8:00 Quiz 5

Statistics:

(including 5-point curve)

mean     22.317 (915.000/41)
stddev   5.425
median   22.000
midrange 19.000-25.000

# avg
1 6.66 / 10
2 4.83 / 10
3 5.83 / 10
 + 5-point curve

Question 5-1

Select which of the following best describes the purpose of each of the circled pieces of the code. (Note that some choices may not appear at all, and others may occur multiple times.)

A. local variable declaration
B. class variable declaration
C. instance variable declaration
D. constructor method definition
E. class method definition
F. instance method definition

Question 5-2

Complete the method body so that it removes all strings in list beginning with the letter initial. It should assume that list contains only String objects.

public static void removeAll(ArrayList list, char initial) {



}

Recall the following instance methods defined for the ArrayList class.

Object get(int index): Returns the object at index index of the list.
Object remove(int index): Removes the object at index index of the list, shifting objects at that index and beyond to fill the gap. Returns the removed object.
int size(): Returns the number of elements in the list.

Also recall the following instance method defined for the String class.

char charAt(int pos): Returns the character at index pos of the string.

Question 5-3

Define a class NumberIterator for iterating through a sequence of numbers. It should support the following methods.

NumberIterator(int start, int stop): (Constructor method) Constructs an object for iterating through the integers beginning at start and going up to stop. The constructor assumes that start is less than stop.
boolean hasMoreNumbers(): Returns true if there are more numbers remaining in the sequence.
int nextNumber(): Returns the current number in the sequence and steps the iterator forward, so that the next call to this method returns the following number in the sequence. This method initiates a NoSuchElementException if the sequence has no more elements remaining.

For example, if you defined this class properly, I should be able to write the following class to test it. When executed, its run() method would print ``5 6 7 8''.

public class NumberIteratorTest {
    public static void run() {
        NumberIterator it = new NumberIterator(5, 8);
        IO.print(it.nextNumber());
        while(it.hasMoreNumbers()) {
            int i = it.nextNumber();
            IO.print(" " + i);
        }
    }
}

Solutions

Solution 5-1

a. C. instance variable
b. C. instance variable
c. D. constructor method
d. F. instance method
e. A. local variable
f. F. instance method

Solution 5-2

This is slightly tricky in that you can't just go through the list removing each element beginning with the letter: Say two adjacent elements begin with the letter. When we remove the first one, the second element shifts into the slot from which we just removed. The program needs some way of checking that element again.

My technique for handling this is to go through the list backwards. It's also possible to address the problem by using a while loop within the for loop (so you repeatedly check whether the current element begins with the letter) or by decrementing the counter when an item is removed (so that the next iteration revisits that position after re-incrementing the counter to its previous position).

public static void removeAll(ArrayList list, char initial) {
    for(int i = list.size() - 1; i >= 0; i--) {
        String elt = (String) list.get(i);
        if(elt.charAt(0) == initial) {
            list.remove(i);
        }
    }
}

Solution 5-3

public class NumberIterator {
    private int current;
    private int last;

    public NumberIterator(int start, int stop) {
        current = start;
        last = stop;
    }

    public boolean hasMoreNumbers() {
        return current <= last;
    }

    public int nextNumber() throws NoSuchElementException {
        if(current <= last) {
            current++;
            return current - 1;
        } else {
            throw new NoSuchElementException();
        }
    }
}