CS 100J Prelim 2 Spring 2006 Answers
Question 0. (answer omitted)
Question 1. (a) All parameters and local
variables are created when the frame for the
call is drawn, before execution of the method
body.
(b) y= y + (h – 1) + 101;
(c) 1. How does it start? (How do you truthify
the invariant?) 2. When can it stop? 3. How
does the repetend make progress toward ter-
mination? 4. How does the repetend maintain
the invariant?
Question 2.
true, Error, 20, 30, “meow”, “” , “meow”,
Error, “”, Error.
Question 3.
/** = ob is a non-null dog with the same
fields as this Dog */
public boolean equals(Object ob) {
if (ob == null) return false;
if (!(ob instanceof Dog)) return false;
Dog dob= (Dog) ob;
return getName().equals(dob.getName())
&& getAge() == dob.getAge()
&& breed.equals(dob.breed);
}
/** Constructor: a Dog that is a terrier, has
name Spot, and is y years old */
public Dog(int y) {
super("Spot", y);
breed= "terrier";
}
Question 4.
/** Remove all CATs from v and return a
Vector that contains the removed CATs */
public static Vector<Cat> remCats(
Vector <Animal> v) {
Vector<Cat> res= new Vector<Cat>();
int k= 0;
// inv: v[0..k-1] contains no Cats and
// res contains the Cats that have been
// removed from v[0..k-1]
while (k != v.size()) {
Animal a= v.get(k);
if (a instanceof Cat) {
Cat c= (Cat)a;
res.add(c);
v.remove(k);
}
else k= k+1;
}
return res;
}
Question 5.
/** Draw a sierpinski carpet of width and
height w with top left corner at (x, y).
Precondition: w is a power of 3
Precondition: the square to be drawn on
is black and the pen color is white. */
public static void sc(int x, int y, int w) {
if (w < 3)
return;
int wn= w/3;
for (int k= 0; k != 9; k= k+1) {
if (k != 4) {
int i= k/3;
int j= k%3;
sc(x+i*wn, y+j*wn, wn);
}
else {
graphics.fillRect(
x+wn, y+wn, wn, wn);
}
}
}