8:00 Quiz 4

Statistics:

(including three-point curve)

mean     22.250 (979.000/44)
stddev   6.906
median   23.000
midrange 18.000-28.000

#  avg
1  6.57 / 10
2  6.52 / 10
3  6.16 / 10
 + 3-point curve

Question 4-1

Suppose we add the following method definition to our Bank class.

public class Bank {
    // : (definitions of other methods and variables)

    public void remove(int i) throws NoSuchElementException {
        if(i < 0) {
            throw new NoSuchElementException("negative index");
        } else if(i >= num_accts) {
            throw new NoSuchElementException("index too large");
        } else {
            num_accts--;
            accts[i] = num_accts;
        }
    }
}

Suppose we have a Bank variable named first. How would you use this method to remove the third account from first? Be sure to handle any exceptions that the method might initiate; for full credit, your code display the message associated with the exception (like ``negative index'').

public static void run() {
    Bank first = new Bank();
    // : (code that inserts accounts into the bank)



}

Question 4-2

Suppose we have the following two class definitions.

class A {
  public int f(int x) {
    return 2 * x;
  }
  public int g(int x) {
    return f(x * 3);
  }
}

class B extends A {
  public int f(int x) {
    return 5 * x;
  }
  public int h(int x) {
    return f(7 * x);
  }
}

And suppose we execute the following run method.

public static void run() {
  B b = new B();
  IO.println(b.f(1) + " " + b.g(1) + " " + b.h(1));
  A a = b;
  IO.println(a.f(1) + " " + a.g(1));
}

What would the method print?

Question 4-3

Define a class PassCount to track whether all the students of a class have passed a test. It should support the following methods.

PassCount(): (Constructor method) Constructs an object representing an empty class.
void addGrade(double grade): Adds grade into the class.
boolean isAnyFailing(): Returns true only if there is somebody in the class who received a grade of less than 60.

For example, if you defined this class properly, I should be able to write the following class to test it.

public class PassCountTest {
    public static void run() {
        PassCount a = new PassCount();
        a.addGrade(45.0);
        a.addGrade(76.0);
        IO.println(a.isAnyFailing()); // should print ``true''

        PassCount b = new PassCount();
        b.addGrade(60.0);
        IO.println(b.isAnyFailing()); // should print ``false''
    }
}

Note that, to accomplish this, a PassCount object need not remember every single grade --- that is, you do not need to use arrays: It just needs to remember whether any of the ones it has seen were below 60.

Solutions

Solution 4-1

public static void run() {
    Bank first = new Bank();
    // : (code that inserts accounts into the bank)

    try {
        first.remove(2);
    } catch(NoSuchElementException e) {
        IO.println(e.getMessage());
    }
}

Solution 4-2

5 15 35
5 15

Solution 4-3

public class PassCount {
    private boolean found_fail;

    public PassCount() {
        found_fail = false;
    }

    public void addGrade(double grade) {
        if(grade < 60.0) {
            found_fail = true;
        }
    }

    public boolean isAnyFailing() {
        return found_fail;
    }
}