Lab 2 In-Class: Variables, Expressions and Types
As usual, start Emacs, Firefox, and an xterm window.
In the browser go to the class page and
open this document.
Some System Hints
A handy system feature is the ability to put one or more commands
into a file and then execute the file, instead of giving all of the
to the shell. This is called a shell script. For example,
we can use a shell script to keep
from having to type that long print command (nenscript -2rG filename)
by putting the command,
slightly modifed (see below), into a file called print (or
something else if you prefer) in your home directory. After changing the
permissions on the file to make it executable (text files aren't
executable by default),
you can refer to it instead of the whole command. Here are the steps:
You can now use this script to print from any directory with the
- In Emacs, open a new file named print in your home directory.
- Type the following into the file:
nenscript -2rG $1
This looks just like the print command you have been using, but it has a $1
instead of the filename.
is an argument, or parameter;
it tells the command to use whatever you type after
the name of this script file in the place of the $1.
- Save your script using C-x C-s.
- Go to an xterm and change to your home directory. Type the following
ls -l print
Look at the permissions; you should see that no one has execute permission.
To change this, issue the following command:
chmod a+x print
The Unix command chmod changes the permissions, or mode, of a file. The
"a+x" part means for all(a) users, add(+) the execute(x) privilege. The
last part (print) is just the name of the file. If you look at the
permissions again, you'll see that the file is now executable.
This tells it to use the print command in your home directory to print
On to Java
In the xterm, change to your labs
create a subdirectory called lab2, and change to that directory.
A variable is a name for a memory location that holds a value. The
value that is stored in this location can be changed, hence the name
variable. In Java, a variable must be declared before it
can be used. The declaration gives the type of value that will be stored so
that the compiler knows how much space to allocate for it. By convention,
Java variables start with
a lower case letter.
A constant is simply a name for a value. As its name implies, its
value cannot be changed. In
Java, a constant declaration looks just
like a variable declaration except it has the reserved word final in front.
By convention, constants are written in all capitals so that they are
easily distinguished from variables.
Study the program below, which uses both variables and constants:
// File: Circle.java
// Purpose: Print the area of a circle with two different radii
public class Circle
public static void main(String args)
final double PI = 3.14159;
int radius = 10;
double area = PI * radius * radius;
System.out.println("The area of a circle with radius " + radius +
" is " + area);
Some things to notice:
- The first three lines inside main are declarations for
PI, radius, and area. Note that the type for each is given in these
lines: final double for PI, since it is a floating point constant;
radius, since it is an integer variable, and double for area, since it
will hold the product of the radius squared and PI,
resulting in a floating point value.
- These first three lines also hold initializations for PI, radius, and
area. These could have been done separately, but it is often convenient
to assign an initial value when a variable is declared.
- The next line is simply a print statement that shows the area for a
circle of radius 10.
Save this program, which is in file Circle.java,
into your lab2 directory, open it in emacs and modify it as follows:
- Put your name in the header documentation!
- Currently each variable is declared and initialized
in the same statement. However, often it is easier to
read and modify a program in which the variables are declared first,
and assignments and calculations are done later. Modify the program by
separating the declarations of variables radius and area from their
initializations. In particular, declare both variables first (after
the declaration/initialization of the constant PI which should stay
the same), then put the two assignment statements to assign values
to each variable. Compile and run the program to make sure you
did things correctly.
- The circumference of a circle is 2 times the product of Pi and the radius.
Add statements to this program so that it computes and prints
the circumference for the circle.
You will need to do the
- Declare a new variable to store the
circumference. Put this declaration immediately after your
declaration of area.
- Compute the circumference and store the value in your new variable.
(Decide where this should go in the program.)
- Add a print statement
to print the circumference (clearly labeled, of course).
- When the radius of a circle doubles, what happens to its circumference
and area? Do they double as well? You can determine this by doubling
the radius, then computing the area and circumference again and
dividing the new values by the old.
To do this you'll need new variables to hold the second area and
circumference, since you need both areas and both circumferences
to see how the value has changed. However, the new radius can
(and should) be stored
in the same variable as the old radius.
Modify the program as follows:
Look at the results. Is this what you expected? (You should be able
to figure out mathematically what the answers should be. If your results
aren't correct, check your program.)
- Add declarations for a new area variable and a new circumference variable. Be sure
their names are different from the first area and circumference
- After you calculate and print the initial area and
circumference, assign the
radius a value that it is twice its original value (note: your assignment
statement should work for any value stored in the radius variable, not
just 10). Now calculate
and circumference again -- storing them in the new variables -- and
- Compute the change in area by dividing the
second area by the first area. This gives you the factor by which the
area grew. You can print this value directly (by doing the division in
a print statement) or store it in a variable and then print it.
- Compute and print the change in circumference similarly.
- In the program above, you showed what happened to the circumference and
area of a circle when the radius went from 10 to 20. Does the same thing
happen whenever the radius doubles, or were those answers just for those
particular values? To figure this out,
you can write a program that reads in a value for the radius from
the user instead
of having it written into the program ("hardcoded"). Modify your program as follows:
Print your Circle.java to turn in. Use the command
contains the partial program below, which when complete
will calculate the amount
of paint needed to paint the walls of a room of the given
length and width. It
assumes that the paint covers 350 square feet per gallon.
// File: Paint.java
// Purpose: Determine how much paint is needed to paint the walls
// (not including the floor or ceiling) of a room given its length,
// width, and height
public class Paint
public static void main(String args)
final double COVERAGE = 350.0; //paint covers 350 sq ft/gal
//declare integers length, width, and height
//declare integers sideWallArea, endWallArea, and totalArea
//declare double paintNeeded
// Create a Scanner object (named scan)
//Prompt for and read in the length of the room
//Prompt for and read in the width of the room
//Prompt for and read in the height of the room
//Compute the area of a side wall (running the length
//of the room) in square feet.
//Compute the area of an end wall (running the width
//of the room) in square feet.
//Compute the total square feet to be painted (4 walls!)
//Compute the amount of paint needed
//Print the length, width, and height of the room, the total
//area, and the number of gallons of paint needed.
Save this file
to your lab2 directory, open it in emacs, put your name in the
header documentation and
fill in the missing statements (the comments
provide a guide) so that the program does what it is supposed
to. Compile and run the program and correct any
Print your Paint.java to turn in.
Integer Arithmetic and Data Conversion
The file Change.java contains a
program to compute the amount of change to be returned to a
customer after a purchase. Save the program to your
lab2 directory, open it in emacs and study it noting the following:
- The program currently takes the
amount of the purchase and the amount of cash tendered as input.
It then computes the amount of change to be returned to the
The goal is to have the program break that change
down into the number of dollars, the number of quarters, the
number of dimes, the number of nickels, and the number of
pennies (using the smallest number of coins) to be returned.
- Note that the number of dollars is already computed - it is
the integer part of the change and that is computed by
casting the double value in the variable totalChange
to an int (using the cast operator (int)).
Compile and run the program to see if the output matches
what you would expect.
Now add to the program as follows:
- Add an assignment statement to compute the number of
cents left over and store it in the variable cents
that has already been declared. (So if total change is 13.79 then the number
of dollars is 13 and the number of cents is 79 - an int).
- Add a statement to print the number of cents after the
number of dollars.
- Compile and run what you have so far to make sure it is
- Now add code to break the number of cents down into the
number of quarters, dimes, nickels, and pennies. You will need
to declare variables for each of these, write assignment
statements to compute each, and print statements to print
the results. Note that in your calculations integer division (the
/ operator with integer operands) and the remainder operator (%)
come in handy. It would be a good idea to add code for one
calculation at a time and check your results before going on. Which
value should you compute first - quarters, dimes, nickels, or pennies?.
Be sure your name is in the header documentation then
print your Change.java program to turn in.
Making a tar file
An easy way to send someone several files at once is to make a tar file.
(Tar stands for tape archive, a rather archaic name for this
To "tar up" everything in your lab2 directory, do the
When you're done, do an ls and you should see lab2.tgz in your
- Be sure you are in your lab2 directory.
- Type the following command:
tar czf lab2.tgz .
This strange looking command can be understood as follows:
c -- "create" a file containing the tarred up files
z -- "zip" this file, that is, compress it so it takes less space
f -- use the next argument to name this file
lab2.tgz -- the name to be used for the tar file
. -- tar up everything in the current directory
What to turn in
- Turn in printouts of Circle.java, Paint.java, and Change.java.
Be sure your
name is in the header of each of your programs. (Write it in if
you didn't type it, but in the future ALWAYS type your name in!)
- E-mail your tar file (lab2.tgz)
Remember to put cpsc120 lab2 in the subject of your message.