CSc 10: Introduction to Computing

Final Exam sample questions and answers

 

Answer the following T or F.  Explain why or why not.  (3 each, 45 total)

See http://www.cse.lehigh.edu/~glennb/um/book/exsolnew.htm.

 

What is an advantage of value parameters?     

Avoiding hidden side effects, because pass by value copies an actual parameter to a formal parameter, any changes to the formal parameter in the callee method have no effect on the actual parameter in the caller. 

 

What are two advantages of  reference parameters?

1)      Allowing multiple outputs from a method, as side effects of any changes to a parameter within a callee (unlike pass by value).

2)      Passing large objects efficiently, since no copying of values occurs.

 

Write a method which, given an String, converts all the ‘.’ (periods) into ‘.’ (commas) and returns the new String.    Note: do not use any library functions other than length, charAt and setCharAt.

 

public class test

{                                                                  

  /** convert commas to periods */

  static String convert(String in)

  { StringBuffer out = new StringBuffer(in); //Modify as a StringBuffer

    int i=0;

    while (i < out.length())

    //Loop boundary invariant: 0 <= i < in.length

    { if (out.charAt(i) == ',')  //Is it a comma?

        out.setCharAt(i,'.');    //Make it a period

      i++;

    }

    return out.toString(); //Return as String

  }

 

  public static void main(String args[])                     

  {

    String test="1,000,000";

    System.out.println("test="+test + " convert="+convert(test));

   

    int stop = Input.getInt();

  }                                            

}

 

In each of the following code fragments: a) find and explain a bug, b) fix it (by marking up the code on this sheet), and c) show your corrected version's output.

            const int Sentinel = -1;

int count=0;

float sum=0;

int score;

System.out.println("Enter scores, stopping with –1:");

while (score != Sentinel)

{ score = Input.getInt();

  count = count + 1;

  sum = sum + score;

}

System.out.println(count + " scores averaging " + sum / count);

//Assume user types in: 70 81 -1

a)      The erronerous output is: 3 scores averaging 53.333333

The bug is that the sentinel value, -1, gets counted as an actual score.

b)      The fix is to use the sentinel loop idiom, moving the input statement so that it occurs just before the while loop test:

score = Input.getInt();

while (score != Sentinel)

{ count = count + 1;

  sum = sum + score;

  score = Input.getInt();

}

c)      2 scores averaging 75.5

 

static void switchEm(int x,int y)

{ int temp=x;

  x = y;

  y = temp;

}

public static void main(String args[])

{ int a=1,b=2;

       switchEm(a,b);

  System.out.println("Now " + a + " is bigger than " + b);

}

a) The erroneous output is: Now 1 is bigger than 2

          The bug is that the parameters get passed by value, so the changes to the formal parameters,            x and y, don’t affect the actual parameters a and b.

      b) Without explicit pass by reference, the fix isn’t as easy in Java as in C++.  Pass in an object, such as an array, and set its values (passing an object just copies the object’s handle, not the values):

     static void switchEm(int a[])

{ int temp=a[0];

  a[0] = a[1];

  a[1] = temp;

}

public static void main(String args[])

{ int a[] = new int[2];

  a[0] = 1; a[1] = 2;

       switchEm(a);

  System.out.println("Now " + a[0] + " is bigger than " + a[1]);

}

Explain why multiprogramming can improve the performance or Athroughput@ of each of the following types of systems:

a) Batch operating systems

            While one job is waiting for file processing, another job can run, maximizing CPU usage.

 

b) Time-sharing operating systems

While one user is inactive, the processor can switch to another user, maximizing usage.

 

6. Consider the following snippet from the atm_oop program:

public class Account

{ public Account() {}

  public int getPIN() { return PIN; }    

  void setPIN(int pin) { PIN = pin; }

  protected int PIN; //Personal Identification Number

}

 

a) Given the above class definition, define a variable, myAccount, of this type, then assign it a PIN.

      Account myAccount = new Account();  //construct an Account

      myAccount.setPIN(2468);

 

b) Why is the following snippet illegal?  Why is making it illegal a good idea?

      if (myAccount.PIN == 2468)

Because PIN is private.  Making it illegal protects the meaning of the data.  Though it doesn’t in the implementation above, setPIN could include an assertion that makes sure that PIN only gets a valid value.

 

c) Derive another class, Savings, from class Account.  Declare a constructor for your new class which requires a PIN number.

            class Savings extends Account

     { Savings(int pin) { PIN = pin; }

            //Note that the above statement assumes that PIN is protected, not private in Account.

 

d) How do classes illustrate the power of abstraction?

Classes let their clients focus on a public interface, while suppliers can worry about implementation details.

 

e) How does class inheritance further illustrate abstraction?

Inheritance pools everything (methods an variables) that a set of classes may have in common in one place, so implementors subclasses don’t have to keep re-inventing the wheel.

 

Th.. th.. th.. that's all folks!