CSE143 Exam with answers Problem numbering may differ from the test as given. Midterm #2 February 16, 2001

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1 CSE143 Exam with answers Problem numbering may differ from the test as given. All multiple choice questions are equally weighted. You can generally assume that code shown in the questions is intended to be syntactically correct, unless something in the question or one of the answers suggests otherwise. Midterm #2 February 16, IntSet implements a Set ADT holding integer elements. Set follows the usual mathematical notions of a set, with the programming interface shown below. After the execution of the following statements, what values are in the set S3? intset S1, S2, S3; //construct empty sets S1.add(1); S1.add(2); S1.add(3); S2.add(3); S2.add(4); S1.setIntersection(S2); S3 = S1; Here is a partial definition of intset: class intset{ // not shown ; intset(); // return false if structure is full bool add(int toadd); bool memberof(int query); // return false on failure bool setunion(const intset & other); bool setintersection(const intset & other); intset & operator=(const intset & other); A. 3 B C D. 1 2 E. 0 3

2 Page 2 2. Suppose you are defining and implementing a class named EconModel. Of the following 5 statements, how many are true? C++ requires that the class declaration go in a file named EconModel.h C++ requires that the implementation of the class methods be placed in a file named EconModel.cpp C++ requires that all member variables of EconModel be private C++ requires that all methods of EconModel be public. C++ requires that a constructor for EconModel be defined (by the programmer). A. 0 B. 2 C. 3 D. 4 E Mindreading You are hired by a company to figure out some code left incomplete by your precedessor. You have this much of a class declaration: class screen { int pixels; int brightness; screen (void); // method "darken" would be declared here... ; You also have this much of the implementation for "darken": { if (pixels > 10000) brightness = brightness - dimmer; return brightness; Which of the following is the most likely missing first line of the implementation of darken? [You can assume the programmer wouldn t use the same identifier for two different purposes.] A. brightness screen::darken (const int i) B. int screen::darken (const int dimmer) C. int screen::darken (int dimmer) const D. int darken (int brightness, int dimmer) E. screen darken (const brightness) ANSWER: B

3 Page 3 4. The following operations are performed on an initially empty queue of ints What is the contents of the queue afterwards? The queue is displayed with the front to the left. q.enqueue (1) q.enqueue (2) i=q.getfront () q.enqueue (3) q.enqueue (4) q.enqueue (5) i=q.dequeue () q.enqueue (6) Notes: getfront operates analogously to the stack method "top()". enqueue and dequeue operate as one would expect standard queue operations with those names. A B C D E ANSWER: C 5. What will be the postfix expression resulting from the following infix expression (using the algorithm learned this quarter)? (4 + 3) + 2 As a reminder, here is the infix to postfix algorithm as it appears in the course packet: Read a token If operand, output it immediately If (, push the '(' on stack If operator: if stack top is an op of => precedence: pop and output; stop when ( is on top or stack empty push the new operator If ), pop and output until ( has been popped Repeat until end of input pop rest of stack A B. (4 3 + ) 2 + C D E ANSWER: C

4 Page 4 6. "X inherits from Y" is generally an appropriate implementation choice if A. X is a kind of Y B. Y is a kind of X C. X has a Y D. Y has a X E. X is the poor cousin and Y is the rich uncle 7. "B is a base class of A" implies A. A is a superclass of B B. A is a derived class of B C. B is a derived class of A D. B is a subclass of A E. A and B are inherited from a common class ANSWER: B 8. class B { int x; protected: int y; int z; ; class N: public B { int w; void display ( ); ; void N::display ( ) { cout << x; //line 1 cout << y; //line 2 cout << z; //line 3 cout << w; //line 4 Which line(s) of the display method contain an error? A. line 1 only B. line 2 only C. line 3 only D. line 4 only E. lines 1, 2, and 3 only

5 Page 5 9. Given the following code fragment, what value is printed by main()? int fiddle (int a) { if (a == 1 a == 2) return a; if (a > 0) return fiddle(a-2); if (a <= 0) return fiddle(a+2); foodle (int val) { if (val < 0) return fiddle(val + 5); else return fiddle(val - 5); int main(void) { cout << foodle(-9) << endl; return 0; A. 2 B. an unknown garbage value C. 1 D. the program will loop forever and never display anything E. -9

6 Page What is the final value displayed on the screen when the following code fragment executes? [The Stack is pretty standard; details below.] Stack a_stack; a_stack.push(5); a_stack.push(10); a_stack.push(17); a_stack.push(a_stack.pop()); a_stack.push(a_stack.pop()+a_stack.pop()); a_stack.push(a_stack.top()); a_stack.pop(); int result = a_stack.pop() - a_stack.pop(); cout << "And the value is: " << result << endl; Here is the Stack definition, almost identical to the one given in homework #4: class Stack { Stack (void); // constructor that initializes an empty stack void push(int item); // pushes item onto the stack int pop(void); // pop the top element off the stack and return the // value of the popped element int top(void); // return the value of the top element, but leave // the stack unmodified ; A. 17 B. there is an error because the program tries to pop something off an empty stack C. 27 D. 22 E. -22 ANSWER: D 11. Which of the following statements about recursion are true? I. Any recursive function can be implemented using iteration (without recursion). II. Any iterative function can be implemented using recursion (without iteration). III. Recursion is always more efficient and easier to implement than iteration, so you should use it in preference iteration A. I only B. II only C. III only D. I and II only E. All of I, II, and III ANSWER: D

7 Page Calculator is a class which has a destructor. The following is a method of Calculator class that correctly compiles. Notice that it refers to three different instances of Calculator: A. *this B. modelcalc C. tempcalc When this method is called, which of these instances has had its destructor called by the time the method completes executing? keypad Calculator::copyKeypad(const Calculator & modelcalc) { Calculator tempcalc; if (this!= &modelcalc) tempcalc.keyboard = modelcalc.keyboard; else tempcalc.keyboard = (*this).keyboard; return tempcalc.keyboard; A. A only B. B only C. C only D. A and B only E. All of A, B, and C ANSWER: C 13. A user is entering numbers at the keyboard. A program reads the characters one at a time and converts the sequence to the proper integer value. For example, if the user has so far entered the three characters (no spaces between them) 1 2 3, the program interprets this as the integer value 123. At a certain point in the execution, the user has entered N numeric characters and the integer value so far is in the variable val. Then the user enters another numeric character which is stored in the char variable ch. What is the new correct value for val? A. val = val + (int) ch; B. val = val*10 + (ch - 0 ); C. val = val + 10*(ch - 0 ); D. val = val*(n-1) + ch*(n); E. val = val*n + ch*(n-1); ANSWER: B 14. If binary search is used on a sorted array of events, about how many events need to be examined to see if the desired event occurs in the array? A B C. 500 D. 50 E. 15 ANSWER: E

8 Page [Worth 4 multiple choice questions] In a banking program, a class is needed to hold a history of all transactions that have been made. Study the class History below, and then add a copy constructor to it in the following two steps: 1) Add the copy constructor prototype to the class declaration below. Make no other changes to the class! 2) Write a full implemention of the copy constructor, so that it does a proper deep copy. If the computer is unable to allocate enough memory for the deep copy, the result should still be a valid History object (with no Transactions in it.) Don t assume there are any other methods or variables except what is shown. Assume that Transaction is a class with an overloaded = operator. class History { History ( ); //copy constructor prototype here: char servicecode; //transactions are held in a dynamically sized array, indexed from 0 to ArraySize-1. Transaction * TArray; int ArraySize; ; //write the implementation here for the deep-copying copy constructor: /************************* sample solution ******************/ //prototype History (const History &); //implementation History::History(const History & other) { servicecode = other.servicecode; ArraySize = other.arraysize; if (ArraySize == 0) TArray = NULL; else { TArray = new Transaction[ArraySize]; if(tarray == NULL){ ArraySize = 0; for (int t = 0; t < ArraySize; t++) TArray[t] = other.tarray[t];