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## COMP 110 Lab 6, more arrays

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Announcements

- Lab 7 due, Friday 2pm

Today in COMP 110

- Lab 6
- More about arrays
- Program 4 exercise?

Why?

- They are somewhat arbitrary right now

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

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

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

Lab 6: making methods

- When you write a method
- The method’s name should tell you the method’s purpose
- The parameter names should be descriptive and support the method’s purpose

publicstaticvoid drawRect(Graphics g, int x, int y, int radius)

when you really meant

publicstaticvoid drawSquare(Graphics g, int x, int y, int side)

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

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

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 0.2374

0.2

0.4

0.6

0.8

0.0

1.0

Lab 6: setRandomColor, technique 1

double rnd = Math.random();

if (rnd >= 0.0 && rnd < 0.2)

g.setColor(Color.RED);

elseif (rnd >= 0.2 && rnd < 0.4)

g.setColor(Color.GREEN);

elseif (rnd >= 0.4 && rnd < 0.6)

g.setColor(Color.BLUE);

elseif (rnd >= 0.6 && rnd < 0.8)

g.setColor(Color.YELLOW);

else

g.setColor(Color.BLACK);

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

0.0

1.0

1.0

0.2

2.0

0.4

3.0

0.6

4.0

0.8

0.0

5.0

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

1.0

2.0

3.0

4.0

0.0

5.0

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);

break;

case 2:

g.setColor(Color.BLUE);

break;

case 3:

g.setColor(Color.YELLOW);

break;

case 4:

g.setColor(Color.BLACK);

break;

}

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?

Lab 6: setRandomColor, 6 colors, technique 1

double rnd = Math.random();

if (rnd >= 0.0 && rnd < (1.0 / 6.0))

g.setColor(Color.RED);

elseif (rnd >= (1.0 / 6.0) && rnd < (2.0 / 6.0))

g.setColor(Color.GREEN);

elseif (rnd >= (2.0 / 6.0) && rnd < (3.0 / 6.0))

g.setColor(Color.BLUE);

elseif (rnd >= (3.0 / 6.0) && rnd < (4.0 / 6.0))

g.setColor(Color.YELLOW);

elseif (rnd >= (4.0 / 6.0) && rnd < (5.0 / 6.0))

g.setColor(Color.YELLOW);

else

g.setColor(Color.BLACK);

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);

break;

case 2:

g.setColor(Color.BLUE);

break;

case 3:

g.setColor(Color.YELLOW);

break;

case 4:

g.setColor(Color.BLACK);

break;

case 5:

g.setColor(Color.MAGENTA);

break;

}

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]);

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);

}

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, y, 20);

}

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 + 100, y, 20);

}

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);

}

}

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

Arrays of objects

Smiley[] smilies = new Smiley[3];

for (int i = 0; i < smilies.length; i++)

{

smilies[i] = new Smiley();

}

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

Arrays are special kinds of objects

- Therefore, they are subject to the same sorts of behaviors as objects

What does this code output?

publicstaticvoid changeArray(int[] arr)

{

int[] newArray = newint[arr.length];

newArray[0] = 12;

arr = newArray;

}

publicstaticvoid 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

What does this code output?

publicstaticvoid changeArray(int[] arr)

{

arr[0] = 12;

}

publicstaticvoid 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

What does this code output?

publicstaticvoid changeArray(int[] arr)

{

arr[0] = 12;

}

publicstaticvoid 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

Friday

- Help with Program 4

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