Chapter 8 Arrays and the ArrayList Class Multi-Dimensional Arrays

2 Contents I. Two-Dimensional Arrays II. Arrays with Three or More Dimensions

3 I. Two-Dimensional Arrays 1. Creating a Two-Dimensional Array 2. Initializing a Two-Dimensional Array 3. The length Field in a Two-Dimensional Array 4. Displaying All the Elements of a Two- Dimensional Arrays 5. Summing All the Elements of a Two- Dimensional Array 6. Summing the Rows of a Two-Dimensional Array

4 I. Two-Dimensional Arrays 7. Summing the Columns of a Two-Dimensional Array 8. Passing Two-Dimensional Arrays to Methods 9. Ragged Arrays

5 I.1. Creating a Two-Dimensional Array Two-dimensional array, which are sometimes called 2D arrays, is an array of arrays. It has rows and columns of elements. Row 0 Row 1 Row 2 Column 0 Column 1 Column 2 Column 3

6 I.1. Creating a Two-Dimensional Array To declare a two-dimensional array: double[][] scores = new double[3][4]; Two set of brackets Two size declarators The first one is for the number of rows: 3 The second one is for the number of columns: 4 Two sets of brackets indicate a two-dimensional array Number of rows Number of columns

7 I.1. Creating a Two-Dimensional Array In the scores array: The elements in row 0: scores[0][0]scores[0][1] scores[0][2]scores[0][3] The elements in row 1: scores[1][0]scores[1][1] scores[1][2]scores[1][3] The elements in row 2: scores[2][0]scores[2][1] scores[2][2]scores[2][3]

8 I.1. Creating a Two-Dimensional Array Row 0 Row 1 Row 2 Column 0 Column 1 Column 2 Column 3 address The scores variable

9 I.1. Creating a Two-Dimensional Array Programs that process two-dimensional arrays can do so with nested loops. final int ROWS = 3; final int COLS = 4; double[][] scores = new double[ROWS][COLS]; for(int row = 0; row < ROWS; row++) { for(int col = 0; col < COLS; col ++) { System.out.print(“Enter a score: “); scores[row][col] = keyboard.nextDouble(); } }

10 I.2. Initializing a Two-Dimensional Array Enclose each row's initialization list in its own set of braces: int[][] numbers = { {1, 2, 3}, {4, 5, 6}, {7, 8, 9} }; Row 0Row 1Row 2

11 I.3. The length Field in a Two- Dimensional Array A two-dimensional array can be considered as an array of one-dimensional arrays. int[][] numbers = new int[3][4]; addres s numbers[1][1]numbers[1][2]numbers[1][3]numbers[1][0] numbers[0][1]numbers[0][2]numbers[0][3]numbers[0][0] numbers[2][1]numbers[2][2]numbers[2][3]numbers[2][0] numbers[ 0] numbers[ 1] numbers[ 2] address The numbers variable numbers.length 3 numbers[0].length 4 numbers[1].length 4 numbers[2].length 4

12 I.3. Displaying All the Elements of a Two-Dimensional Array int[][] numbers = { {1, 2, 3, 4}, {4, 5, 6, 8}, {9, 10, 11, 12} }; //Display all the elements in the array for(int row = 0; row < 3; row++) { for(int col = 0; col < 4; col++) System.out.println(numbers[row][col]); } //A better approach to display all elements in the array for(int row = 0; row < numbers.length; row++) { for(int col = 0; col < numbers[row].length; col++) System.out.println(numbers[row][col]); }

13 I.4. Summing All the Elements of a Two-Dimensional Array int[][] numbers = { {1, 2, 3, 4}, {4, 5, 6, 8}, {9, 10, 11, 12} }; int acc = 0; //Accumulator, set to 0 //Sum the array elements for(int row = 0; row < numbers.length; row++) { for(int col = 0; col < numbers[row].length; col++) acc += numbers[row][col]; } //Display the sum System.out.println(“The total is ” + acc);

14 I.5. Summing the Rows of a Two- Dimensional Array Calculate the sum of each row in a two- dimensional array: int[][] numbers = { {1, 2, 3, 4}, {4, 5, 6, 8}, {9, 10, 11, 12} }; int acc; //Accumulator for(int row = 0; row < numbers.length; row++) { //Set the accumulator to 0 acc = 0; //Sum a row for(int col = 0; col < numbers[row].length; col++) acc += numbers[row][col]; //Display the sum System.out.println(“The total of row is ” + acc); }

15 I.6. Summing the Column of a Two-Dimensional Array The outer loop controls the column subscript and the inner loop controls the row subscript. int[][] numbers = { {1, 2, 3, 4}, {4, 5, 6, 8}, {9, 10, 11, 12} }; int acc; //Accumulator for(int col = 0; col < numbers[0].length; col++) { //Set the accumulator to 0 acc = 0; //Sum a column for(int row = 0; row < numbers.length; row++) acc += numbers[row][col]; //Display the sum System.out.println(“The total of column is ” + acc); }

16 I.7. Passing Two-Dimensional Arrays to Methods When a two-dimensional array is passed to a method, the parameter must be declared as a reference to a two-dimensional array. public static void showArray(int[][] array) The method's parameter, array, is declared as a reference to a two- dimensional array.

17 I.8. Ragged Arrays When the rows of a two-dimensional array are of different lengths, the array is known as a ragged array. Creating a ragged array: //Creating a two-dimensional array, the array has four //rows, but the rows have not yet been created. int[][] ragged = new int[4][]; //Creat the individual rows ragged[0] = new int[3]; //row 0 has 3 elements ragged[1] = new int[4]; //row 1 has 4 elements ragged[2] = new int[5]; //row 2 has 5 elements ragged[3] = new int[6]; //row 3 has 6 elements

18 I.8. Ragged Arrays Processing elements of a ragged array: for(int row = 0; row < ragged.length; row++) { for(int col = 0; col < ragged[row].length; col++) { // Processing the element ragged[row][col] // } }

19 II. Arrays with Three or More Dimensions Java does not limit the number of dimensions that an array may have. A three-dimensional array declaration: double[][][] array = new double[3][5][7]; This array can be though of as three sets of five rows. Each row contains seven elements.