Lab 11 In-Class: Arrays
As usual open an xterm, Emacs, and Firefox and create a lab11 directory
for your work.
The file Salaries.java contains a variation
on the code in pre-lab that read in a list of salaries. The pre-lab
problem asked the user for the number of salaries first then created
the array and used a count-controlled for loop to read in the salaries.
This version is a sentinel controlled that does not ask how many
salaries ahead of time. Instead it lets the user keep entering
salaries until a negative number is read in.
- Download the program and compile it. What is wrong?
- Fix the problem above by creating an array of size 5 (MAX).
- Compile and run the program entering the following numbers
for salaries: 54000, 48375, 15800, 33000, 98600, 21500. What happens?
- Fix the problem above by modifying the loop control condition so that
the index will not go out of bounds. After the loop add an if statement
with the message "Sorry, the program cannot handle more than ___ salaries"
in the case that the index went out of bounds
(insert the MAX number of salaries instead of the blank).
- Run the program again, trying to enter more than 5 salaries. Be
sure that instead of a runtime error you get a message saying the program
cannot handle more than 5 salaries.
- Change MAX to 50, then add code to the existing loop to find
the average of the salaries
entered. Print the average after the loop. TEST your code!
- Add a for loop AFTER the existing code (see the comment in the
program) to print each salary that is above average
and count them. Before the loop print a statement to label
the list - for example "Above Average Salaries". After the
loop print the number of above average salaries, appropriately
- Be sure your program is working correctly and is formatted
correctly (CTRL-x CTRL-p followed by CTRL-ALT-\).
More Basic Array Processing - Grading Quizzes
In this exercise you will write a program that grades arithmetic
quizzes (all the answers are integers). The program will
ask the user how many questions are on the quiz, then it will ask
for and read in the answer key, storing the values in an array of
integers. The program will then read in a student's answers and
compute and print the number correct and the percent correct.
- The file Quiz.java contains an outline of
the program. Download it then use the comments as a
guide to complete the program.
- Test your program using the following input:
Number of Questions: 10
Correct Answers: -3 120 45 76 89 -13 1290 -57 99 0
Student Answers: -3 120 39 76 82 -13 1290 -57 25 0
(The student got 7 right which is 70%)
Number of Questions: 4
Correct Answers: 538 -300 782 45
Student Answers: 500 300 782 450
(The student got 1 right which is 25%)
Some Cryptography - working with type char
One common way to encrypt a message is to replace each letter in the
message with a different letter. This is the way the CryptoQuote puzzles
in the newspaper work. Each letter in the alphabet (called the plain
alphabet) will have a corresponding letter in the cipher alphabet (see
the example below). To encrypt a message (called the plaintext) each
letter is replaced with the corresponding letter in the cipher
alphabet. Using the cipher alphabet below the message "Java is cool"
would become "lxix qy fddg".
In this exercise you will use this technique (called a substitution
cipher) to encrypt messages. The file Encrypt.java
contains a skeleton of the program. Notice that it uses an initializer list
to set up the cipher alphabet in an array of chars. The alphabet is the
same as the example above.
- Open Encrypt.java and add code to perform the encryption. Notice
that there is already code to read in the message and convert it to
lowercase. So you only need to worry about lowercase letters. To do
the encryption for each letter in the message you need to "look up"
letter in the
alphabet array to see what letter to substitute. You MUST do this
using the ideas from the letter counting program on pages 340 - 341
of the text. The main idea is to use arithmetic on characters to index into
the alphabet array. Note that characters that are not lowercase letters
should just be printed out.
- Test your program thoroughly!
Arrays of Objects
File CDCollection.java contains the code
for the CDCollection class (pp. 352 - 353) that we discussed in class. File
CD.java contains the code for the CD class (p. 354)
CDCollection uses, and Tunes.java contains a
slightly modified version of the Tunes class (pp. 350 - 351) that uses
CDCollection. Save these files to your directory.
- Compile and run Tunes.java. You should see a list
of 6 CDs with a total value of $97.69 and an average cost of $16.28 followed by
a menu that lets you add a CD, search for a CD with a given title, or
list all CDs for a given artist. Only the add part has
been implemented, so add a few CDs to see what happens. When you add the 11th CD (total),
you should notice a message that says "Increasing size of array," which comes from
the increaseSize() method. This was added just to help you follow the program's
- Add the findCDIndex method from the prelab to the CDCollection
class. Note that getter methods have been
added to the CD class - you need to use getTitle.
Be sure your code doesn't look through the
array any longer than you have to. A boolean is a nice way to keep track
of whether or not you have found the CD and stop the loop when you do find it.
- In Tunes.java add a call to your method (under option 2). Note
that code is already there to prompt for and read in the title of the
CD to search for. Add code that calls your method and prints a message
indicating whether or not the CD is in the collection.
- Now run Tunes again and choose the search option. Also try adding
some CDs, then searching for ones that you added.
- Add a method public CD getCD (int index) to the CDCollection
class that returns the CD in the collection at the given index.
- Modify Tunes.java so that if the CD with the given title is found
the actual CD information (the CD object) is printed. You should use
the getCD method along with the index returned by your find method.
Run the program again to make sure this works.
- Now add a method listByArtist that takes a String parameter
for the artist, then returns a String consisting of a list of all
CDs by that artist. Use the idea in the toString
method of the CDCollection
class for composing the string to return. Note that toString adds
all CDs to the string. You want to just add the ones with the given
- In Tunes.java add a call to the listByArtist method and print
the list returned. Also remove the 2 "under construction" messages from
- Test your program. Be sure to add more than one CD by the same artist
so you can see if your method is correctly getting all CDs.
To submit your code: Tar the files in your lab11 directory
(be sure to name the file with your names, not lab11.tgz)
and cp the tgz file to the directory: