Briefly explain what undefined behaviour is in the C programming language. Under what circumstance(s) would calling the following C function result in undefined behaviour? (2 marks) int divide(int *a, int *b) { return *a / *b; }

在 C 编程语言中，未定义行为是指程序的行为在语言规范中并未明确定义。

如果调用下面的 C 函数，在以下情况下会导致未定义行为：

1. 当 *b 的值为 0 时，除以 0 是不允许的，将导致未定义行为。

2. 当 *a 的值是最小整数值，并且 *b 的值是 -1 时，结果将溢出，从而导致未定义行为。

注意：C 语言中未定义行为的结果是不可预测的，可能会导致程序终止，也可能会继续执行。因此，编写代码时应避免出现未定义行为。

相关问题

Look at the Account class Account.java and write a main method in a different class to briefly experiment with some instances of the Account class. o Using the Account class as a base class, write two derived classes called SavingsAccount and CurrentAccount. A SavingsAccount object, in addition to the attributes of an Account object, should have an interest variable and a method which adds interest to the account. A CurrentAccount object, in addition to the attributes of an Account object, should have an overdraft limit variable. Ensure that you have overridden methods of the Account class as necessary in both derived classes. o Now create a Bank class, an object of which contains an array of Account objects. Accounts in the array could be instances of the Account class, the SavingsAccount class, or the CurrentAccount class. Create some test accounts (some of each type). o Write an update method in the bank class. It iterates through each account, updating it in the following ways: Savings accounts get interest added (via the method you already wrote); CurrentAccounts get a letter sent if they are in overdraft. o The Bank class requires methods for opening and closing accounts, and for paying a dividend into each account.

Here is an implementation of the requested classes with a brief main method to test the Account class:

class Account {
    private String accountNumber;
    private double balance;

    public Account(String accountNumber, double balance) {
        this.accountNumber = accountNumber;
        this.balance = balance;
    }

    public void deposit(double amount) {
        this.balance += amount;
    }

    public void withdraw(double amount) {
        if (this.balance - amount < 0) {
            System.out.println("Insufficient funds.");
            return;
        }
        this.balance -= amount;
    }

    public void printBalance() {
        System.out.println("Account balance: " + this.balance);
    }
}

class SavingsAccount extends Account {
    private double interestRate;

    public SavingsAccount(String accountNumber, double balance, double interestRate) {
        super(accountNumber, balance);
        this.interestRate = interestRate;
    }

    public void addInterest() {
        double interest = this.getBalance() * this.interestRate;
        this.deposit(interest);
    }
}

class CurrentAccount extends Account {
    private double overdraftLimit;

    public CurrentAccount(String accountNumber, double balance, double overdraftLimit) {
        super(accountNumber, balance);
        this.overdraftLimit = overdraftLimit;
    }

    @Override
    public void withdraw(double amount) {
        if (this.getBalance() - amount < -overdraftLimit) {
            System.out.println("Overdraft limit exceeded.");
            return;
        }
        super.withdraw(amount);
    }
}

class Bank {
    private Account[] accounts;

    public Bank(Account[] accounts) {
        this.accounts = accounts;
    }

    public void update() {
        for (Account account : accounts) {
            if (account instanceof SavingsAccount) {
                ((SavingsAccount) account).addInterest();
            } else if (account instanceof CurrentAccount) {
                if (account.getBalance() < 0) {
                    System.out.println("Sending overdraft letter to account " + account.getAccountNumber());
                }
            }
        }
    }

    public void openAccount(Account account) {
        Account[] newAccounts = new Account[accounts.length + 1];
        System.arraycopy(accounts, 0, newAccounts, 0, accounts.length);
        newAccounts[accounts.length] = account;
        accounts = newAccounts;
    }

    public void closeAccount(Account account) {
        Account[] newAccounts = new Account[accounts.length - 1];
        int j = 0;
        for (int i = 0; i < accounts.length; i++) {
            if (accounts[i] != account) {
                newAccounts[j] = accounts[i];
                j++;
            }
        }
        accounts = newAccounts;
    }

    public void payDividend(double amount) {
        for (Account account : accounts) {
            account.deposit(amount);
        }
    }
}

public class Main {
    public static void main(String[] args) {
        Account account1 = new Account("A001", 1000);
        SavingsAccount account2 = new SavingsAccount("A002", 2000, 0.01);
        CurrentAccount account3 = new CurrentAccount("A003", 5000, 1000);

        account1.deposit(500);
        account1.printBalance();
        account1.withdraw(2000);
        account1.printBalance();

        account2.addInterest();
        account2.printBalance();

        account3.withdraw(2000);
        account3.printBalance();
        account3.withdraw(5000);
        account3.printBalance();

        Account[] accounts = {account1, account2, account3};
        Bank bank = new Bank(accounts);

        bank.update();

        bank.openAccount(new Account("A004", 1500));
        bank.closeAccount(account1);

        bank.payDividend(100);
        bank.update();
    }
}

This code creates three accounts of different types, performs some transactions on them, creates a Bank object with an array of accounts, and calls some methods on the Bank object to update accounts, open and close accounts, and pay dividends.

For a given PNP transistorExplain briefly what voltages need to be applied so that this transistor will work in the active (linear) region. Sketch a circuit showing the polarity of your power supplies.

To operate a PNP transistor in the active (linear) region, the following voltages need to be applied:

1. The base-emitter junction should be forward-biased, which means the voltage at the base should be more negative than the voltage at the emitter.

2. The collector-base junction should be reverse-biased, which means the voltage at the collector should be more positive than the voltage at the base.

A simple circuit to demonstrate this would be:

      +Vcc
        |
       / \
      /   \ R1
     /     \
     \     /
      \   /
       \ /
     |/ \| PNP
      |
      _
      |
      -
      |
      0V

In this circuit, the positive power supply is connected to the collector of the PNP transistor, and the negative power supply is connected to the emitter. The base is biased with a resistor R1 and a voltage source can be used to set the base voltage. The resistor R1 limits the current flowing into the base to protect the transistor from damage.

By applying the correct voltages to the base, collector, and emitter, the transistor will operate in the active (linear) region and can be used for amplification or switching purposes.