Lab 7 In-Class: More Selection

This lab is designed to give you more practice writing conditional statements. As usual, create a subdirectory for the lab, open up the Web version of this handout in Mozilla, and open emacs.

Exercise #1: Activities at Lake Lazydays

Print your final program to turn in.

The Switch Statement

System.out.println("Enter size box you want");
int boxSize = scan.nextInt();
double price;

if (boxSize == 1)
{
   System.out.println("Small");
   price = 2.5;
}
else if (boxSize == 2) 
{
   System.out.println("Medium");     
   price = 3.75;
}
else if (boxSize == 3)
{
   System.out.println("Large");
   price = 5.0;
}
else
   price = -1;

switch (boxSize)
{
   case 1:
      System.out.println("Small");
      price = 2.5;
      break;
   case 2:
      System.out.println("Medium");
      price = 3.75;
      break;
   case 3:
      System.out.println("Large");
      price = 5.0;
      break;
   default:
      price = -1;
}

if (price > 0)
   System.out.println("Price is $" + price);
else
   System.out.println("Sorry, we don't have that size");

A few things to note about switch statements:

• The expression you are switching on (boxSize in the example above) must have an integer or character value. It cannot be a boolean, floating point, or object.
• The expression is evaluated once and then is matched against each of the cases, starting at the top. When a case matches, all the rest of the code including later cases is executed. However, very often you will only want to execute the code for a single case. To do this, put a break statement after the code for that case; this will break control out of the switch statement, sending it to the next statement in the program.
• If no other case matches, default always matches (like else in an if statement).

Exercise #2: Rock, Paper, Scissors

$ java Rock
Enter your play: R, P, or S
r
Computer play is S
Rock crushes scissors, you win!

When your program works, modify it so that if either the user or the computer does not have a legal play (R,P,S), it prints a message telling which is wrong (user or computer) and terminates without playing the game. (Note that this will mean modifying the switch statement so that if the value is not 0,1, or 2 it assigns a different letter, maybe I for illegal, to computerPlay. Of course, if the computer's play is illegal that means your statement that generates the random number is wrong!) If both plays are ok, go ahead and print the computer's (legal) play and give the winner. Think about the condition for this: the user's play should be R or P or S; if it's not one of these, there's a problem. Similar reasoning holds for the computer's play. So the structure of the last part of your program should now look like this:

  if (the computer's play is illegal)
     print message
  else if (the person's play is illegal)
     print message
  else
  {
     print computer play
     determine and print winner 
  }

Print your completed program to turn in.

Exercise #3: Date Validation

1. In the validDate method in the Date class add the following as indicated by the comments (ignore the comment about valid day for now) in the program:
   • An assignment statement that sets monthValid to true if the month entered is between 1 and 12, inclusive.
   • An assignment statement that sets yearValid to true if the year is between 1000 and 1999, inclusive.
   • Change the return statement (which currently just returns true) to return true if the month and year are valid (and false otherwise). Note that your return statement should just return a boolean expression.

2. In TestDates.java do the following (as indicated by the comments):
   • Write a statement that instantiates the someDay object.
   • Write an if statement that prints either a "Date is valid" or "Date is not valid" message. You need to invoke the validDate method of the Date class.

3. Compile and run the TestDates program to see if it works so far. Enter some months and years that are not valid and some that are.

4. Add a method boolean leapYear() to the Date class that returns true if the year is a leap year. Here is the leap year rule: A year is a leap year if a) it's divisible by 400, or b) it's divisible by 4 and it's not divisible by 100. Your leapYear method should have a single return statement that returns the boolean condition describing a leap year.

5. In the if statement in TestDates.java, in the case that the date is valid add an if to test whether or not the year is a leap year. Print out an appropriate message (is/is not a leap year).

6. Compile and run TestDates.java to test your leapYear method. Some years that are leap years: 1600, 1996, 1776; some years that aren't: 1998, 1900, 1494.

7. In the daysInMonth method in the Date class add an if statement that determines the number of days in the month entered and stores that value in variable numDays. If the month entered is not valid, numDays should get 0. Note that to figure out the number of days in February you'll need to check if it's a leap year (that is, you will need to call the leapYear method).

8. In the validDate method add an assignment statement to set dayValid to true if the day is a valid day in the given month and then modify the return statement to return a boolean expression that is true if the date is valid.

9. Compile and run TestDates to test your latest changes. Be sure to test all months to see if the last day in the month is correct.

10. Complete the toString method in the Date class by completing the switch statement that assigns a name to the month and change the return statement to return a string for the date in the standard way we write dates (for example, October 25, 1996).

11. Modify the print statements in TestDates to print the Date object (implicitly using toString) along with the message. For example, if the date entered is 3, 2, 1993 the message should be "March 2, 1993 is a valid date." but if 3, 32, 1993 is entered the message should be "March 32, 1993" is not a valid date."

Exercise #4: Flipping a Coin - An Introduction to Loops

     int numTimes, count;

     System.out.print ("How many times do you want the loop to go? ");
     numTimes = scan.nextInt();

     // Initialize the loop count
     count = 0;

     // Here's the loop... It is controlled by the condition count < numTimes
     while (count < numTimes) 
     {
        // This is the body of the loop - the statements executed repeatedly
        count++;
        System.out.println ("Looping... " + count + " time(s)!");
     }

     // After the loop
     System.out.println ("Out of the loop!");

• The setup, or initialization. This is before the actual loop and is where variables are initialized in preparation for the loop.
• The condition which is a boolean expression that controls the loop. If it evaluates to true, the body of the loop is executed, and then the condition is evaluated again; if it evaluates to false, the loop terminates.
• The body of the loop. This is where the work is done. In the above example the count variable is incremented and a message is printed.

1. Save CoinFlips.java to your directory then compile and run it to see how it works.
2. Now change the program to one that repeatedly flips a coin and counts the heads and tails. The program needs to use the Coin class defined in your textbook (pages 213 - 215) and in the file Coin.java. Save Coin.java to your directory (you don't need to change it at all). Do the following to CoinFlips.java (as indicated in the comments in the program).
   • Declare and instantiate a Coin object named myCoin
   • Before the loop initialize the variable numHeads by setting it to 0.
   • In the body of the loop flip the coin (note that you use a method to do this!).
   • After you flip the coin, add an if to determine if the coin is heads (again - there is a method!). If it is, increment the numHeads variable.
   • Modify the print statement so that it prints out the value of the Coin object (see the toString method in the Coin class - it determines what is printed) along with the count. For example, if the coin turned up tails on the 3rd flip, it should print the message "Flip 3 is Tails". Your print statement should directly print the myCoin object to get the Heads or Tails part of this message.
   • After the loop put a print statement that prints out the number of times the coin was heads AND the percentage of times (so if the coin turned up heads 12 times in 20 flips it was heads 60% of the time).
3. Compile and run the program. Test it thoroughly. Print out CoinFlips.java to hand in.

What to Hand In

• Printouts of Temp.java, Rock.java, Date.java, TestDates.java, and CoinFlips.java.
• Tar up your lab7 directory and e-mail it to your instructor at roanoke.edu. Put cpsc120 lab7 in the subject line.