Lab 9 In-Class: More Loops

As usual, create a lab9 subdirectory for today's lab, open up Mozilla and the Web version of this handout, and open emacs.

  1. Program contains a simplified version of the Account class from an earlier lab. As you probably recall, the Account class models a simple bank account. In this exercise you will write a program that instantiates an Account object then lets the user deposit money, withdraw money, and check the balance as much as he/she wishes.

    1. First open and modify the withdraw method to make sure the amount to withdraw is positive and the balance is sufficient to cover the withdrawal amount. If there is not enough money in the account print an "Insufficient funds" message and don't withdraw any money; similarly, if the amount is negative print an error message (and do nothing else); otherwise, deduct the amount from the balance.

    2. The file contains a skeleton of the program described above. Complete the program as follows (as indicated by the comments in the program):
      • Declare and instantiate an Account object with the information read in.
      • Add a do...while loop that executes until the user chooses to quit. The loop should print out a menu of options in the following format:
                 1. Deposit
                 2. Withdraw
                 3. Check the balance
                 4. Quit
                 Enter your choice (1,2,3, or 4): 
        Then read in the user's choice (in the choice variable already declared) and do what the user requests (which requires using the appropriate method from the Account class). Note that if the user chooses deposit or withdraw, the program must ask for and read in the amount. Also, if the user types in an incorrect choice (such as a 6) the program should print a message telling the user to try again.

    3. Thoroughly test your program.

    Print and to turn in.

  2. The Coin class from Listing 5.4 in the text is in the file Copy it to your directory, then write a program to find the length of the longest run of heads in 100 flips of the coin. A skeleton of the program is in the file You will add to the skeleton as follows (see the comments in the program):
    1. Create a coin object.
    2. Inside the loop, use the flip method to flip the coin, the toString method (implicitly -- just print the coin object, and it will call the toString method automatically) to print the results of the flip, and the isHeads method to see if the result was HEADS.
    3. To keep track of the current run length (the number of times in a row that the coin was HEADS), note that if the coin is HEADS on this flip, the current run is one longer than it was; if it's not HEADS, then the counter starts over. Be sure to update the maxRun variable as necessary.
    4. While you're checking for HEADS, if it's HEADS also print the number it is in the current run. For example, your output might start like this, indicating a longest run (so far) of 4:
      Heads 1
      Heads 2
      Heads 1
      Heads 2
      Heads 3
      Heads 4
      Heads 1
      You'll have to work a little on the formatting to get this to come out right (think about where you'll use print and where you'll use println).

    5. After the loop, print the length of the longest run (clearly labeled, of course).

    Print to hand in.

  3. Write a modified version of the Stars program (Listing 5.14 on page 250) that prints out the following pattern. The number of rows should be an input variable rather than a constant so the program should work for any number of rows.
                        * *
                       * * *
                      * * * *
                     * * * * *
                    * * * * * *
    Note: You need a loop to print out some spaces in each row. Carefully plan how many you need!

    Print a copy of your final program to hand in.

  4. Write a program that reads in a phrase or sentence and determines the percentage of letters in the phrase that are vowels. The program must count all the letters in the phrase and count the vowels (a, e, i, o, and u). The file contains the skeleton of the program. Open it and do the following as indicated in the comments:
    1. Add declarations for any additional variables you need and initialize them as appropriate.
    2. Change the phrase read in to all lower case for easier processing (so you don't have to test for both upper and lower case vowels).
    3. Write a for loop to go through the phrase character by character. In the body of the for loop you need to determine if the current character is a letter. If it is you need to count it and then see if it is a vowel. There are several ways to determine if a character is a letter including different ways of comparing characters. However, another way is to use the isLetter method from the Character class. The signature of the method is as follows:
            public static boolean isLetter(char c)
      (See page 764.) Recall that a static method is called using the class name. Hence, a call would have the form
      where ch is a variable of type char. Since a boolean is returned the above call should be placed in an appropriate place for a boolean such as an if.

    4. After the loop print out the number of vowels and the total number of letters, appropriately labeled.

    5. Also compute and print the percentage of letters that are vowels. Use a percent formatter object from the NumberFormat class to print the percent. To do this you need to do the following (see pages 131-132 for an example).
      • Add an import statement to import java.text.NumberFormat.
      • Just before you print, get a format object (named fmt for example) as follows:
              NumberFormat fmt = NumberFormat.getPercentInstance();
      • Then, in your print statement use the format method to print the percent (note: the formatter multiplies by 100 for you):
         System.out.println ("Percentage of vowels: " + fmt.format(whatever));
    6. It would be nice to have the program let the user keep entering phrases rather than having to restart the program every time. To do this we need another loop surrounding the current code. That is, the current loop will be nested inside the new loop. Add an outer while loop that will continue to execute as long as the user does NOT enter Q or q for the phrase. Note that this makes the outer loop a sentinel controlled loop -- the program is processing phrases and a special value Q will stop it. The outline of the program is as follows:
           read in the first phrase
           while the phrase is not "Q"
             initialize the counters etc. to set up the for loop
             for loop to count letters and vowels in the phrase
             print the number of vowels & letters and percentage of vowels
             read in the next phrase or "Q" to quit
      Note that all you need to do is add the sentinel controlled while loop around the code you already have. Be sure the code to initialize the counters is inside the while loop (you start the counters over for each phrase).

      Be sure to go through the program and properly indent after adding code (with nested loops the inner loop should be indented). Emacs will do the work for you! Select the region you want to indent (with the mouse or keys), then press Alt-Ctrl-\ ("indent region"). Voila!

    Print the final version of your program

Hand in: