This week, you will download the BlueJ project Week4.zip and write methods to manipulate arrays. Two methods are given to you complete. You don’t have to do anything to them.

The method readArray

public int[] readArray()

reads a list of numbers, one per line, ending with a blank line, and returns an array containing the numbers. Don’t try to understand the details of this method. It uses things that you’ll learn about later.

The method printArray takes an integer array parameter and prints it to the terminal

public void printArray(int[] R)

The project provides templates for other methods that you have to write. Your job is to work out the code that goes inside them.

The first method, arrayAdd, takes two arrays as parameters and returns a new array that is the sum of the first two.

public int[] arrayAdd(int[] R1, int[] R2)

The addition of two arrays yields an array whose elements are the sum of the corresponding elements in the parameter arrays, i.e.

R1[0] + R2[0], R1[1] + R2[1], R1[2] + R2[2], ....

The dotProduct of two arrays returns a single integer.

public int dotProduct(int[] R1, int[] R2)

It is found by multiplying corresponding elements and adding them all up, i.e.

R1[0] * R2[0] + R1[1] * R2[1] + R1[2] * R2[2], ....

Finally, you should write a method, main,

public void main()

that reads two arrays, then:

• calls arrayAdd and calls printArray to print the resulting array
• calls dotProduct and prints the result

The exercises below use the Tut3.zip project for BlueJ.

Data types

What data type would you use to represent the following values? Why?

1. The length of a piece of string.
2. The number of students at UNSW.
3. A person’s age.
4. The mass of the sun.
5. The exact number of stars in the Milky Way.
6. The mathematical constant π.

Expressions

What are the value and type of the following expressions? Test your answers by typing them into the Code Pad in BlueJ. Can you explain the result?

1. 5 / 2
2. 5 % 2
3. 5.0 / 2
4. 5 / 2.0
5. 5.0 / 2.5
6. 5.0f / 2
7. 1.5 % 1
8. 1 / 0
9. 1.0 / 0
10. 10000000000 + 1
11. 10000000000L + 1
12. 10000000000.0 + 1
13. 10000000000.0f + 1

Circle area

The circleArea method in the Tut3 class calculates the area of a circle with a given radius.

• Create a new Tut3 object and try calling this method with different input values.
• Add breakpoint on the first line and step through the code. Watch the ‘local variable’ panel to see variables being created and change value.
• Change the method to calculate the area of an ellipse of given width and height

Hand shaking

If there are five people Alan, Bel, Cameron, Dan, and Emily in the room and they each have to shake hands with everyone else, how many handshakes occur? What if there were 10 people? What if there were only two? Work out a general formula.

Complete the ‘handshakes’ method in the Tut3 project to use your formula to compute the number of handshakes for a given size group.

The Math Class

For more complicated mathematical expressions, Java has a lot of built in methods in the Math class. Take a look at the documentation to see what is available. Try typing the following
in CodePad. Explain the results.

• Math.sqrt(2.0)
• Math.round(3.1)
• Math.round(3.7)
• Math.min(1, 2)
• Math.max(1, 2)

You will be working in Quad lab 7. Experiment with the BlueJ project we saw in lectures.

1. Login with your zPass login and password.
2. Download the ‘Turtle’ project zip file from here .
3. Unzip both files in your “My Documents” folder.
4. In the ‘Start’ menu, select ‘Programs’ and click on BlueJ.
5. Select ‘Open Project’ from the File menu and open the ‘Turtle’ project.
6. Create a new ‘MyTurtle’ object by right clicking on the MyTurtle class and selecting new MyTurtle()
7. Give your turtle a name and press OK.

Simple shapes

1. Right click on the red MyTurtle object and select the void drawSquare(int sideLength) method.
2. Enter a side length of 100 and press OK.
3. Observe what happens in the World window.
4. Try drawing squares of different sizes.
5. What direction does the turtle face before and after you call the method?
6. Double-click on the MyTurtle class to open the source-code.
7. Read the drawSquare method. Can you explain why the turtle has a different direction after the method is called?
8. What could you do to fix this so the turtle is facing the same way before and after the method?
9. Change the code so that it draws a triangle instead of a square.

Using the debugger

1. Read the method drawPolygon and try to understand how it works.
2. Run it with different parameters to see if it does what you expect.
3. In the source window, click on the line number for the line “if (numberOfSides < 3)“. A little red stop sign should appear.
4. Run the method again with numberOfSides set to 6 and sideLength set to 100. The Debugger window should appear.
5. Press the Step button and watch how the program steps through the code.
6. Run the method again with numberOfSides set to 2. Can you predict what will happen?

Spirals

1. Read the method drawSpiral and try to understand how it works.
2. Try drawing a spiral with length = 10 on a piece of paper following these instructions.
3. Use the debugger to step through the method. Does it do what you expected?
4. Change the line ‘length = length – 2‘ to ‘length = length – 4‘. How will this change the spiral? Run it and see.
5. Change the line to ‘length = length + 4‘. What happens now? Why?
6. Change the line ‘rotate(90)‘ to ‘rotate(88)‘ and see what happens.
7. Can you make other pretty patterns by making a few simple changes?

Instead of running tutorials this week, we will be holding consultations during the normal tutorial timeslots with the respective tutor. You are not required to attend a consultation, and no lab marks will be recorded.You may attend any consultation.
All consulatations will be held in the room K17 403.