COMP 110 Lab 6, more arrays

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COMP 110Lab 6, more arrays

• Luv Kohli
• November 12, 2008
• MWF 2-250 pm
• Sitterson 014

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Announcements

• Lab 7 due, Friday 2pm

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Questions?
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Today in COMP 110

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

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Extra credit points on Program 4 lowered
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Why?

• They are somewhat arbitrary right now

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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
  youre on the border

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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

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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

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Lab 6 making methods

• When you write a method
• The methods name should tell you the methods
  purpose
• The parameter names should be descriptive and
  support the methods 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)

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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

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Lab 6 setRandomColor

• Math.random() returns a value in the range 0.0,
  1.0)
• In other words
• 0.0 lt Math.random() lt 1.0

0.0
1.0
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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
0.0
1.0
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Lab 6 setRandomColor, technique 1

• double rnd Math.random()
• if (rnd gt 0.0 rnd lt 0.2)
• g.setColor(Color.RED)
• else if (rnd gt 0.2 rnd lt 0.4)
• g.setColor(Color.GREEN)
• else if (rnd gt 0.4 rnd lt 0.6)
• g.setColor(Color.BLUE)
• else if (rnd gt 0.6 rnd lt 0.8)
• g.setColor(Color.YELLOW)
• else
• g.setColor(Color.BLACK)

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Lab 6 setRandomColor, technique 2

• Scale the range, and then divide it into
  subranges
• What if we multiply rnd by 5?
• Then 0.0 lt rnd lt 5.0

0.0
1.0
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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

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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

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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?

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Lab 6 setRandomColor, 6 colors, technique 1

• double rnd Math.random()
• if (rnd gt 0.0 rnd lt (1.0 / 6.0))
• g.setColor(Color.RED)
• else if (rnd gt (1.0 / 6.0) rnd lt (2.0 / 6.0))
• g.setColor(Color.GREEN)
• else if (rnd gt (2.0 / 6.0) rnd lt (3.0 / 6.0))
• g.setColor(Color.BLUE)
• else if (rnd gt (3.0 / 6.0) rnd lt (4.0 / 6.0))
• g.setColor(Color.YELLOW)
• else if (rnd gt (4.0 / 6.0) rnd lt (5.0 / 6.0))
• g.setColor(Color.YELLOW)
• else
• g.setColor(Color.BLACK)

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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

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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(colorschoice)

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Lab 6 think about nested loops

• for (int i 0 i lt 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 lt 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 lt 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)

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Lab 6 nested loops

• for (int count 0 count lt 3 count)
• for (int i 0 i lt 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)

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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 shouldnt try to explain things on the
  whiteboard unless Ive rehearsed what Im
  explaining

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Arrays of objects

• Smiley smilies new Smiley3
• for (int i 0 i lt smilies.length i)
• smiliesi new Smiley()

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Arrays of objects

• When you create an array of objects like this
• Student students new Student35
• Each of the elements of students is not yet an
  object
• You have to instantiate each individual one
• students0 new Student()
• students1 new Student()
• or do this in a loop

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Arrays are special kinds of objects

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

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What does this code output?

• public static void changeArray(int arr)
• int newArray new intarr.length
• newArray0 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
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What does this code output?

• public static void changeArray(int arr)
• arr0 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
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What does this code output?

• public static void changeArray(int arr)
• arr0 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
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Friday

• Help with Program 4