Nested loops

1
Nested loops

• "Nested" means "one inside the other"
• Let's say we wanted to make a 10x10 times table
• 1 2 3 4 5 6 7 8 9 10
• 2 4 6 8 10 12 14 16 18 20
• 3 6 9 12 15 18 21 24 27 30
• 4 8 12 16 20 24 28 32 36 40
• 5 10 15 20 25 30 35 40 45 50
• 6 12 18 24 30 36 42 48 54 60
• 7 14 21 28 35 42 49 56 63 70
• 8 16 24 32 40 48 56 64 72 80
• 9 18 27 36 45 54 63 72 81 90
• 10 20 30 40 50 60 70 80 90 100

2
Nested loops

• Let's start with the first row
• 1 2 3 4 5 6 7 8 9 10
• 2 4 6 8 10 12 14 16 18 20
• 3 6 9 12 15 18 21 24 27 30
• 4 8 12 16 20 24 28 32 36 40
• 5 10 15 20 25 30 35 40 45 50
• 6 12 18 24 30 36 42 48 54 60
• 7 14 21 28 35 42 49 56 63 70
• 8 16 24 32 40 48 56 64 72 80
• 9 18 27 36 45 54 63 72 81 90
• 10 20 30 40 50 60 70 80 90 100
• It would be easy enough to write a loop that
  makes the numbers 1through 10

3
Nested loops

• Let's start with the first row
• 1 2 3 4 5 6 7 8 9 10
• 2 4 6 8 10 12 14 16 18 20 x2
• 3 6 9 12 15 18 21 24 27 30 x3
• 4 8 12 16 20 24 28 32 36 40 x4
• 5 10 15 20 25 30 35 40 45 50 x5
• 6 12 18 24 30 36 42 48 54 60 x6
• 7 14 21 28 35 42 49 56 63 70 x7
• 8 16 24 32 40 48 56 64 72 80 x8
• 9 18 27 36 45 54 63 72 81 90 x9
• 10 20 30 40 50 60 70 80 90 100 x10
• We'd put that loop inside another loop, that
  "multiplies" it

4
Nested loops

• //inside loop counts columns 1 to 10
• 1 2 3 4 5 6 7 8 9 10
• 2 4 6 8 10 12 14 16 18 20 x2
• 3 6 9 12 15 18 21 24 27 30 x3
• 4 8 12 16 20 24 28 32 36 40 x4
• 5 10 15 20 25 30 35 40 45 50 x5
• 6 12 18 24 30 36 42 48 54 60 x6
• 7 14 21 28 35 42 49 56 63 70 x7
• 8 16 24 32 40 48 56 64 72 80 x8
• 9 18 27 36 45 54 63 72 81 90 x9
• 10 20 30 40 50 60 70 80 90 100 x10

5
Nested loops

• //outside loop counts rows 1 to 10
• //(inside loop column)outside row
• 1 2 3 4 5 6 7 8 9 10
• 2 4 6 8 10 12 14 16 18 20 x2
• 3 6 9 12 15 18 21 24 27 30 x3
• 4 8 12 16 20 24 28 32 36 40 x4
• 5 10 15 20 25 30 35 40 45 50 x5
• 6 12 18 24 30 36 42 48 54 60 x6
• 7 14 21 28 35 42 49 56 63 70 x7
• 8 16 24 32 40 48 56 64 72 80 x8
• 9 18 27 36 45 54 63 72 81 90 x9
• 10 20 30 40 50 60 70 80 90 100 x10

6
In Class

• Write an application that generates a 10x10 times
  table using nested loops and displays it using
  System.out.print()
• Hints
• Start RealJ, choose New Project Application
  Project
• Delete the two import statements, extends Frame
  and the class constructor
• Use "\t" to space numbers
• Use an empty System.out.println() to end the line

7
In Class

• Next, Modify your application so that it produces
  the following output
• (1,1) (1,2) (1,3) (1,4) (1,5) (1,6) (1,7) (1,8) (1
  ,9) (1,10)
• (2,1) (2,2) (2,3) (2,4) (2,5) (2,6) (2,7) (2,8) (2
  ,9) (2,10)
• (3,1) (3,2) (3,3) (3,4) (3,5) (3,6) (3,7) (3,8) (3
  ,9) (3,10)
• (4,1) (4,2) (4,3) (4,4) (4,5) (4,6) (4,7) (4,8) (4
  ,9) (4,10)
• (5,1) (5,2) (5,3) (5,4) (5,5) (5,6) (5,7) (5,8) (5
  ,9) (5,10)
• (6,1) (6,2) (6,3) (6,4) (6,5) (6,6) (6,7) (6,8) (6
  ,9) (6,10)
• (7,1) (7,2) (7,3) (7,4) (7,5) (7,6) (7,7) (7,8) (7
  ,9) (7,10)
• (8,1) (8,2) (8,3) (8,4) (8,5) (8,6) (8,7) (8,8) (8
  ,9) (8,10)
• (9,1) (9,2) (9,3) (9,4) (9,5) (9,6) (9,7) (9,8) (9
  ,9) (9,10)
• (10,1) (10,2) (10,3) (10,4) (10,5) (10,6) (10,7) (
  10,8) (10,9) (10,10)
• Which loop gives the row? Which loop gives the
  column? Change your variable names so that they
  match.

8
2 dimensional arrays

• Can be thought of as a grid with rows and
  columns

column 0 1 2
-3
-1
5
row 0
7
12
13
row 1
9
2 dimensional arrays

• int naNums -3, -1, 5,
• 7, 12, 13
• /In this form, Java automatically sizes the
  array to 2 rows and 3 columns
• Note rows first, then columns /
• System.out.println(naNums12)
• //displays 13
• System.out.println(naNums21)
• //Out of bounds exception!

column 0 1 2
-3
-1
5
row 0
7
12
13
row 1
10
2 dimensional arrays

• int naNums
• /In Java, arrays are objects. If we declare and
  initialize the array seperately, we need to use
  "new" /
• naNums new int23

11
2 dimensional arrays

• int naNums
• naNums new int23
• Typically, nested loops are used to initialize a
  2d array. The outside loop controls the row, and
  the inside the column
• for(int nRow 0 nRow lt 2 nRow)
• for(int nCol 0 nCol lt 3 nCol)
• naNumsnRownCol nCol(3nRow)

column 0 1 2
0
1
2
row 0
3
4
5
row 1
12
A one dimensional array of buttons

• import java.awt.
• import java.awt.event.
• import java.applet.
• public class Buttons extends Applet implements
  ActionListener
• Button buttonArray
• int nRows 3
• int nColumns 4
• int nTotalButtons nRows nColumns
• public void init()
• //declare a grid layout with no space
  between buttons
• setLayout(new GridLayout(nRows, nColumns,
  0, 0))
• //declare a new array of buttons
• buttonArray new ButtonnTotalButtons
• //initialize each of the buttons in the
  array

13
A one dimensional array of buttons

• import java.awt.
• import java.awt.event.
• import java.applet.
• public class Buttons extends Applet
• implements ActionListener
• Button buttonArray
• int nRows 3
• int nColumns 4
• int nTotalButtons nRows nColumns
• public void init()
• //declare a grid layout with no space
  between buttons
• setLayout(new GridLayout(nRows, nColumns,
  0, 0))
• //declare a new array of buttons
• buttonArray new ButtonnTotalButtons

14
A one dimensional array of buttons

• import java.awt.
• import java.awt.event.
• import java.applet.
• public class Buttons extends Applet
• implements ActionListener
• Button buttonArray
• int nRows 3
• int nColumns 4
• int nTotalButtons nRows nColumns
• public void init()
• //grid layout with no space between buttons
• setLayout(new GridLayout(nRows,nColumns,0,0))
• //declare a new array of buttons
• buttonArray new ButtonnTotalButtons
• //initialize each of the buttons in the array
• //with an empty label

15
A one dimensional array of buttons

• import java.awt.
• import java.awt.event.
• import java.applet.
• public class Buttons extends Applet
• implements ActionListener
• Button buttonArray
• int nRows 3
• int nColumns 4
• int nTotalButtons nRows nColumns
• public void init()
• //grid layout with no space between buttons
• setLayout(new GridLayout(nRows,nColumns,0,0))
• //declare a new array of buttons
• buttonArray new ButtonnTotalButtons
• //initialize each of the buttons in the array
• //with an empty label

16
A one dimensional array of buttons

• public void actionPerformed(ActionEvent e)
• //get number of button from getActionCommand
• //convert it to int and store in nButtonNumber
• int nButtonNumber
• Integer.parseInt(e.getActionCommand())
• //display button Number at bottom of screen
• showStatus("Button number " nButtonNumber)
• //change background of buttons
  if(buttonArraynButtonNumber.getBackground()
• ! Color.red)
  
• buttonArraynButtonNumber.
• setBackground(Color.red)
• else
• buttonArraynButtonNumber.
• setBackground(Color.blue)

17
How would we modify the example for minesweeper?

• When a button is pressed, we need to know two
  numbers the row and column
• We also need to know if the button is hiding a
  mine

18
extending the Button class

• class Cell extends Button
• private boolean bIsBomb
• private int myRow, myCol
• . . .

19
extending the Button class

• class Cell extends Button
• private boolean bIsBomb
• private int myRow, myCol
• public Cell(??)
• ??

20
extending the Button class

• class Cell extends Button
• private boolean bIsBomb
• private int myRow, myCol
• public Cell(??)
• ??
• public int getRow()return myRow

21
In actionPerformed

• To find out the row (and column) of the button
  that was pressed
• public void actionPerformed(ActionEvent e)
• int nRow ((Cell)e.getSource()).getRow()

22
In actionPerformed

• To find out the row (and column) of the button
  that was pressed
• //Get the object that generated the event
• e.getSource()
• //Cast it as a Cell
• (Cell)e.getSource()
• //Then ask it what row it came from
• ((Cell)e.getSource()).getRow()

23
Recursion Reverse Digits

• public class ReverseDigits
• public static void main(String args)
• reverse(321)
• public static void reverse(int nNum)
• ???

24
Recursion Reverse Digits

• public class ReverseDigits
• public static void main(String args)
• reverse(321)
• public static void reverse(int nNum)
• System.out.print(nNum10)
• ??

25
Recursion Reverse Digits

• public class ReverseDigits
• public static void main(String args)
• reverse(321)
• public static void reverse(int nNum)
• System.out.print(nNum10)
• if(nNumgt10)
• ??

26
Recursion Reverse Digits

• public class ReverseDigits
• public static void main(String args)
• reverse(321)
• public static void reverse(int nNum)
• System.out.print(nNum10)
• if(nNumgt10)
• reverse(nNum/10)

27
Recursion

• What will this recursive program display?
• public class RecursiveDemo
• static String saWords
• "one","two","three","."
• public static void main(String args)
• stackWords(0)
• public static void stackWords(int nPosition)
• if(saWordsnPosition.equals("."))
• System.out.println("end")
• else
• stackWords(nPosition 1)
• System.out.println(saWordsnPosition)

28
Recursion

• When we run this program, the output is
  backwards
• end
• .
• three
• two
• one
• Why? Each time the recursive call to stackWords()
  is made, execution goes back to the start of a
  new method call.
• The computer must "remember" to complete all the
  method calls

29
Recursion

• public static void stackWords(int nPosition)
• if(saWordsnPosition.equals("."))
• System.out.println("end")
• else
• stackWords(nPosition 1)
• System.out.println(saWordsnPosition)
• Because we can't finish the call until make
  another, the calls "stack up"
• Call saWordsnPosition
• stackWords(0) one

30
Recursion

• public static void stackWords(int nPosition)
• if(saWordsnPosition.equals("."))
• System.out.println("end")
• else
• stackWords(nPosition 1)
• System.out.println(saWordsnPosition)
• Call saWordsnPosition
• stackWords(1) two
• stackWords(0) one

31
Recursion

• public static void stackWords(int nPosition)
• if(saWordsnPosition.equals("."))
• System.out.println("end")
• else
• stackWords(nPosition 1)
• System.out.println(saWordsnPosition)
• Call saWordsnPosition
• stackWords(2) three
• stackWords(1) two
• stackWords(0) one

32
Recursion

• public static void stackWords(int nPosition)
• if(saWordsnPosition.equals("."))
• System.out.println("end")
• else
• stackWords(nPosition 1)
• System.out.println(saWordsnPosition)
• Once we get to the period, we stop making calls,
  and the computer can finish the calls on the
  stack
• This is called the Base Case
• Call saWordsnPosition
• stackWords(3) .
• stackWords(2) three
• stackWords(1) two
• stackWords(0) one
• Output
• end
• .

33
Recursion

• public static void stackWords(int nPosition)
• if(saWordsnPosition.equals("."))
• System.out.println("end")
• else
• stackWords(nPosition 1)
• System.out.println(saWordsnPosition)
• Once we get to the period, we stop making calls,
  and the computer can finish the calls on the
  stack
• This is called the Base Case
• Call saWordsnPosition
• stackWords(2) three
• stackWords(1) two
• stackWords(0) one
• Output
• end
• .
• three

34
Recursion

• public static void stackWords(int nPosition)
• if(saWordsnPosition.equals("."))
• System.out.println("end")
• else
• stackWords(nPosition 1)
• System.out.println(saWordsnPosition)
• Once we get to the period, we stop making calls,
  and the computer can finish the calls on the
  stack
• This is called the Base Case
• Call saWordsnPosition
• stackWords(1) two
• stackWords(0) one
• Output
• end
• .
• three
• two

35
Recursion

• public static void stackWords(int nPosition)
• if(saWordsnPosition.equals("."))
• System.out.println("end")
• else
• stackWords(nPosition 1)
• System.out.println(saWordsnPosition)
• Once we get to the period, we stop making calls,
  and the computer can finish the calls on the
  stack
• This is called the Base Case
• Call saWordsnPosition
• stackWords(0) one
• Output
• end
• .
• three
• two
• one

36
Problem

• Write a simple application that finds the
  factorial of an integer
• 3! 3 2 1
• Hint You will need a method like
• public static int factorial(int nInt)

37
Recursion in 2d arrays

• Problem Write a "Remove Blob" method. A Blob
  is a collection of one or more dark adjacent
  cells.

• There are 4 Blobs on the right
• Clicking on any dark cell should remove the
  entire Blob

38
A BlobCell class

• class BlobCell extends Button
• private int myRow, myCol
• public BlobCell
• (int nR, int nC, String sText)
• super(sText)
• myRow nR
• myCol nC
• public int getRow()return myRow
• public int getCol()return myCol

39
The "Applet" class

• public class RemoveBlobDemo extends Applet
• implements ActionListener
• BlobCell buttonArray
• final static int nROWS 9
• final static int nCOLUMNS 9
• static Random randGen new Random()
• // init() not shown
• public void actionPerformed(ActionEvent e)
• int nRow
• ((BlobCell)e.getSource()).getRow()
• int nCol
• ((BlobCell)e.getSource()).getCol()
• removeBlob(nRow,nCol)

40
The removeBlob method

• public class RemoveBlobDemo extends Applet
• implements ActionListener
• . . .
• public void removeBlob(int nRow,int nCol)
• if(buttonArraynRownCol.getBackground()
• .equals(Color.black))
• buttonArraynRownCol.
• setBackground(Color.white)
• . . . .

41
The removeBlob method

• public class RemoveBlobDemo extends Applet
• implements ActionListener
• . . .
• public void removeBlob(int nRow,int nCol)
• if(buttonArraynRownCol.getBackground()
• .equals(Color.black))
• buttonArraynRownCol.
• setBackground(Color.white)
• //Check 8 neighbors
• if(nRow gt 0 nCol gt 0 )
• removeBlob(nRow-1,nCol-1)

42
Finishing MBSCS Chapter 2

• page 26
• What will the following code do?
• Location loc1 new Location(7,3)
• Location loc2 new Location(7,4)
• Direction dir1 env.getDirection(loc1,loc2)
• Direction dir2 dir1.toRight(90)
• Direction dir3 dir2.reverse()
• Location loc3 env.getNeighbor(loc1,dir3)
• Location loc4 env.getNeighbor(new
  Location(5,2), dir1)
• We can write a simple program (a driver) to find
  out

43
Finishing MBSCS Chapter 2

• page 26
• public class WhatOutput
• public static void main(String args)
• BoundedEnv env new BoundedEnv(10, 10)
• Location loc1 new Location(7,3)
• Location loc2 new Location(7,4)
• Direction dir1 env.getDirection(loc1,lo
  c2)
• Direction dir2 dir1.toRight(90)
• Direction dir3 dir2.reverse()
• Location loc3 env.getNeighbor(loc1,dir3
  )
• Location loc4 env.getNeighbor(new
• 
  Location(5,2), dir1)
• System.out.println(loc1)
• //and so on . . .

44
Finishing MBSCS Chapter 2

• page 33
• Both the Fish and Environment classes keep track
  of where a fish is. If they agree it called
  consistent, if they disagree it is called
  inconsistent.
• If you were to remove a fish from the
  environment, the fish would still think it was
  still at it's old location. The environment
  wouldn't.

45
Finishing MBSCS Chapter 2

• page 37
• Here's a driver that finds out how many neighbors
  (0,0) has. Why is it only two?
• public class WhatOutput
• public static void main(String args)
• BoundedEnv env new BoundedEnv(10, 10)
• Location loc1 new Location(0,0)
• System.out.println(env.neighborsOf(loc1))
• / Sample Output
• (0, 1), (1, 0)
• Exit code 0
• No Errors /

46
Finishing MBSCS Chapter 2

• page 40
• Modify the Fish class by adding a changeColor
  method
• public class Fish implements Locatable
• public void changeColor(Color newColor)
• myColor newColor
• //and so on. . .

47
Finishing MBSCS Chapter 2

• pages 42-43 are confusing
• On page 42, there are 10 "bulleted" statements.
  Those are the "black box test cases"
• page 43, question 2 "Which black box test cases
  cover the last three cases for the loop. . ."
• means which of the bulleted statements will test
  what happens for "multiple neighboring
  locations"? The 3rd bullet, a single fish, is
  one correct answer

48
Finishing MBSCS Chapter 2

• pages 45 For 2, you could write code like

49
Practice Quiz Questions

• Which of the following calls to this method
  create infinite recursion?
• public void mystery(int nNum1, int nNum2)
• if(nNum1 ! nNum2)
• mystery(nNum1 1, nNum2 - 1)
• mystery(2,3)
• mystery(3,3)
• mystery(2,4)
• none of the above

50
Practice Quiz Questions

• Complete the following method so that clicking on
  a black button changes it's background to white
  AND the background of all the adjacent buttons to
  the right

public void removeRowRight(int nRow,int nCol)
if(buttonArraynRownCol.getBackground(
).equals(Color.black))
buttonArraynRownCol.setBackground(Color.white)
if(__________________________)
_________________________________________

51

• A class of 30 students rated their cs teacher on
  a scale of 1 to 10. The response array is a 30
  element integer array of the student reponses.
  An 11 element array will count the number of
  occurrences of each response. For example
  freq6 will count the number of students who
  responded 6. The quantity freq0 will not be
  used. Here is a program that counts the
  students' responses and outputs the results
• public class StudentEvaluations
• public static void main(String args)
• int responses 6,6,7,8,10,1,5,4,6,7,5,4,
  3,4
• ,4,9,8,6,7,10,6,7,8,8,9,6,7,8,9
  ,2
• int freq new int11
• for(int i0iltresponses.lengthi)
• freqresponsesI
• Suppose the last entry in the initializer list
  for the responses array was incorrectly typed as
  12 instead of 2. What would be the result of
  running the program?
• A rating of 12 would be listed with a frequency
  of 1
• A rating of 1 would be liste with a frequency of
  12
• An ArrayIndexOutOfBoundsException would be thrown
• A StringIndexOutOfBoundsException would be thrown
• A NullPointerException would be thrown