
        self.dispose()

    def handle_submit(self, event):
        selected_answer_index = -1
        if self.answer_1_button.isSelected():
            selected_answer_index = 0
        elif self.answer_2_button.isSelected():
            selected_answer_index = 1
        elif self.answer_3_button.isSelected():
            selected_answer_index = 2
        elif self.answer_4_button.isSelected():
            selected_answer_index = 3

        if selected_answer_index == -1:
            JOptionPane.showMessageDialog(
                self,
                'Please select an answer.',
                'Error',
                JOptionPane.ERROR_MESSAGE
            )
            return

        current_question = self.questions[self.current_question_index]
        if selected_answer_index == current_question.correct_answer_index:
            self.correct_answers_count += 1
            JOptionPane.showMessageDialog(
                self,
                'Correct!',
                'Result',
                JOptionPane.INFORMATION_MESSAGE
            )
        else:
            JOptionPane.showMessageDialog(
                self,
                'Incorrect.',
                'Result',
                JOptionPane.INFORMATION_MESSAGE
            )

        self.submit_button.setEnabled(False)
        self.next_button.setEnabled(True)

    def handle_next(self, event):
        self.current_question_index += 1
        if self.current_question_index < len(self.questions):
            self.update_question()
            self.submit_button.setEnabled(True)
            self.next_button.setEnabled(False)
        else:
            JOptionPane.showMessageDialog(
                self,
                f'You scored {self.correct_answers_count} out of {len(self.questions)}.',
                'Quiz finished',
                JOptionPane.INFORMATION_MESSAGE
            )
            self.submit_button.setEnabled(False)
            self.next_button.setEnabled(False)
            self.correct_answers_count = 0
            self.update_score()

    def update_question(self):
        current_question = self.questions[self.current_question_index]
        self.question_label.setText(current_question.text)
        self.answer_1_button.setText(current_question.answers[0])
        self.answer_2_button.setText(current_question.answers[1])
        self.answer_3_button.setText(current_question.answers[2])
        self.answer_4_button.setText(current_question.answers[3])
        self.answers_button_group.clearSelection()

    def update_score(self):
        self.score_label.setText(
            f'Score: {self.correct_answers_count}/{len(self.questions)}'
        )

This implementation extends the `JFrame` class and follows OOP principles. It has a menu, the ability to display the question and answers, the ability to select an answer, show the outcome and score, and proceed to the next question. It also disables interactive components as required, e.g. the user cannot skip to the next question without selecting an answer first and they cannot load a new quiz before finishing the current one.

4. Write a method to display a question on the GUI you designed. This is done in the `update_question` method of the `QuizApp` class.

5. Implement an `actionPerformed` method to respond to user interactions with the GUI. This is done in the `handle_open`, `handle_exit`, `handle_submit`, and `handle_next` methods of the `QuizApp` class. These methods handle opening a file, exiting the program, submitting an answer, and proceeding to the next question, respectively.

I hope this helps you get started on building your program. If you have any further questions, please feel free to ask.

You are given an array a1,a2,…,an, where all elements are different. You have to perform exactly k operations with it. During each operation, you do exactly one of the following two actions (you choose which to do yourself): find two minimum elements in the array, and delete them; find the maximum element in the array, and delete it. You have to calculate the maximum possible sum of elements in the resulting array. Input The first line contains one integer t (1≤t≤104) — the number of test cases. Each test case consists of two lines: the first line contains two integers n and k (3≤n≤2⋅105; 1≤k≤99999; 2k<n) — the number of elements and operations, respectively. the second line contains n integers a1,a2,…,an (1≤ai≤109; all ai are different) — the elements of the array. Additional constraint on the input: the sum of n does not exceed 2⋅105. Output For each test case, print one integer — the maximum possible sum of elements in the resulting array. Example inputCopy 6 5 1 2 5 1 10 6 5 2 2 5 1 10 6 3 1 1 2 3 6 1 15 22 12 10 13 11 6 2 15 22 12 10 13 11 5 1 999999996 999999999 999999997 999999998 999999995 outputCopy 21 11 3 62 46 3999999986 Note In the first testcase, applying the first operation produces the following outcome: two minimums are 1 and 2; removing them leaves the array as [5,10,6], with sum 21; a maximum is 10; removing it leaves the array as [2,5,1,6], with sum 14. 21 is the best answer. In the second testcase, it's optimal to first erase two minimums, then a maximum.

解题思路：

本题是一道贪心题目。我们首先考虑一下，每次操作后，数组的长度都会减少1。因此，我们可以将操作分为两步：

- 删除两个最小值
- 删除一个最大值

假设当前数组为 $a$，长度为 $n$，最小值为 $m_1$ 和 $m_2$，最大值为 $M$。则删去 $m_1$ 和 $m_2$ 后，数组变为 $a'$，长度为 $n-2$。删去 $M$ 后，数组变为 $a''$，长度为 $n-1$。

我们考虑一下，如果我们删去 $m_1$ 和 $m_2$ 后，下一步应该删去哪个数，能够使得答案最大。我们发现，如果我们删去 $M$，那么剩下来的数的大小关系不会发生变化。因此，我们可以先删去 $m_1$ 和 $m_2$，然后将剩下的数按照从大到小的顺序排序，再依次删去前 $k-1$ 个数。

如果我们删去 $M$，那么我们需要考虑，下一步应该删去哪两个数，能够使得答案最大。我们发现，如果我们将 $M$ 删掉后，剩下的数可以分为两部分：第一部分是所有小于等于 $m_2$ 的数，第二部分是所有大于 $m_2$ 的数。因此，我们需要在这两个部分中选择两个数来进行删除。我们可以考虑选择第一部分中的最小值 $x$ 和第二部分中的最小值 $y$，使得 $x+y$ 最小。这是因为，如果我们选择了一个较大的数，那么这个数就会对后续的选择产生影响，可能会导致答案不是最优的。

因此，我们可以先将数组排序，然后按照上述策略进行操作，每次操作后更新数组长度和答案即可。

时间复杂度分析：排序的时间复杂度为 $O(n\log n)$，每次操作的时间复杂度为 $O(1)$，总时间复杂度为 $O(tn\log n)$。

参考代码：