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Multi-dimensional Arrays

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Introduction

A multidimentional array is an array that contains a number of other arrays of the same type. They are declared, constructed and initialised in a simular way to one-dimensional arrays. In the first bracket we specify the number of arrays we are going to create in our array.

Array Declaration

To declare a three dimesional array we specifiy the size of the array as follows.

int [][] scores= new int [3][];

This array can contain up to three int arrays. The three arrays are defined by assigning a new array to each element.

scores[0] = new int[4]; // An int array of 4 elements
                        // is allocated to the first element
scores[1] = new int[6]; // An int array of 6 elements
                        // is allocated to the second element
scores[2] = new int[1]; // An int array of 1 elements
                        // is allocated to the third element

Array Initialisation

Initializing an array gives its elements a value other than the default value. Array elements are initialised by assigning a compatible value.

The contents of each array are allocated as follows:

scores[0][0] = 6; // Allocates 6 to the first element in the first array.
scores[0][1] = 1; // Allocates 1 to the second element in the first array.
scores[1][0] = 3; // Allocates 3 to the first element in the second array.
scores[2][0] = 2; // Allocates 2 to the first element in the third array.

The first square brackets refers to the array, while the second square brackets refers to the element in the array refered to by the first square brackets.

Shortcut Declaration, Construction and Initialization

An array can be declared, constructed and initialised all at once.

int[][] scores = {{5,2,4,7}, {9,2}, {3,4}};

this is the same as doing:

int[][] scores = new int[3][];
scores[0][] = {5,2,4,7};
scores[1][] = {9,2};
scores[2][] = {3,4};

A two-dimensional array is created because we have int[][] scores on the left-hand side. The contents inside {} creates and allocates arrays. In this case three.

The size of the array is determined by the number of comma-separated items.

Notes

The dimensions must be declared from left to right: int[ ] a[ ] = new int[4][ ] ; and int[ ][ ] a = new int[5][4] ; However this is wrong: int a[ ][ ] = new int [ ][4] ;

The first element in the array is: i[0], this refers to the first array reference, the second element is: i[1], this refers to the second array reference. Therefore it is possible to do the following assignments:

int [ ] y = i [ 0 ];
int [ ] x = i [ 1 ];

It is not necessary to put the extra [] because we only want to refer to the array reference not the contents of these arrays.

In the array declaration int i[][] = new int[4][4]; there are two arrays and each can hold 5 elements. The first array is at i[0] and the second array is at i[1].

In an array that looks like this:

   0    1
 0 [x] [ ]
 1 [ ] [ ]
 2 [ ] [ ]
 3 [ ] [y]
 4 [ ] [ ]

To refer to x we would write: i[0][0] to refer to y: i[1][3]. The first number is the index of the array while the second number is the index of the element: i[index of array][index of element].

If you liked this tutorial about multi-dimensional arrays you might like my tutorial on one-dimensional arrays.

I often post about arrays and core Java on Twitter. Follow me here

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