Introduction

Data in a table or a matrix can be represented using a two dimensional array.

The preceding ___ introduces how to use one-dimensional arrays to store linear collections of elements. You can use a two-dimensional array to store a matrix or a table. For example, the following table that lists the distances between cities can be stored using a two-dimensional array named distances.

The two dimensional array equivalent:

double[][] distances = {
    {0, 983, 787, 714, 1375, 967, 1087},
    {983, 0, 214, 1102, 1763, 1723, 1842},
    {787, 214, 0, 661, 781, 810},
    {714, 1102, 888, 0, 661, 781, 810},
    {1375, 1763, 1549, 661, 0, 1426, 1187},
    {967, 1723, 1548, 781, 1426, 0, 239},
    {1087, 1842, 1627, 810, 1187, 239, 0},
};

Two Dimensional Array Basics

An element in a two dimensional array is accessed through a row and a column index.

How do you declare a variable for two dimensional arrays? How do you make a two dimensional array? How do you access elements in a two dimensional array? This section addresses these issues.

Declaring Variables of and Making Two Dimensional Arrays

The syntax for declaring a two-dimensional array is:

elementType[][] arrayRefVar;

or

elementType arrayRefVar[][]; // Allowed, but not preferred style of syntax

As an example, here is how you would declare a two-dimensional array variable matrix of int values:

int[][] matrix;

or

int matrix[][]; // Allowed, but not preferred style of syntax

You can make a two dimensional array of 5-by-5 int values and assign it to matrix using this syntax:

matrix = new int[5][5];

Two subscripts are used in a two dimensional array, one for the rows and another for the columns. As in a one dimensional array, the index for each subscript is of the int type and starts from 0, as show below in table (a).

To assign the value 7 to a specific element at row 2 and column 1, as shown in table (b), you can use the following syntax:

matrix[2][1] = 7;

To make the a point clear: the 1st bracket [] is references the rows. The 2nd [] is references the columns.

*Note that a common mistake is to use matrix[2, 1] to access the element at row 2 and column 1. In Java, each subscript must be enclosed in a pair of square brackets.

You can also use an array initializer to declare, make, and initialize a two dimensional array. For example, the following code:

int[][] array = {
    {1, 2, 3},
    {4, 5, 6},
    {7, 8, 9},
    {10, 11, 12},
};

makes an array with specified initial values, as shown in table (c). This is code is equivalent to:

int[][] array = new int[4][3];
array[0][0] = 1; array[0][1] = 2; array[0][2] = 3;
array[1][0] = 4; array[1][1] = 5; array[1][2] = 6;
array[2][0] = 7; array[2][1] = 8; array[2][2] = 9;
array[3][0] = 10; array[3][1] = 11; array[3][2] = 12;

Obtaining the Lengths of Two-Dimensional Arrays

A two-dimensional array is actually an array in which each element is a one-dimensional array. The length of an array x is the number of elements in the array, which can be obtained using x.length. x[0], x[1],…, and x[x.length-1] are arrays. Their lengths can be obtained using x[0].length, x[1].length,…, and x[x.length-1].length.

In other words, Number of Rows = x.length and Number of Columns = x[x.length-1].length.

For example, suppose x = new int[3][4], x[0], x[1], and x[2] are one dimensional arrays and each contains four elements. x.length is 3, and x[0].length, x[1].length, and x[2].length are 4. Here’s a diagram showing the lengths of a two dimensional array:

Ragged Arrays

Each row in a two dimensional array is itself an array. Thus, the rows can have different lengths. An array of this kind is known as a ragged array. Here is an example of making a ragged array:

As you can see, triangleArray[0].length is 5, traingleArray[1].length is 4, triangleArray[2] is 3, triangleArray[3] is 2, and triangleArray[4] is 1. If you don’t know the values in a ragged array in advance, but do know the sizes- say, the same as before- you can make a ragged array using the following syntax:

You can now assign values to the array. For example,

triangleArray[0][3] = 50;
traingleArray[4][0] = 45;

*Note that the syntax new int[5][] for making an array requires the first index to be specified. So new int[][] would be wrong.

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