CIS110 Labs

In lab, we will practice the basics of designing classes. Recall that classes are blueprints for objects. Objects in turn contain state (fields) and behavior (methods).

Imagine that you are writing a Nintendogs + cats clone. A reasonable design might involve writing a class that represents the pet the player takes care of in the game.

Please sketch out either a Dog or Cat class (your preference) that you might use for this program. In your sketch, you should call out in bulleted-list form the state and behavior of your class. You do not need to write any code, but for the state, please include the type and name of the field. And for methods, please include the name and a brief (one sentence) description of what the method does.

For example, to get you started, you might start your sketch out for a Cat class like this:

• State:
  • int age
• Behavior:
  • meow: makes the cat meow (to the console)

In your design, you should include at least 3 pieces of state and 3 pieces of behavior. In lab, we'll talk about how we take sketches like this and translate them into code.

Exam #2

Time to prep for the second midterm! Please review the sample midterm briefly and write down the one question you most want to answer in section about it.

In lab this week, we are drilling hard on how to traverse and manipulate arrays. As warm-up, consider the following problem:

[1][2][3][4][5][6][7][8][9]

Then the result should be:

[0][1][2][3][4][5][6][7][8]

Now consider the following code that attempts to implement shiftRight:

public static void shiftRight(int[] arr) {
    arr[0] = 0;
    for (int i = 0; i < arr.length - 1; i++) {
        arr[i+1] = arr[i];
    }
}

Answer the following questions about this implementation:

• Does this method work?
• If not, what is broken about it?

We'll start off lab by discussing the answers to these questions.

So far in the class we've focused on control issues in programming. However, we've talked relatively little about the data we manipulate and how we represent is in our programs. As we learn this week, files are one way to organize large amounts of data. However, in order to access this data effectively, we need to be mindful of how we layout our data in files.

In lab, we'll practice our file input and output skills, but as a warm-up lets consider how we might arrange a file ourselves given a set of data. For example, consider a file that contains a series of student and faculty records. Each record contains the following information:

• PennID #
• PennID username
• Student or faculty

If the record belongs to a student, then it also contains the following data:

• Class standing
• GPA

If the record belongs to a faculty member, then it also contains the following data:

• Department
• Tenured?

Write down how you would layout this information in a file. Each line of your file should contain a single record, so you should describe the components of the record on each line including their expected types. For example, if the file contained only PennID #s and GPAs, then I would specify something like:

<PennID #> <gpa>

Since there are two kinds of records, you will need a "tag" of sorts in your record to differentiate between the two kinds.

Understanding programming assertions and establishing invariants is a critical part of programming in an imperative style, i.e., where we can change the contents of variables.

Consider the following piece of code:

Scanner in = new Scanner(System.in);
int val = 0;
for (int i = 0; i < 10; i += 2) {
    // POINT A
    val += i * 2;
}
val = val * in.nextInt();
// POINT B

Answer the following questions about the code fragment above, choosing one of always, never, or sometimes for each part. You'll discuss the results in labs along with other problems similar to this.

1. At point A:
   • Is i % 2 == 0 always/never/sometimes true?
   • Is i < 6 always/never/sometimes true?
   • Is val < 0 always/never/sometimes true?
2. At point B:
   • Is i < 10 always/never/sometimes true?
   • Is val >= 0 always/never/sometimes true?

Sketch a solution to the following programming problem. Like the previous problems we've tackled, you shouldn't start coding immediately, instead you should take a step back, analyze the problem, and decompose it. Your sketch should be an outline of how you break down this problem. The format of the sketch is not particularly important. However, your sketch should call out what static methods you need to make to solve the problem.

In section, you will take your sketch and create a complete solution from it.

This program converts celsius to fahrenheit and vice versa.
Are you providing a celsius or fahrenheit value [C or F]? C
Provide the degree value to convert. 0
The converted amount is 32.0.
===
This program converts celsius to fahrenheit and vice versa.
Are you providing a celsius or fahrenheit value [C or F]? F
Provide the degree value to convert. 120.50
The converted amount is 49.166666666666664.

1. Please fill out the post-exam #1 survey found on SurveyMonkey.
2. Each of the following code snippets requests input from the user. Based off of the names of the variables involved, come up with a list of possible "bad inputs" that the user might provide. That is, create of list of example inputs from the user that would be accepted by the code but would not be "correct" in some sense. For each such bad input, suggest a check you could write to detect such input.

   For example, if we requested the age of the user, a bad input would be a negative number like -50. The check we could make would be to ensure that age > 0.

   • Scanner in = new Scanner(System.in);
     int monthOfBirth = in.nextInt();
         

   • Scanner in = new Scanner(System.in);
     double amountToWithdrawFromBankAccount = in.nextDouble();
         

   • Scanner in = new Scanner(System.in);
     String rockPaperOrScissors = in.nextLine();
         

Exam #1

Assigned: Monday, September 26

Write down the output of the following program. (Note that this is problem 3.5 from the text if you're looking for more practice for the exam!)

public class MysteryTouch {
    public static void main(String[] args) {
        String head = "shoulders";
        String knees = "toes";
        String elbow = "head";
        String eye = "eyes and ears";
        String ear = "eyes";

        touch(ear, elbow);
        touch(elbow, ear);
        touch(head, "elbow");
        touch(eye, eye);
        touch(knees, "Toes");
        touch(head, "knees " + knees);
    }

    public static void touch(String elbow, String ear) {
        System.out.println("touch your " + elbow + " to your  " + ear);
    }
}

Assigned: Monday, September 19

Trace the execution of these loops.

// Loop #1
for (int i = 1; i < 256; i *= 2) {
  System.out.println(i);        // HERE
}

// Loop #2
for (int i = 0; i < 5; i++) {
  for (int j = i; j > 0; j--) {
    System.out.println(i - j);  // HERE
  }
}

Create a table (ala lecture) for each loop showing the value of the loop variables (i in #1, i and j in #2) and the text printed after executing the line marked "HERE".

Assigned: Wednesday, September 12

Here is a diagram similar to the ones we'll be drawing initially in lecture and lab:

+-----+
|     |
|     |
+-----+
+-----+
|~~~~~|
|~~~~~|
+-----+
+-----+
|     |
|     |
+-----+

How can we can decompose this problem into multiple static methods? Do not create the actual program and hand it in at lab time (although if you have the time, it's a good practice problem). Instead, imagine the static methods you might make to solve this problem, the method calls you would make in your main method, and list those instead. For example, if you envision that your program would require a method called foo twice, that would in turn call a method bar, then you would write:

• foo
  • bar
• foo
  • bar

Please turn in this bulleted list of method calls at the beginning of lab.

First week of class.