Chapter 11—Arrays and ArrayLists

The Art and Science of

An Introductionto Computer ScienceERIC S. ROBERTS

Java

Arrays and ArrayListsC H A P T E R 1 1

Little boxes, on a hillside, little boxes made of ticky-tackyLittle boxes, little boxes, little boxes, all the sameThere’s a green one and a pink one and a blue one and a yellow oneAnd they're all made out of ticky-tacky and they all look just the same

—Malvina Reynolds, “Little Boxes,” 1962

CS101 @ Özyeğin University

Slides are adapted from the originals available athttp://www-cs-faculty.stanford.edu/~eroberts/books/ArtAndScienceOfJava/

Lecture Slides, Part IV

1

Multidimensional Arrays• Because the elements of an array can be of any Java type,

those elements can themselves be arrays. Arrays of arrays are called multidimensional arrays.

• In Java, you can create a multidimensional array by using multiple brackets in both the type and the initialization parts of the declaration. For example, you can create array space for a 3 x 3 tic-tac-toe board using the following declaration:

char[][] board = new char[3][3];

• This declaration creates a two-dimensional array of characters that is organized like this:

board[0][0] board[0][1] board[0][2]

board[1][0] board[1][1] board[1][2]

board[2][0] board[2][1] board[2][2]

Memory Layoutchar[][] board = new char[3][3];

0

1

2

board

0 1

0 0 02

0 1

0 0 02

0 1

0 0 02

Memory Layoutchar[][] board = new char[3][3];board[0][0] = ‘X’;

0

1

2

board

0 1

‘X’ 0 02

0 1

0 0 02

0 1

0 0 02

Memory Layoutchar[][] board = new char[3][3];board[0][0] = ‘X’;board[1][0] = ‘O’;

0

1

2

board

0 1

‘X’ 0 02

0 1

‘O’ 0 02

0 1

0 0 02

Memory Layoutchar[][] board = new char[3][3];board[0][0] = ‘X’;board[1][0] = ‘O’;board[2][1] = ‘X’;

0

1

2

board

0 1

‘X’ 0 02

0 1

‘O’ 0 02

0 1

0 ‘X’ 02

Exercise 1int[][] matrix = new int[3][4];

0

1

2

matrix

0 1

0 0 02

0 1

0 0 02

0 1

0 0 02

03

03

03

0

1

2

matrix

0 1

0 0 02

0 1

0 0 02

0 1

0 0 02

3

0 1

0 0 02

(A) (B)

Exercise 2int[][] matrix = new int[3][4];matrix[3][4] = 42;println(matrix[0][0] + matrix[3][4]);

(B) 42(A) 0

(C) Error

Exercise 3int[][] matrix = new int[3][4];println(matrix.length);

(B) 3(A) 4

(C) 12

(D) 7

Exercise 4int[][] matrix = new int[3][4];println(matrix[0].length);

(B) 3(A) 4

(C) 12

(D) 7

Exercise 5int[][] matrix = new int[3][4];println(matrix[2].length);

(B) 3(A) 4

(C) 12

(D) 7

Traversing a two-dimensional arrayint[][] matrix = new int[3][4];

for(int i = 0; i < matrix.length; i++) { for(int j = 0; j < matrix[0].length; j++) { matrix[i][j] = i * j; }}

0

1

2

matrix

0 1

0 0 02

0 1

0 1 22

0 1

0 2 42

03

33

63

Exercise 6int[][] matrix = new int[3][4];

for(int i = 0; i < matrix.length; i++) { for(int j = 0; j < matrix[0].length; j++) { matrix[i][j] = ???; }}

0

1

2

matrix

0 1

0 1 22

0 1

1 2 32

0 1

2 3 42

33

43

53

(B)i * j

(A)i + j

(C)matrix.length + j

(D)matrix[0].length + i

Exercise 7int[][] matrix = new int[3][4];

for(int i = 0; i < matrix.length; i++) { for(int j = 0; j < matrix[0].length; j++) { matrix[i][j] = ???; }}

0

1

2

matrix

0 1

0 1 22

0 1

4 5 62

0 1

8 9 102

33

73

113

(B)matrix.length*i + j

(A)i + j

(C)matrix[0].length*i+j

(D)matrix.length*j + i

Exercise 8int[][] matrix = new int[3][4];

for(int i = 0; i < matrix.length; i++) { for(int j = 0; j < matrix[0].length; j++) { matrix[i][j] = ???; }}

0

1

2

matrix

0 1

0 3 62

0 1

1 4 72

0 1

2 5 82

93

103

113

(B)matrix.length*i + j

(A)matrix[0].length*j+i

(C)matrix[0].length*i+j

(D)matrix.length*j + i

Exercise 9int[][] matrix = new int[3][3];

for(int i = 0; i < matrix.length; i++) { for(int j = 0; j < matrix[0].length; j++) { if(i >= j) { matrix[i][j] = 1; } }}1 1 1

0 1 1

0 0 1

1 0 0

1 1 0

1 1 1

0 1 1

0 0 1

0 0 0

0 0 0

1 0 0

1 1 0

(D)

(B)

(C)

(A)

Image Processing• Images consist of a set of pixels

arranged in a two-dimensional array, as shown in the expanded image.

• In the ACM library, images are represented using the GImage class.

GImage logo = new GImage("JTFLogo.gif");add(logo);

Pixel Values• Each individual element in a pixel array is an int in which

the 32 bits are interpreted as follows:

• The first byte of the pixel value specifies the transparency of the color.

1 1 1 1 1 1 1 1 1 0 0 1 1 0 0 1 0 1 1 0 0 1 1 0 0 0 1 1 0 0 1 1

• The next three bytes indicate the amount of red, green, and blue in the pixel, in which each value varies from 0 to 255. Together, these three bytes form the RGB value of the color, which is typically expressed using six hexadecimal digits. The color in the example has the RGB value 0x996633, which is a light brown:

transparency (α) red green blue

Transparency• The first byte of the pixel value specifies the transparency of

the color, which indicates how much of the background shows through. This value is often denoted using the Greek letter alpha (α).

• Transparency values vary from 0 to 255. The value 0 is used to indicate a completely transparent color in which only the background appears. The value 255 indicates an opaque color that completely obscures the background. The standard color constants all have alpha values of 255.

• Fully transparent colors are particularly useful in images, because they make it possible to display images that do not have rectangular outlines. For example, if the gray pixels in the corners of the JTFLogo.gif image have an alpha value of 0, the background will show through those parts of the logo.

Image Manipulation

pixel at (i,j) pixel at (height-i-1,j)

public int[][] mirrorVertically(int[][] pixels) { int w = pixels[0].length; int h = pixels.length; int[][] result = new int[h][w]; for(int i = 0; i < result.length; i++) { for(int j = 0; j < result[0].length; j++) { result[h - i - 1][j] = pixels[i][j]; } } return result;}

public void run() { // Images must be placed under the following folder: // workspace -> CS101 -> bin // assuming that your project is called "CS101" GImage yoda = new GImage("yoda.jpg"); // centralize the location of the image yoda.setLocation((getWidth() - yoda.getWidth())/2, (getHeight() - yoda.getHeight())/2); add(yoda); pause(1000); int[][] pixels = getImagePixels(yoda); int[][] newPixels = mirrorVertically(pixels); setImagePixels(yoda, newPixels);}

/********************************************************* * Methods given below are helper methods. * Do not try to understand their details. *********************************************************/

/* This method sets the pixels of the given GImage object * to the given pixel array. * Ignore the implementation details. */public void setImagePixels(GImage img, int[][] newPixels) { int w = newPixels[0].length; int h = newPixels.length; int[] pixels = new int[w*h]; for(int i = 0; i < h; i++) { for(int j = 0; j < w; j++) { pixels[i * w + j] = newPixels[i][j]; } }

BufferedImage image = new BufferedImage(w, h, BufferedImage.TYPE_INT_RGB); image.setRGB(0, 0, w, h, pixels, 0, w); img.setImage(image); img.setBounds((getWidth() - w)/2, (getHeight() - h)/2, w, h);}

/* This method reads the pixels of the given GImage * and returns them as a two-dimensional array. * Ignore the implementation details */public int[][] getImagePixels(GImage img) { int w = (int)img.getWidth(); int h = (int)img.getHeight(); int[] pixels = new int[w*h]; PixelGrabber pg = new PixelGrabber(img.getImage(), 0, 0, w, h, pixels, 0, w); try { pg.grabPixels(); } catch (InterruptedException e) { println("Problem occurred. Contact the course staff!"); } int[][] result = new int[h][w]; for(int i=0; i < h; i++) { for(int j=0; j < w; j++) { result[i][j] = pixels[i*w + j]; } } return result;}

public void init() { setSize(600, 450);}

Rotate 180 Degrees

pixel at (i,j) pixel at (height-i-1, width-j-1)

public int[][] rotate180Degrees(int[][] pixels) { int w = pixels[0].length; int h = pixels.length; int[][] result = new int[h][w]; for(int i = 0; i < result.length; i++) { for(int j = 0; j < result[0].length; j++) { result[h - i -1][w - j - 1] = pixels[i][j]; } } return result;}

Mirror Horizontally (self-exercise)

pixel at (i,j) pixel at (?,?)

Rotate Clockwise (self-exercise)

pixel at (i,j) pixel at (?,?)

What’s the width and height of the new image?

Mirror Diagonally (self-exercise)

Half-Mirror Horizontally (self-exercise)

Green Screen Replacement

F B R

if F(i,j) is not green R(i,j) = F(i,j)else R(i,j) = B(i,j)

A color (r,g,b) is consideredgreen when r < 50, b < 50,and g > 100.

public int[][] greenScreenEffect(int[][] foreground, int[][] background) { ???}

public void run() { GImage superman = new GImage("superman.jpg"); GImage istanbul = new GImage("istanbul.jpg"); // centralize the location of the image superman.setLocation((getWidth() - superman.getWidth())/2, (getHeight() - superman.getHeight())/2); add(superman); pause(1000); int[][] supermanPixels = getImagePixels(superman); int[][] istanbulPixels = getImagePixels(istanbul); int[][] newSupermanPixels = greenScreenEffect(supermanPixels, istanbulPixels); setImagePixels(superman, newSupermanPixels);}

/********************************************************* * Methods given below are helper methods. * Do not try to understand their details. *********************************************************/ /* Returns the red color component of a given pixel value * Ignore the implementation details. */ public int getRedComponent(int pixel) { return (pixel >> 16) & 0xff; }

/* Returns the green color component of a given pixel value * Ignore the implementation details. */ public int getGreenComponent(int pixel) { return (pixel >> 8) & 0xff; }

/* Returns the blue color component of a given pixel value * Ignore the implementation details. */ public int getBlueComponent(int pixel) { return pixel & 0xff; }

Black and Whiten (self-exercise)

Use the getRed/Green/BlueComponent methods to decide if a pixel is dark or light.Use pixel value 0xFFFFFF for white, 0 for black.

Lights Out• http://en.wikipedia.org/wiki/Lights_Out_%28game%29

public void toggleLight(int i, int j) { if(i >= 0 && i < lights.length && j >= 0 && j < lights[0].length) { boolean isOff = lights[i][j].isFilled(); lights[i][j].setFilled(!isOff); }}

public void pressOnLight(int i, int j) { toggleLight(i,j); toggleLight(i-1, j); toggleLight(i+1, j); toggleLight(i, j-1); toggleLight(i, j+1); if(isPuzzleSolved()) { GLabel lbl = new GLabel("SOLVED!", 0, 0); lbl.setFont("Courier-36"); lbl.setColor(Color.WHITE); lbl.setLocation((getWidth()-lbl.getWidth())/2, getHeight()/2); add(lbl); }}

public boolean isPuzzleSolved() { for(int i=0; i < lights.length; i++) { for(int j=0; j < lights[0].length; j++) { if(!lights[i][j].isFilled()) { // light is lit return false; } } } return true;}

// This method is invoked whenever the user presses the mouse button// The method finds out which light is pressed on// and invokes the pressOnLight() methodpublic void mousePressed(MouseEvent e) { int x = e.getX(); int y = e.getY(); int i = y / LIGHT_SIZE; int j = x / LIGHT_SIZE; pressOnLight(i, j);}

public void init() { setSize(DIMENSION * LIGHT_SIZE, DIMENSION * LIGHT_SIZE); addMouseListeners();}

Heat Transfer• A space can be considered as a collection of cells.• Each cell has a temperature. • Each cell transfers heat from/to its neighbors.

• Compute this equation for each cell, forever.

Tnew = Told + k(TN – Told)ΣN is a neighbor

High k: quick transferLow k: slow transfer

Initializing Multidimensional Arrays• You can initialize a multidimensional array when you declare

it by using nested braces to reflect the levels of array nesting.• For example, you can declare and initialize a multiplication

table for the digits 0 to 9 like this:

private static int[][] MULTIPLICATION_TABLE = { { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 }, { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 }, { 0, 2, 4, 6, 8, 10, 12, 14, 16, 18 }, { 0, 3, 6, 9, 12, 15, 18, 21, 24, 27 }, { 0, 4, 8, 12, 16, 20, 24, 28, 32, 36 }, { 0, 5, 10, 15, 20, 25, 30, 35, 40, 45 }, { 0, 6, 12, 18, 24, 30, 36, 42, 48, 56 }, { 0, 7, 14, 21, 28, 35, 42, 49, 56, 63 }, { 0, 8, 16, 24, 32, 40, 48, 56, 64, 72 }, { 0, 9, 18, 27, 36, 45, 54, 63, 72, 81 } };

Exercise: Multidimensional ArraysWrite a single Java statement that declares and initializes a two-dimensional array named chessboard so that the array contains a representation of the initial position in a game of chess:

The individual elements should be characters using the standard symbols for the pieces:

Solution: Chessboard Problem

private char[][] chessboard = { { 'r', 'n', 'b', 'q', 'k', 'b', 'n', 'r' }, { 'p', 'p', 'p', 'p', 'p', 'p', 'p', 'p' }, { ' ', ' ', ' ', ' ', ' ', ' ', ' ', ' ' }, { ' ', ' ', ' ', ' ', ' ', ' ', ' ', ' ' }, { ' ', ' ', ' ', ' ', ' ', ' ', ' ', ' ' }, { ' ', ' ', ' ', ' ', ' ', ' ', ' ', ' ' }, { 'P', 'P', 'P', 'P', 'P', 'P', 'P', 'P' }, { 'R', 'N', 'B', 'Q', 'K', 'B', 'N', 'R' }, };