CSCI A201/A597 Lecture Notes 15 Spring 2000

1. A Note on the Practical

First a note on the practical exam, most of you did very well on it, and grades have been posted already. There's one thing that you need to remember from this experience: don't stop when you hit a roadblock. Go around it, invent a bridge, simulate the road. Here's an example.

A few of you got stuck in the syntax of

Integer.parseInt(args[0])

public class Practical {
  public static void main(String[] args) {
    int size = Integer.parseInt(args[0]), i, j;
      for (i = 0; i < size; i++) {
        for (j = 0; j < size; j++) {
          if (j == 0 || i == size - 1 && j <= (size - 1) / 2) {
            System.out.print("*"); 
          } else { 
            System.out.print(" "); 
          } 
        }
      System.out.println(); 
    }
  }
}

public class Practical {
  public static void main(String[] args) {
    int size = 10, i, j;
      for (i = 0; i < size; i++) {
        for (j = 0; j < size; j++) {
          if (j == 0 || i == size - 1 && j <= (size - 1) / 2) {
            System.out.print("*"); 
          } else { 
            System.out.print(" "); 
          } 
        }
      System.out.println(); 
    }
  }
}

This way you get as much partial credit as you deserve. And, perhaps by the time you're done finishing testing and developing the program this way you may even actually remember the exact syntax of the line that you were missing.

2. Midterm Review

I include below a number of exercises, with solutions, similar to the ones that you will see on the midterm exam. The midterm will be composed of only objective items, that is items that are either multiple-choice, or for which there is just one answer that you need to provide.

The questions on the midterm will test your overall basic understanding of your basic Java knowledge. Here's what it will cover:

• values, types, operators, expressions
• variables, assignment statements
• control structures: if statements
• control structures: while and for loops

Example 1

Here's a question similar to the first question on quiz 4 that was due last week. Assume option can be only 1, 2 or 3 before each of the code fragments illustrated below. Which ones set i to the value that option has? We will look at this kind of problems with the help of diagrams.

Code	Diagram
if (option == 1) i = 1; else if (option == 2) i = 2; else i = 3;
In this particular case it's easy to see that the code sets i to the same value as option (given the assumption about the possible values that option can have).

For the remaining of these notes we will draw the diagrams and fill them with text in class.

Code	Diagram
i = 1; if (option >= 2) i = i + 1; if (option == 3) i = i + 1;

Code	Diagram
if (option == 1) i = 1; if (option == 2) i = 2; else i = 3;

Code	Diagram
i = 1; if (option > 1) if (option > 2) i = 3; else i = 2;

Code	Diagram
i = 1 if (option == 1); i = 2 if (option == 2); i = 3 if (option == 3);	Incorrect syntax, no diagram.

Example 2

Consider this code fragment

if (x > y)
  z = z + 1;
else 
  z = z + 2;

Code	Diagram
if (x > y) z = z + 1; if (x <= y) z = z + 2;

Code	Diagram
if (! (x > y)) z = z + 2; else z = z + 1;

Code	Diagram
z = z + 1; if (x <= y) z = z + 1;

Code	Diagram
if (x > y) z = z + 1; else if (x <= y) z = z + 2;

Code	Diagram
z = z + 2; if (!(x > y)) z = z - 1;

Two code fragments will be considered equivalent when they behave in the same way (same output, same internal state) for identical inputs, for all possible inputs.

Other kinds of problems that you can see on the exam could be like the following two.

Problem 1 Consider the following two program fragments:

Fragment 1	Fragment 2
if (x == 5) x = x + 1; else x = 8;	if (x == 5) x = x + 1; if (x != 5) x = 8;

Check all that apply:

• The two fragments are logically equivalent
• Fragment two contains a syntax error.
• If x is 6 initially then
  • the value in x is 8 after executing fragment one, and
  • the value in x is 6 after executing fragment two.
• x always has the value 8 after executing fragment two.
• x has either the value 5 or the value 8 after executing fragment one.

Problem 2

Assume that x and y are integer variables. Consider the following nested if statement.

if (x > 3)
  if (x <= 5) 
    y = 1;
  else if (x != 6) 
    y = 2;
  else 
    y = 3;
else 
  y = 4;

Problem 3

Assume that x and y are integer variables. and the code fragment shown below:

if (x > 3) { 
  if (x <= 5) 
    y = 1;
  else if (x != 6) 
    y = 2;
} else 
  y = 3;

1. If x is 1 before the fragment gets executed what's the value of y after the fragment is executed?

2. Now erase the curly braces. What value does x need to have before the fragment gets executed for y to be 3 at the end of the fragment?

Remember that assignment 6 consists of 6 batches of about 10 problems each, all objective, and open-ended, graded automatically by QuizSite. Some are true-false or multiple-choice. Note the due date: Thursday, before the midterm, at 10am.

After the midterm in lab eight we start on assignment 7 which will be due at the end of the lab next week (but you can turn it in earlier). Turning in will done through the handin program as before the QuizSite exercises.

Please bring these notes with you to class tomorrow.