Two-Dimensional Arrays

Two-Dimensional Arrays

When working with two dimensional arrays, it is beneficial to think of them as arrays holding arrays. For example, a statement declaring a 2D array with an initializer list would look like:

int[ ][ ] int2D = {{1, 2, 3, 4}, {5, 6, 7, 8}, {9, 0, 1, 2}};

However, consider the following representation of this array:

The size of this array is 3 rows and 4 columns. In a mathematics class, we would call this a 3 by 4 matrix. In addition, we should be able to identify the indices for each element of the array as an ordered pair. For example, the number 3 has location 0, 2. This means that int2D[0][2] = 3. The array below shows the index location for each element of the array int2D.

Practice

Evaluate the following:

[1] int2D[1][3] = [2] int2D[3][0] =

Answers:

[1] int2D[1][13] = 8

[2] int2D[3][0] = undefined, causes an ArrayIndexOutOfBoundsException error

An ArrayIndexOutOfBoundsException error occurs when you try to access a location that does not exist in a given array.

2D Array Lengths

When considering 2D arrays as arrays holding arrays it makes it easier to evaluate the length field.

int2D.length = ???

Remember int2D is an array holding arrays, so what is the length of int2D? Once you’ve found the length of int2D, next evaluate:

int2D[0].length = ???

All of the properties, techniques, and characteristics of one-dimensional arrays can be extended to two-dimensional arrays with some modifications in coding. For example, the for loop will need to vary for two-dimensional arrays depending on which part of the array the code is iterating. Consider the following statements and when to use each.

for(int i = 0; i < int2D.length ; i++)

vs.

for(int i = 0; i < int2D[0].length ; i++)

Recall, it is beneficial to think of two-dimensional arrays as "arrays" holding "arrays". With this in mind then int2D.length counts the number of arrays being stored, and int2D[0] counts how many values are stored by the first array, i.e. the array stored at the zero index of int2D.