CSC/ECE 517: Object-Oriented Languages and Systems Summer 2008 Test 1 with Answers

Transcription

1 CSC/ECE 517: Object-Oriented Languages and Systems Summer 2008 Test 1 with Answers This was a 95-minute open-book test. There were a total of 102 points on this test. If you answered more than 100% worth of questions correctly, your score was 100%. Question 1. (2 points each) Suppose we execute the following Ruby statements: a = [11, "eleven"] a << [12, "twelve"] a << 14 Then what is returned by each of the following? (a) a.size Answer: 4 (b) a[1] Answer: "eleven" (c) a[-2] Answer: [12, "twelve"] (d) a[2][1] Answer: "twelve" (e) a.slice 1, 2 Answer: ["eleven", [12, "twelve"]] Question 2. (4 points)) What is returned by this Ruby code? hold = {21, 22, {23, 24}, 25} hold Answer: {{23=>24}=>25, 21=>22} Question 3. Suppose we have this Ruby block: def ablock i = 1 j = 2 yield(i, j) (a) (4 points) Suppose we want to invoke this block to print 1 2 1

2 How can we do this? Answer: ablock { x, y puts x, y} (b) (4 points) How can we invoke ablock to print 5 without using any constants in the invocation? Answer: ablock { x, y puts x + y*y} (c) (3 points) Suppose now that i or j is redefined in the block, e.g., def ablock i = 1 j = 2 yield(i, j) i = 3 j = 4 In this case, what would be printed by the code that you gave as the answer for part (a)? Answer: 1 2 It doesn t make any difference, because the code in the block is executed when the yield statement is reached. Question 4. (10 points)) Write an iterative method fact n to compute a factorial in Ruby. Recall that a Ruby method returns the value of the last expression executed in the method. Answer: def fact n i = 1 for k in 1.. n i = i * k i Question 5. Here is a class that implements a tree. Each node in the tree may have any number of children. (a) (3 points) Fill in the three blanks in the code below. class Tree attr_reader :value def = = [] def <<(value) subtree = << subtree 2

3 return subtree Here s code to create a specific Tree t: t = Tree.new("Parent") child1 = t << "Child 1" child1 << "Grandchild 1.1" child1 << "Grandchild 1.2" child2 = t << "Child 2" child2 << "Grandchild 2.1" (b) (4 points) Now we would like to define an iterator that will yield at each node of the Tree, traversing each node before its children. Fill in the blanks. class Tree def each yield do child_node child_node.each { e yield e } (c) (5 points) Write a statement that invokes the each method on tree t to print out the name of each node (e.g., Parent, Child 1, etc.) Answer: t.each { x puts x} (d) (3 points) Finish this list of nodes traversed by the each method. # Parent # Child 1 # Grandchild 1.1 # Grandchild 1.2 # Child 2 # Grandchild 2.1 Question 6. Here is Ruby code that defines some modules and uses them as mixins. module HelloModule def hello_world puts "Hello, World!" module GoodbyeModule def goodbye_world puts "Goodbye, World." 3

4 class BaseClass def class_method puts "In base class method" class Greet include HelloModule def howdy_world puts Howdy, World! class Subclass < BaseClass include HelloModule include GoodbyeModule (a) (3 points) List all the kinds of objects that can be created from this code using the new method (e.g., BaseClass.new). Answer: BaseClass, Greet, Subclass (b) (5 points, ¼ point per blank) For each module and class, what methods are available in them? If a method is available, put a check box in the corresponding square. HelloModule GoodbyeModule BaseClass Greet Subclass hello_world howdy_world goodbye_world class_method (c) (2 points) When the following code is executed, what will be printed? h = Subclass.new h.class_method h.hello_world Answer: In class method Hello, world! Question 7. Here is a short Ruby program that implements a design pattern. class Duck def beaduck() fly quack 4

5 class Mallard < Duck def fly() puts "I'm flying" def quack() puts "Quack" class RubberDucky < Duck def fly() puts "I can't fly" def quack() puts "<<silence>>" daffy =.new donald = [daffy, donald].each do d d.beaduck (a) (3 points) Which design pattern does it implement? Answer: Strategy. (b) (3 points) Fill in the blanks above so that the output from this program will be I m flying Quack I can't fly <<silence> Answer: daffy = Mallard.new donald = RubberDucky.new (c) (4 points) If we were to try to implement the same functionality in Java (to iterate over a data structure containing different kinds of ducks), we would find that we could not do so without implementing or reimplementing additional methods in some classes. Which method(s) and which class(es)? Answer: We would need to define the fly() and quack() methods in the Duck class, or otherwise we couldn t have a single method invocation call those methods on RubberDuckys and Mallards. (d) (2 points) What is the feature of Ruby called that permits this code to be written without extra method definitions? Answer: Unbounded polymorphism. Question 8. (1 point each) For each of the following statements, tell whether it is most closely associatedi with a model (M), view (V), or controller (C). (a) The files that make up this part of a Ruby on Rails application are organized into multiple directories. Answer: V (b) This part of a Rails application contains.rhtml files. Answer: V (c) This part of a Rails application contains a method for each action that the user can perform. Answer: C (d) This part of a Rails application makes most frequent use of ActiveRecord. Answer: C 5

6 (e) The relationship among tables in the database ts to be defined in this part of a Rails application. Answer: M (f) This part of a Rails application determines the URL for performing a certain action. Answer: C (g) This part of a Rails application ts to contain the smallest files. Answer: M (h) This part of a rails application contains methods for creating, updating, and deleting items stored in the database. Answer: C (i) One-to-many relationships are defined in this part of a Rails application. Answer: M (j) Standard layout is defined in this part of a Rails application. Answer: V Question 9. (a) (12 points) Write Ruby code to define a Student class, where every student has a name, major, hometown, and gpa. Make sure that <, =, >, etc. methods are defined for students, such that one student is greater than another student if (s)he has a higher GPA. Do this in the standard way. Answer: class Student include Comparable attr_reader :gpa def initialize(name, hometown, major, = = = = gpa def <=> other self.gpa <=> other.gpa (b) (3 points) Write code to instantiate at least three students with different GPAs. s = Student.new("Joe College", "Springfield", "CS", 2.0) t= Student.new("Able Axel", "Aarhus", "CS", 3.3) u = Student.new("Betty Beezer", "Blackburn", "IT", 3.2) (c) (5 points) Write code to sort the students in order of increasing GPA. Do this as concisely as possible (writing code that will work for arbitrary students). Answer: [s, t, u].sort Grade summary High 97 Mean Low 45 Std. Dev Quartiles (67, 77, 90) 6