1. CMSC 150 MULTI-DIMENSIONAL ARRAYS CS 150: Mon 27 Feb 2012

2. Motivation  A digital image is a two- dimensional collection of pixels (picture elements)  Each pixel has an associated color (green, brown, pink, yellow)  Taken collectively, the pixels form the image

3. Motivation

4. pixel

5. 2D Array  can be primitive or class type  S yntax:  type[][] variableName;  type[][] variableName = new type[numRows][numCols];  Examples: Color[][] pixels = new Color[100][256]; boolean[][] missingTeeth = new boolean[2][16]; String[][] firstLastNames = new String[NUM_STUDENTS][2];

6. public class 2DArrayExample { public static void main(String[] args) { int[][] myArray; myArray = new int[2][3]; myArray[0][0] = 77; myArray[0][1] = 42; myArray[0][2] = 101; myArray[1][0] = 77 + 77; myArray[1][1] = 42 + 42; myArray[1][2] = 101 + 101; } In Memory myArray null

7. In Memory 0x12AB79 [0][0] [0][1] [0][2] public class 2DArrayExample { public static void main(String[] args) { int[][] myArray; myArray = new int[2][3]; myArray[0][0] = 77; myArray[0][1] = 42; myArray[0][2] = 101; myArray[1][0] = 77 + 77; myArray[1][1] = 42 + 42; myArray[1][2] = 101 + 101; } myArray [1][0] [1][1] [1][2]

8. In Memory 0x12AB79 [0][0] [0][1] [0][2] public class 2DArrayExample { public static void main(String[] args) { int[][] myArray; myArray = new int[2][3]; myArray[0][0] = 77; myArray[0][1] = 42; myArray[0][2] = 101; myArray[1][0] = 77 + 77; myArray[1][1] = 42 + 42; myArray[1][2] = 101 + 101; } myArray [1][0] [1][1] [1][2] row 0 row 1

9. In Memory 0x12AB79 77 public class 2DArrayExample { public static void main(String[] args) { int[][] myArray; myArray = new int[2][3]; myArray[0][0] = 77; myArray[0][1] = 42; myArray[0][2] = 101; myArray[1][0] = 77 + 77; myArray[1][1] = 42 + 42; myArray[1][2] = 101 + 101; } myArray [0][0] [0][1] [0][2] [1][0] [1][1] [1][2]

10. In Memory 0x12AB79 77 42 public class 2DArrayExample { public static void main(String[] args) { int[][] myArray; myArray = new int[2][3]; myArray[0][0] = 77; myArray[0][1] = 42; myArray[0][2] = 101; myArray[1][0] = 77 + 77; myArray[1][1] = 42 + 42; myArray[1][2] = 101 + 101; } myArray [0][0] [0][1] [0][2] [1][0] [1][1] [1][2]

11. In Memory 0x12AB79 77 42 101 public class 2DArrayExample { public static void main(String[] args) { int[][] myArray; myArray = new int[2][3]; myArray[0][0] = 77; myArray[0][1] = 42; myArray[0][2] = 101; myArray[1][0] = 77 + 77; myArray[1][1] = 42 + 42; myArray[1][2] = 101 + 101; } myArray [0][0] [0][1] [0][2] [1][0] [1][1] [1][2]

12. In Memory 0x12AB79 77 42 101 public class 2DArrayExample { public static void main(String[] args) { int[][] myArray; myArray = new int[2][3]; myArray[0][0] = 77; myArray[0][1] = 42; myArray[0][2] = 101; myArray[1][0] = 77 + 77; myArray[1][1] = 42 + 42; myArray[1][2] = 101 + 101; } myArray [0][0] [0][1] [0][2] [1][0] [1][1] [1][2] 154

13. In Memory 0x12AB79 77 42 101 public class 2DArrayExample { public static void main(String[] args) { int[][] myArray; myArray = new int[2][3]; myArray[0][0] = 77; myArray[0][1] = 42; myArray[0][2] = 101; myArray[1][0] = 77 + 77; myArray[1][1] = 42 + 42; myArray[1][2] = 101 + 101; } myArray [0][0] [0][1] [0][2] [1][0] [1][1] [1][2] 154 84

14. In Memory 0x12AB79 77 42 101 public class 2DArrayExample { public static void main(String[] args) { int[][] myArray; myArray = new int[2][3]; myArray[0][0] = 77; myArray[0][1] = 42; myArray[0][2] = 101; myArray[1][0] = 77 + 77; myArray[1][1] = 42 + 42; myArray[1][2] = 101 + 101; } myArray [0][0] [0][1] [0][2] [1][0] [1][1] [1][2] 154 84 202

15. In Memory 0x12AB79 77 42 101 public class 2DArrayExample { public static void main(String[] args) { int[][] myArray; myArray = new int[2][3]; myArray[0][0] = 77; myArray[0][1] = 42; myArray[0][2] = 101; myArray[1][0] = 77 + 77; myArray[1][1] = 42 + 42; myArray[1][2] = 101 + 101; } myArray [0][0] [0][1] [0][2] [1][0] [1][1] [1][2] 154 84 202 myArray[0] myArray is an int[][] myArray[0] is an int[] myArray[0][0] is an int myArray is an int[][] myArray[0] is an int[] myArray[0][0] is an int

16. In Memory 0x12AB79 77 42 101 public class 2DArrayExample { public static void main(String[] args) { int[][] myArray; myArray = new int[2][3]; myArray[0][0] = 77; myArray[0][1] = 42; myArray[0][2] = 101; myArray[1][0] = 77 + 77; myArray[1][1] = 42 + 42; myArray[1][2] = 101 + 101; } myArray [0][0] [0][1] [0][2] [1][0] [1][1] [1][2] 154 84 202 myArray[1] myArray is an int[][] myArray[1] is an int[] myArray[1][0] is an int myArray is an int[][] myArray[1] is an int[] myArray[1][0] is an int

17. In Memory 0x12AB79 public class 2DArrayExample { public static void main(String[] args) { String[][] strArray; strArray = new String[2][3]; for (int i = 0; i < strArray.length; i++) { for (int j = 0; j < strArray[i].length; j++) { strArray[i][j] = new String((i+1) + “ and “); } strArray [0][0] [0][1] [0][2] [1][0] [1][1] [1][2] 0x33DB26 0x33DC01 0x33DD25 0x33DE04 0x33DE29 0x33DF22

18. In Memory 0x12AB79 public class 2DArrayExample { public static void main(String[] args) { String[][] strArray; strArray = new String[2][3]; for (int i = 0; i < strArray.length; i++) { for (int j = 0; j < strArray[i].length; j++) { strArray[i][j] = new String((i+1) + “ and “); } strArray [0][0] [0][1] [0][2] [1][0] [1][1] [1][2] 0x33DB26 0x33DC01 0x33DD25 0x33DE04 0x33DE29 0x33DF22 # rows # cols

19. Managing 2D Arrays  Typically use nested for loops: int[][] randomNumbers = int[1024][2048]; for (int i = 0; i < randomNumbers.length; i++) { for (int j = 0; j < randomNumbers[i].length; j++) { randomNumbers[i][j] = generator.nextInt(10000); }

20. Managing 2D Arrays  Typically use nested for loops: int[][] randomNumbers = int[1024][2048]; for (int i = 0; i < randomNumbers.length; i++) { for (int j = 0; j < randomNumbers[i].length; j++) { randomNumbers[i][j] = generator.nextInt(10000); } int numberOfOdds = 0; for (int i = 0; i < randomNumbers.length; i++) { for (int j = 0; j < randomNumbers[i].length; j++) { if ( randomNumbers[i][j] % 2 != 0 ) { numberOfOdds++; } }

21. Let’s Try Some… Photo by Gary Mueller, from www.allaboutbirds.orgwww.allaboutbirds.org (Cornell Laboratory of Ornithology)