Two-Dimensional Arrays - University of Arizona

Chapter 11

Two-Dimensional Arrays

This chapter introduces Java arrays with two subscripts for managing data logically stored in a table-like format in rows and columns. This structure proves useful for storing and managing data in many applications, such as electronic spreadsheets, games, topographical maps, and student record books.

11.1 2-D Arrays

Data that conveniently presents itself in tabular format can be represented using an array with two subscripts, known as a two-dimensional array. Two-dimensional arrays are constructed with two pairs of square brackets to indicate two subscripts representing the row and column of the element.

Chapter 11: Tw0-Dimensional Arrays

General Form: A two-dimensional array consruction (all elements set to default values)

type[][] array-name = new type [row-capacity][column-capacity]; type[][] array-name = { { element[0][0], element[0][1], element[0][2], ... } ,

{ element[1][0], element[1][1], element[1][2], ... } , { element[2][0], element[2][1], element[2][2], ... } };

type may be one of the primitive types or the name of any Java class or interface identifier is the name of the two-dimensional array rows specifies the total number of rows columns specifies the total number of columns

Examples:

double[][] matrix = new double[4][8];

// Construct with integer expressions int rows = 5; int columns = 10; String[][] name = new String[rows][columns];

// You can use athis shortcut that initializes all elements int[][] t = { { 1, 2, 3 }, // First row of 3 integers

{ 4, 5, 6 }, // Row index 1 with 3 columns { 7, 8, 9 } }; // Row index 2 with 3 columns

Referencing Individual Items with Two Subscripts

A reference to an individual element of a two-dimensional array requires two subscripts. By convention, programmers use the first subscript for the rows, and the second for the columns. Each subscript must be bracketed individually.

General Form: Accessing individual two-dimensional array elements

two-dimensional-array-name[rows][columns]

rows is an integer value in the range of 0 through the number of rows - 1 columns is an integer value in the range of 0 through the number of columns - 1

Examples: String[][] name = new String[5][10]; name[0][0] = "Upper Left"; name[4][9] = "Lower Right"; assertEquals("Upper Left", name[0][0]);

// name.length is the number of rows, // name[0].length is the number of columns assertEquals("Lower Right", name[name.length-1][name[0].length-1]);

Nested Looping with Two-Dimensional Arrays

Nested looping is commonly used to process the elements of two-dimensional arrays. This initialization allocates enough memory to store 40 floating-point numbers--a two-dimensional array with five rows and eight columns. Java initializes all values to 0.0 when constructed.

int ROWS = 5; int COLUMNS = 8; double[][] table = new double[ROWS][COLUMNS]; // 40 elements set to 0.0

These nested for loops initialize all 40 elements to -1.0.

// Initialize all elements to -1.0 for (int row = 0; row < ROWS; row++) {

for (int col = 0; col < COLUMNS; col++) { table[row][col] = -1.0;

} }

Self-Check

Use this construction of a 2-D array object to answer questions 1 through 8:

int[][] a = new int[3][4];

11-1 What is the value of a[1][2]? 11-2 Does Java check the range of the subscripts when referencing the elements of a? 11-3 How many ints are properly stored by a? 11-4 What is the row (first) subscript range for a? 11-5 What is the column (second) subscript range for a? 11-6 Write code to initialize all of the elements of a to 999. 11-7 Declare a two-dimensional array sales such that stores 120 doubles in 10 rows. 11-8 Declare a two-dimensional array named sales2 such that 120 floating-point numbers can be stored

in 10 columns.

A two-dimensional array manages tabular data that is typically processed by row, by column, or in totality. These forms of processing are examined in an example class that manages a grade book. The data could look like this with six quizzes for each of the nine students.

Quiz #0 0 67.8 1 76.4 2 87.8 3 86.4 4 72.8 5 94.4 6 85.8 7 76.4 8 57.9

1 56.4 81.1 76.4 54.0 89.0 63.0 95.0 84.4 49.5

2 88.4 72.2 88.7 40.0 55.0 92.9 88.1 100.0 58.8

3 79.1 76.0 83.0

3.0 62.0 45.0 100.0 94.3 67.4

4 90.0 85.6 76.3

2.0 68.0 75.6 60.0 75.6 80.0

5 66.0 85.0 87.0

1.0 77.7 99.5 85.8 74.0 56.0

This data will be stored in a tabular form as a 2D array. The 2D array will be processed in three ways:

1. Find the average quiz score for any of the 9 students 2. Find the range of quiz scores for any of the 5 quizzes 3. Find the overall average of all quiz scores

Here are the methods that will be tested and implemented on the next few pages:

// Return the number of students in the data (#rows) public int getNumberOfStudents()

// Return the number of quizzes in the data (#columns) public int getNumberOfQuizzes()

 ................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download