Lab 6 In-Class: More Selection

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

Exercise #1: Activities at Lake Lazydays

Write a program Temp.java that prompts the user for a temperature, then prints out an appropriate activity as described in #1 of the prelab. As noted in the prelab, you should use a single cascading if and should keep your conditions as simple as possible.

When this works, add the extreme temperature condition from #2 in the prelab. Add a boolean variable extremeTemp and give it a value as described in the prelab; then modify your if statement so that if the temperature is too low or too high, the program prints "Visit our shops!" instead of any of the other activities. Print your final program to turn in.

The Switch Statement

The Java switch statement (p. 144-148) is another conditional statement. Switch provides a compact, efficient syntax that is useful when you would otherwise have a cascading if statement in which each condition checks the value of the same expression. For example, the if and switch statements below do the same thing -- either could be used as shown in the program:

System.out.println("Enter size box you want");
int boxSize = Keyboard.readInt();
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).

You will use a switch statement (as well as a nested if) in the next exercise.

Exercise #2: Rock, Paper, Scissors

Program Rock.java contains a skeleton for the game Rock, Paper, Scissors. Open it and save it to your lab6 directory. Add statements to the program as indicated by the comments so that the program asks the user to enter a play, generates a random play for the computer, compares them and announces the winner (and why). For example, one run of your program might look like this:

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

Note that the user should be able to enter either upper or lower case r, p, and s. The user's play is stored as a string to make it easy to convert whatever is entered to upper case. Use a switch statement to convert the randomly generated integer for the computer's play to a string.

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.) 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

In this exercise you will write a program that checks to see if a date entered by the user is a valid date in the second millenium. A skeleton of the program is in Dates.java. Open this program and save it to your lab6 directory. As indicated by the comments in the program, fill in the following:

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.
An assignment statement that sets leapYear to true if the year is a leap year. Here is the leap year rule (there's more to it than you may have thought!):
- If the year is divisible by 400, it's a leap year.
- Otherwise, if the year is divisible by 100 it is not a leap year.
- If neither of the above applies and the year is divisible by 4, then it is a leap year.
Put another way, it's a leap year if a) it's divisible by 400, or b) it's divisible by 4 and it's not divisible by 100.
An if statement that determines the number of days in the month entered and stores that value in variable daysInMonth. If the month entered is not valid, daysInMonth 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.
An assignment statement that sets dayValid to true if the day entered is legal for the given month and year.
If the month, day, and year entered are all valid, print "Date is valid" and indicate whether or not it is a leap year. If any of the items entered is not valid, just print "Date is not valid" without any comment on leap year.

Print your completed program to turn in.

What to Hand In

Printouts of Temp.java, Rock.java, and Dates.java
Tar up your lab6 directory and e-mail it to your instructor at roanoke.edu. Put cpsc120 lab6 in the subject line.