cs151 Fall 1999
Exam 1 Solutions

Part One: vocabulary (10 points)

A logical error is sometimes called a
semantic error because it pertains to the meaning of the program.

Any kind of expression can be used as an argument,
even a method invocation.

Just like a local variable, a
parameter only exists inside the method where it is declared.

A class is a named collection of methods, and a
method is a named collection of statements.

Part Two: debugging (6 points)

For each of the following code fragments, identify any
errors and explain in a sentence or two what the
problem is.  There may be more than one error in a given
fragment, or none!

  public static void printIntTwice (int fizz) {
    System.out.print (int fizz);
    System.out.println (int fizz);
  }  

When you provide arguments, you don't have to specify their
type.  If you do, the compiler gets upset, because you are
insulting its intelligence.

     int x = 5;
     String y = x + "banana";
     String z = x + Math.cos (x);

In both cases, the addition is legal.  The first converts x
to a String and then does String concatenation.  The second
converts x to double and then does floating-point addition.
But in the second case, the assignment of a double to a String
is illegal.

     int a;
     int b;
     a = 7;
     b+5 = a;
     System.out.println (b);

The left side of an assignment must be a variable, because
it specifies the location where the result will be stored.

Part Three: programming (8 points)

A friend of yours shows you the following method and
explains that if {\tt number} is any two-digit number, the program
will output the number backwards.  He claims that if {\tt number} is
17, the method will output {\tt 71}.

     int number = 17;
     int lastDigit = number%10;
     int firstDigit = number/10;
     System.out.println (lastDigit + firstDigit);

The problem is that the + sign here will perform integer
addition rather than the String concatenation that would
be desired.  One solution is to add an empty String to the
expression, as shown below:

Encapsulate your corrected program in a method named
printBackwards:

  public static void printBackwards (int number) {
     int lastDigit = number%10;
     int firstDigit = number/10;
     System.out.println (lastDigit + "" + firstDigit);
  }

The key to this question is to realize that part of what
I mean by "encapsulation" is the definition of the interface
of the method, which in this case includes one parameter,
the number we are printing backwards.

Part Four: flow of execution (8 points)}

Write the number 1 next to the first statement of this
program that gets executed.  You don't have to number the rest
of the statements.

   As always, the first statement in main is the first statement
   that gets executed.

When clink invokes zoop, what is the value of the
second argument?

       4

What is the output of this program?

just for
any not more
It's breakfast
!

I was hoping that someone would see that output and decide
that the right answer must be, "It's not just for breakfast
any more!" but no one did.


Part Five: programming (10 points)

Please write a method named checkFermat that takes four
integers as parameters---a, b, c and n---and
that checks to see if Fermat's theorem holds.  If
n is greater than 2 and it turns out to be true that 

a^n + b^n = c^n

the program should print ``Holy smokes, Fermat was wrong!''
Otherwise the program should print ``No, that doesn't work.''

  public static void checkFermat (int a, int b, int c, int n) {
    String message = "No, that doesn't work.";
    if (n <= 2) {
      System.out.println (message);
      return;
    }
    if (raiseToPow (a, n) + raiseToPow (b, n) == raiseToPow (c, n)) {
      System.out.println ("Holy smokes, Fermat was wrong!");
    } else {
      System.out.println (message);
    }
  }

One key to this problem was to read the problem specification clearly.
It says that we should print the "Holy smokes" message if (n>2) AND
(a^n + b^n = c^n), and that it should print the other message
OTHERWISE.  In this case, OTHERWISE means "if either of the specified
conditions are false."

Since the same message gets printed twice, I put it into a temporary
variable.

Part Six: short answer (8 points)

1) Why do methods have parameters?

Parameters make methods more general.  Instead of solving a
specific problem, they allow a method to solve a general class
of problems.

Many people answered a different question, which is "Why do
you have to provide arguments when you invoke a method?"
The answer is that you have to provide the information the
method needs to do its job, but that answer does not address
the question.


1) When people write contracts and laws, they use a stilted
form of English that is like a formal language in many ways.
What are the characteristics of a formal language that make it
useful for legal applications?

Formal languages are unambiguous, which is very desirable
for contracts and laws.  The clearer the law/contract, the
less likely that it will lead to disputes.

They tend to be more concise than natural languages, although it is
questionable whether that is true of legalese.

Formal languages are hard to understand, which may be a good
thing from the point of view of a lawyer, since it provides
job security.

Formal languages do not evolve over time the way natural
languages do, so there is less chance that the meaning of a
law will drift.

For this question, many people mentioned the attributes of
formal languages but failed to relate them to the application
domain in question.  In other words, it is not enough to
say that formal languages are literal---you have to say why
that is a good thing for legal applications.