public static<e> List<E> removeoccurrences(list<e> origlist, E remove) {

Transcription

1 CS 201, Fall 2008 Nov 19th Exam 2 Name: Question 1. [10 points] Complete the following generic method. It should return a list containing all of the elements in origlist, in order, except the elements equal in value to the value remove. The method should not modify the list passed as its parameter. Example test: List<String> names = new ArrayList<String>(); names.add("alice"); names.add("bob"); names.add("alice"); names.add("carl"); names.add("donald"); names.add("emma"); names.add("alice"); List<String> trimmed = removeoccurrences(names, "Alice"); assertequals("bob", trimmed.get(0)); assertequals("carl", trimmed.get(1)); assertequals("donald", trimmed.get(2)); assertequals("emma", trimmed.get(3)); public static<e> List<E> removeoccurrences(list<e> origlist, E remove) {

2 Question 2. [8 points] What output is printed by the following statements? Map<String, List<String>> m = new HashMap<String, List<String>>(); List<String> list = new ArrayList<String>(); list.add("a"); list.add("b"); list.add("c"); m.put("foo", list); m.put("bar", list); m.get("bar").remove(2); m.get("bar").add("d"); System.out.println(m.get("foo").get(2)); m.get("foo").remove(2); System.out.println(m.get("foo").get(1)); m.get("foo").add("e"); System.out.println(m.get("bar").get(2)); Question 3. [6 points] What output is printed by the following statements? TreeSet<Integer> s = new TreeSet<Integer>(); s.add((integer) 33); s.add((integer) 2); s.add((integer) 27); for (Integer i : s) { System.out.println(i);

3 Question 4. [10 points] Complete the following generic method. It should return a list containing an interleaving of the elements from the two collections c1 and c2. Specifically, the first element in the returned list is the first element in c1, the second element in the returned list is the first element in c2, etc. You may assume that c1 and c2 contain the same number of elements. Example test: TreeSet<String> s = new TreeSet<String>(); s.add("a"); s.add("b"); LinkedList<String> ll = new LinkedList<String>(); ll.add("c"); ll.add("d"); List<String> result = interleave(s, ll); assertequals("a", result.get(0)); assertequals("c", result.get(1)); assertequals("b", result.get(2)); assertequals("d", result.get(3)); public static<e> List<E> interleave(collection<e> c1, Collection<E> c2) {

4 Question 5. [8 points] What is the big-o upper bound on the worst-case running time of the following method? Let the problem size N be the number of elements in the list. Explain briefly. public static<e extends Comparable<E>> boolean containsduplicates(list<e> list) { TreeSet<E> set = new TreeSet<E>(); for (E elt : list) { set.add(elt); if (list.size() == set.size()) { return false; else { return true; Question 6. [8 points] What is the big-o upper bound on the worst-case running time of the following method? Let the problem size N be the length of the array. Explain briefly. public static int mystery(int[] arr) { int sum = 0; for (int j = 0; j < arr.length; j++) { int max = j + 100; if (max > arr.length) { max = arr.length; for (int i = j; i < max; i++) { sum += arr[i]; return sum;

5 Question 7. [10 points] Complete the following recursive method. Given a string s, it should return a string containing all of the characters of the original string, doubled. Example test: String str = "Recursion is fun"; assertequals("rreeccuurrssiioonn iiss ffuunn", doublecharacters(str)); Note: your method must use recursion. Do not use a loop. public static String doublecharacters(string s) { Question 8. [10 points] Complete the following recursive method. Given a string str and a character value c, the method should return true if the string contains at least one occurrence of the character. Example test: String str = "Recursion is fun"; asserttrue(containschar(str, s )); assertfalse(containschar(str, x )); Note: your method must use recursion. Do not use a loop. Also, do not call any methods other than the length, charat, and substring methods on the string. public static boolean containschar(string str, char c) {

6 Question 9. [10 points] Use proof by induction to show that for all n 1, the sum of the squares of the integers 1, 2, 3,..., n can be determined using the following formula: n f(n) = i 2 = n 2 = i=1 n(n + 1)(2n + 1) 6 Basis step. Show that the formula works when n = 1. Induction step. Show that if the formula works for n, then it also works for n + 1. Remember to state the expectation for f(n + 1), and also to state a recurrence defining f(n + 1) in terms of f(n). Show that if the occurrence of f(n) is expanded in the recurrence, that the formula for f(n + 1) matches the expectation.

7 Question 10. [10 points] What output is printed by the following program (which begins on the left and ends on the right)? public class Q10 { private static<e> void mix( LinkedList<E> list) { Stack<E> s = new Stack<E>(); for (E elt : list) { s.push(elt); list.clear(); int count = 0; while (!s.isempty()) { if (count % 2 == 0) { // add to beginning of list list.addfirst(s.pop()); else { // add to end of list list.addlast(s.pop()); count++; public static void main(string[] args) { LinkedList<String> ll = new LinkedList<String>(); ll.add("alice"); ll.add("bob"); ll.add("charlie"); ll.add("debbie"); ll.add("eloise"); mix(ll); for (String s : ll) { System.out.println(s); Question 11. [5 points] What output is printed by the following sequence of statements? Queue<Integer> q = new LinkedList<Integer>(); q.add((integer) 42); q.add((integer) 17); q.add((integer) 121); q.remove(); q.remove(); System.out.println(q.remove());

8 Question 12. [5 points] Maps are similar to array lists in some ways. An array list associates a range of integer index values with arbitrary element values (of a specified element type). A map associates an arbitrary set of keys (of a specified key type) with arbitrary element values (of a specified value type). Say that we want to associate integer values with string values. Under what circumstances would it be better to use an instance of Map<Integer, String> rather than an instance of ArrayList<String>? Explain briefly. Bonus Question. [5 points] In order for instances of a class to be stored in a HashSet, or used as keys in a HashMap, a class must override the int hashcode() method defined in the Object class. Let s say that the class Frotz has the following hashcode() method: public int hashcode() { return 42; Will a HashSet of Frotz elements work correctly? (In other words, will the methods implementing set operations work as intended?) Explain briefly.