CS 100J Prelim 2 Fall 2006 Answers
Question 1. (a) Local variable: A variable declared in
the body of a method. It is created when the frame for a
call is created, before execution of the method body.
(b) // Store in c the number of positions k in Strings s1
// and s2 such that s1[k] == s2[k]
int n= Math.min(s1.length(), s2.length());
int c= 0;
// invariant: c = no. of positions i in s1[0..k-1] and
s2[0..k-1] such that s1[i] == s2[i]
for (int k= 0; k < n; k= k+1) {
if (s1.charAt(k) == s2.charAt(k))
c= c+1;
}
// c = no. of positions i in s1[0..n-1] and s2[0..n-1]
// such that s1[i] == s2[i]
Question 2. (a) c.Bigger(a) : true,
(b) a.getJob(): ERROR. Apparent class is Student.
(c) c.getJob(): “work-study”,
(d) d.getJob()!: “Cabbie”,
(e) c.getSound(): “I'm new here.”,
(f) d.getSound(): “”,
(g) ((Frosh)a).getSound(): “I'm new here.”,
(h) ((Frosh)d).getSound(): ERROR,
(i) ((Senior)d).getSound(): “”,
(j) ((Senior)a).getSound(): ERROR.
Question 3.
/** Constructor: a Senior with major "CS",
name m, and gpa of 3.8. */
public Senior(String m) {
super(m, 3.8); major= "CS";
}
/** = “ob is a non-null Senior with the same
fields as this Senior” */
public boolean equals(Object ob) {
if (ob == null) return false;
if (!(ob instanceof Senior)) return false;
Senior obs= (Senior) ob;
return this.major.equals(obs.major) &&
this.getName().equals(obs.getName()) &&
this.getGpa() == obs.getGpa();
}
Question 4. /** Edit string s as described in q. 4 */
public static String edit(String s) {
String res= "";
// inv: res contains the processed s[0..k-1]
for (int k= 0; k != s.length(); k= k+1) {
// If res is empty or ends in ". " (but not "i.e."),
// append capitalized s[k] to res;
// otherwise append s[k] to res.
if ((res.length() == 0 || res.endsWith(". ")) &&
!res.endsWith("i.e. ") ) {
res= res +
Character.toUpperCase(s.charAt(k));
}
else res= res + s.charAt(k);
if (res.endsWith(".\""))
res= res.substring(0, res.length()-2) + "\".";
}
return res;
}
Question 5. (a) /** = i as a string but with leading 0's,
if necessary, so that it is 3 digits
Precondition: 0 <= i < 1000 */
public static String digit3(int i) {
if (i < 10) return "00" + i;
if (i < 100) return "0" + i;
return "" + i;
}
/** = A string that contains a representation of i, but
with a comma every three digits.
Precondition: i > 0.
Example: toString(5243642) is "5,243,642"*/
public static String toString(int i) {
if (i < 1000) return "" + i;
return toString(i/1000) + "," + digit3(i%1000);
}
(b) /** = no. elephants in this Elephant’s family tree */
public int treeSize() {
int size= 1; // for this elephant.
if (father != null) size= size + father.treeSize();
if (mother != null) size= size + mother.treeSize();
return size;
}