Java

TOPIC 08 – MULTIDIMENSIONAL ARRAYS

LESSON NOTE

JAVA CODE

One can create a multidimensional array in the same way as one creates a normal array with one simple exception. You simply need to include square brackets for each dimension that you want. Inside each bracket, you must also include the size of the dimension.

EXAMPLES

Example 1 – Create a two dimensional integer array that is 2 by 3 in size.

int[][] x = new int[2][3];

Example 2 – Create a four dimension String array that is 3 by 5 by 2 by 4 in size.

String[][][][] sa = new String[3][5][2][4];

NUMBER OF ELEMENTS

To calculate the number of elements in a multidimensional array, we simply multiply all of the sizes of all the dimensions together.

Examples

The number of elements in the array x above is 6 because 2 x 3 = 6.

The number of elements in the array sa above is 120 because 3 x 5 x 2 x 4 = 120.

INDEXES

For each dimension, the index work like before. They range from 0 to its size-1.

EXAMPLES – ASSIGNING VALUES

Example 1 – Here is the code that is creating a 2 by 4 2D array and giving values to each element.

int[][] arr = new int[2][4];
arr[0][0] = 3;
arr[0][1] = 6;
arr[0][2] = 10;
arr[0][3] = -3;

arr[1][0] = 1;

arr[1][1] = 8;

arr[1][2] = 2;

arr[1][3] = 5;

Example 2 – The following assignments statement would cause errors.

arr[0][4] = 2; //the “4” is out of bounds

arr[2][3] = 5; //the “2” is out of bounds

USING LOOPS TO ASSIGN VALUES

We can use loops to assign the values of elements. This works in the same way as the it did with one-dimensional arrays.

Example – Create a 2D array of size 25 by 42 and initialize each value to a random number.

double[][] arr = new double[25][42];

for (int i=0; i < arr.length; i++) //arr.length is 25

{

for (int k=0; k < arr[i].length; k++) //arr.length[i] is 42

{

arr[i][k] = Math.random();

}

}

OUTPUTTING FUNCTION IN ARRAYS

We can output multidimensional arrays to screen by using loops in a very similar way as we used loops to set the value of each element (see above).

However, the Arrays class once again has a useful function that returns a String representation of a multidimensional array.

String s = Arrays.deepToString(arrayName);
System.out.println(s);

Of course, like before, we do need the import statement at the top of our class:

import java.util.Arrays;

VISUAL REPRESENTATION

We often pretend that a 2D array is like a simple grid.

For example, the statement

int board[][] = new int[8][8];

Would be a grid like this:

However, while thinking of 2D arrays like this is probably is fine for most applications, it is not what actually happens in memory.

MULTIDIMENSIONAL ARRAYS IN MEMORY

Multidimensional arrays are arrays of arrays. So they are really a bunch of 1-dimensional arrays that are grouped together.

For example, the board[][] array from above would look like this in memory: