1. Michele Weigle - COMP 14 - Spr 04
COMP 110 Lab 6, more arrays
Luv Kohli
November 12, 2008
MWF 2-2:50 pm
Sitterson 014

2. Michele Weigle - COMP 14 - Spr 04
Announcements
Lab 7 due, Friday 2pm

3. Michele Weigle - COMP 14 - Spr 04
Questions?
Any other questions?

4. Michele Weigle - COMP 14 - Spr 04
Today in COMP 110
Lab 6
More about arrays
Program 4 exercise?

5. Extra credit points on Program 4 lowered

6. Why?
They are somewhat arbitrary right now

7. Extra credit policy
Working on it, but there will probably be some sort of cap
Basic idea (not finalized) is that extra credit can help pull you up half a letter grade if you’re on the border

8. Lab 6 Class solutions posted Cool patterns!
Pat yourself on the back for coming up with interesting stuff
I made a couple of mistakes
radius in drawCircle
setBackground

9. Lab 6: setting a color Set your color before you draw a shape
Imagine being a painter
You put your paintbrush on the palette to mix and pick up a color before you touch the paintbrush to the canvas

10. Lab 6: making methods When you write a method when you really meant
The method’s name should tell you the method’s purpose
The parameter names should be descriptive and support the method’s purpose
public static void drawRect(Graphics g, int x, int y, int radius)
when you really meant
public static void drawSquare(Graphics g, int x, int y, int side)

11. Lab 6: setRandomColor
Important to know how to convert from one range to another
Useful in many situations
Scaling images
Converting mouse clicks to regions on your user interface (for example, the grid cells in Program 4)
Drawing parametric functions, or doing computations with parametric functions

12. Lab 6: setRandomColor
Math.random() returns a value in the range [0.0, 1.0)
In other words
0.0 <= Math.random() < 1.0
0.0
1.0

13. Lab 6: setRandomColor We want 5 colors, chosen randomly
How do we choose them?
Divide our range into 5 subranges
Decide which subrange maps to which color
If we get a random number in a certain range, use the color we decided on for that range
Math.random() returns
0.2
0.4
0.6
0.8
0.0
1.0

14. Lab 6: setRandomColor, technique 1
double rnd = Math.random(); if (rnd >= 0.0 && rnd < 0.2) g.setColor(Color.RED); else if (rnd >= 0.2 && rnd < 0.4) g.setColor(Color.GREEN); else if (rnd >= 0.4 && rnd < 0.6) g.setColor(Color.BLUE); else if (rnd >= 0.6 && rnd < 0.8) g.setColor(Color.YELLOW); else g.setColor(Color.BLACK);

15. Lab 6: setRandomColor, technique 2
Scale the range, and then divide it into subranges
What if we multiply rnd by 5?
Then 0.0 <= rnd < 5.0
0.2
0.4
0.6
0.8
0.0
1.0
0.0
5.0
1.0
2.0
3.0
4.0

16. Lab 6: setRandomColor, technique 2
We could use if/else statements as before
Or we can typecast rnd to an int, and then use a switch statement
What is (int) (rnd * 5) if rnd is…
0.3?
(int) (1.5) is 1
0.1?
(int) (0.5) is 0
0.91?
(int) (4.55) is 4
0.0
5.0
1.0
2.0
3.0
4.0

17. Lab 6: setRandomColor, technique 2
double rnd = Math.random(); int choice = (int) (rnd * 5); switch (choice) { case 0: g.setColor(Color.RED); break; case 1: g.setColor(Color.GREEN); case 2: g.setColor(Color.BLUE); case 3: g.setColor(Color.YELLOW); case 4: g.setColor(Color.BLACK); }

18. Why? Is it any better?
It depends, but imagine if you suddenly decide you want 6 random colors instead of 5
How would you do it with the if/else statements?

19. Lab 6: setRandomColor, 6 colors, technique 1
double rnd = Math.random(); if (rnd >= 0.0 && rnd < (1.0 / 6.0)) g.setColor(Color.RED); else if (rnd >= (1.0 / 6.0) && rnd < (2.0 / 6.0)) g.setColor(Color.GREEN); else if (rnd >= (2.0 / 6.0) && rnd < (3.0 / 6.0)) g.setColor(Color.BLUE); else if (rnd >= (3.0 / 6.0) && rnd < (4.0 / 6.0)) g.setColor(Color.YELLOW); else if (rnd >= (4.0 / 6.0) && rnd < (5.0 / 6.0)) else g.setColor(Color.BLACK);

20. Lab 6: setRandomColor, 6 colors, technique 2
double rnd = Math.random(); int choice = (int) (rnd * 6); switch (choice) { case 0: g.setColor(Color.RED); break; case 1: g.setColor(Color.GREEN); case 2: g.setColor(Color.BLUE); case 3: g.setColor(Color.YELLOW); case 4: g.setColor(Color.BLACK); case 5: g.setColor(Color.MAGENTA); }

21. Lab 6: setRandomColor, how about arrays?
Color[] colors = { Color.RED, Color.GREEN, Color.BLUE, Color.YELLOW, Color.BLACK }; double rnd = Math.random(); int choice = (int) (rnd * 5); g.setColor(colors[choice]);

22. Lab 6: think about nested loops
for (int i = 0; i < 360; i += 10) { int x = (int) (Math.sin(Math.toRadians(i)) * 20); int y = (int) (Math.cos(Math.toRadians(i)) * 20); drawCircle(g, x, y, 20); } drawCircle(g, x + 50, y, 20); drawCircle(g, x + 100, y, 20);

23. Lab 6: nested loops
for (int count = 0; count < 3; count++) { for (int i = 0; i < 360; i += 10) int x = (int) (Math.sin(Math.toRadians(i)) * 20); int y = (int) (Math.cos(Math.toRadians(i)) * 20); drawCircle(g, x + (50 * count), y, 20); }

24. What did we learn earlier?
Arrays can be instance variables
Arrays can be of any base type
Arrays can be method parameters
Arrays can be returned from a method
Lots of stuff about multidimensional arrays
Multidimensional arrays are awesome
I shouldn’t try to explain things on the whiteboard unless I’ve rehearsed what I’m explaining

25. Arrays of objects 1045 2584 2836 ? true GREEN 3 false BLUE 1 false
Smiley[] smilies = new Smiley[3]; for (int i = 0; i < smilies.length; i++) { smilies[i] = new Smiley(); }
1045
2584
2836 ?
true
GREEN 3
false
BLUE 1
false
CYAN 4

26. Arrays of objects When you create an array of objects like this:
Student[] students = new Student[35];
Each of the elements of students is not yet an object
You have to instantiate each individual one
students[0] = new Student();
students[1] = new Student();
…or do this in a loop

27. Arrays are special kinds of objects
Therefore, they are subject to the same sorts of behaviors as objects

28. What does this code output?
public static void changeArray(int[] arr) { int[] newArray = new int[arr.length]; newArray[0] = 12; arr = newArray; } public static void main(String[] args) int[] arr = { 3, 6, 15 }; changeArray(arr); for (int x : arr) System.out.println(x);
Output: 3 6 15
The parameter is local to changeArray, reassigning does not change the argument

29. What does this code output?
public static void changeArray(int[] arr) { arr[0] = 12; } public static void main(String[] args) int[] arr = { 3, 6, 15 }; changeArray(arr); for (int x : arr) System.out.println(x);
Output: 12 6 15
The parameter is local to changeArray, but it contains the address of the array passed in, so we can change its elements

30. What does this code output?
public static void changeArray(int[] arr) { arr[0] = 12; } public static void main(String[] args) int[] arr = { 3, 6, 15 }; int[] newArray = arr; changeArray(newArray); for (int x : arr) System.out.println(x);
Output: 12 6 15
arr and newArray both contain the same address, and therefore refer to the same data in memory

31. Michele Weigle - COMP 14 - Spr 04
Friday
Help with Program 4