Arrays

Chapter 6

Outline

• Array Basics• Arrays in Classes and Methods• Sorting Arrays• Multidimensional Arrays

Introduction to Arrays

• An array is an object used to store a (possibly large) collection of data.

• All the data stored in the array must be of the same type.

• An array object has a small number of predefined methods.

Introduction to Arrays, cont.

• Sometimes you want to know several things about a collection of data:– the average– the number of items below the average– the number of items above the average– etc.

• This requires all the items to be stored as variables of one kind or another.

Introduction to Arrays, cont.

• Arrays satisfy this need.• Even though an array is an object in Java, it

can be convenient to think of it as a collection of variables of the same type.

Array Basics: Outline

• Creating and Accessing Arrays• Array Details• The length Instance Variable• Initializing Arrays

Creating and Accessing Arrays

• exampledouble[] temperature = new double[7];

is like declaring seven variables of type double, namedtemperature[0], temperature[1], temperature[2], temperature[3], temperature[4], temperature[5], temperature[6].

Creating and Accessing Arrays, cont.

• These variables can be used just like any other variables of type double.

• examplestemperature[3] = 32.0;

temperature[6] = temperature[3] + 5;

System.out.println(temperature[6]);

temperature[index+1] = 66.5;

• These variables are called indexed variables, elements, or subscripted variables.

• The integer expression within the square brackets is called the index (or subscript).

Creating and Accessing Arrays, cont.

Creating and Accessing Arrays, cont.

Array Details

• syntax for creating an arrayBase_Type[] Array_Name =

new Base_Type[Length];

• exampleint[] pressure = new int[100];

orint[] pressure;

pressure = new int[100];

Array Details, cont.

• The type of the elements is called the base type.

• The base type of an array can be any type including a class type.– for example,Species[] entry = new Species[3];

• The number of elements is the length or size of the array.

Brackets[]

• Brackets [] serve three purposes:– creating the type name

example: int[] pressure;– creating the new arraypressure = new int[100];

– naming an indexed variable of the arraypressure[3] = keyboard.nextInt();

Array Terminology

The length Instance Variable

• An array has only one public instance variable, namely length.

• The length variable stores the number of elements the array can hold.

• Using Array_Name.length typically produces clearer code than using an integer literal.

The length Instance Variable, cont

Indices and length

• The indices of an array start with 0 and end with Array_Name.length-1.

• When a for loop is used to step through an array, the loop control variable should start at 0 and end at length-1.

• examplefor (lcv = 0; lcv < temperature.length;

index++)

Array Index Out of Bounds

• Every index must evaluate to an integer which is not less than 0 and not greater than Array_Name.length-1.

• Otherwise, the index is said to be out of bounds or invalid.

• An out-of-bounds index will produce a run-time error.

Incomplete Array Processing

• Loops fail to process an entire array correctly when they–begin with an index other than 0–end with an index other than length-1.

• Examples for (i = 1; i < oops.length-1; index++)

for (i = 1; i <= oops.length; index++)

for (i = 0; i <= oops.length; index++)

for (i = 0; i < oops.length-1; index++)

Initializing Arrays

• An array can be initialized at the time it is declared.

• exampledouble[] reading = {3,3, 15.8, 9.7];

– The size of the array is determined by the number of values in the initializer list.

Initializing Arrays, cont.

• Uninitialized array elements are set to the default value of the base type.

• However, it’s better to use either an initializer list or a for loop.int[] count = new int[100];

for (int i = 0, i < count.length, i++)

a[i] = 0;

Arrays in Classes and Methods

• Arrays can be used as instance variables in classes.

• Both an indexed variable of an array and an entire array can be a argument of a method.

• Methods can return an indexed variable of an array or an entire array.

• exampledouble[] a = new double[10];

SampleClass.change(a);

...

public static void change(double[] d)

Array Subtleties

• If the base type of an array is a primitive type, then a method to which an array element is passed creates a copy of the array element, and cannot change the original array element.

• If the base type of an array is a class type, then a method to which an array element is passed creates an alias, and the referenced object can be changed.

Arguments for the Method main

• Recall the heading for method main:public static void main(String[] args)

• Method main takes an array of String values as its argument.

• A default array of String values is when you run a program.

Arguments for the Method main, cont.

• Alternatively, an array of String values can be provided in the command line.

• examplejava TestProgram Mary Lou

– args[0] is set to “Mary”– args[1] is set to “Lou”System.out.println(“Hello “ + args[0] +

“ “ + args[1]);

prints Hello Mary Lou.

Use of = and == with Arrays

• The assignment operator = and the equality operator ==, when used with arrays, behave the same as when used with other objects.– The assignment operator creates an alias, not a copy of the array.

– The equality operator determines if two references contain the same memory address, not if two arrays contain the same values.

Making a Copy of an Array

• exampleint[] a = new int[50];

int[] b = new int[50];

...

for (int j = 0; j < a.length; j++)

b[j] = a[j];

Sorting Arrays: Outline

• Selection Sort• Other Sorting Algorithms

Sorting Arrays

• Sometime we want numbers in an array sorted from smallest to largest, or from largest to smallest.

• Sometimes we want the strings referenced by an array to be in alphabetical order.

• Sorting techniques typically are easy to adapt to sort any type that can be ordered.

Selection Sort

• The selection sort arranges the values in an an array so thata[0] <= a[1] <= a[2] … <= a[a.length-1]

• The selection sort places the smallest item in a[0], the next smallest item in a[1], and so on for all but the last item.for(i = 0; i <a.length-1; i++)

place the ith smallest item in a[i]

Selection Sort, cont.

• Selection sort begins by finding a smallest item in the array and swapping it with the item in a[0].

• Selection sort continue by finding a smallest item in the remainder of the array and swapping it with the next item in array a.

• Selection sort terminates when only one item remains.

Selection Sort, cont.

Selection Sort, cont.

Selection Sort, cont.

Selection Sort, cont.

Swapping Elements

• To swap two elements a[i] and a[j], one of them must be saved temporarily.

Other Sorting Algorithms

• The selection sort is not particularly efficient, but it is easy to code.

• More efficient algorithms, such as quicksort, are more difficult to code.

Multidimensional Arrays

• Introduction to Multidimensional Arrays• Multidimensional-Array Basics• Multidimensional-Array Parameters and

Returned Values• Implementation of Multidimensional Arrays• (optional) Ragged Arrays• Programming Example

Introduction to Multidimensional Arrays

• An array with more than one index sometimes is useful.

• example: savings account balances at various interest rates for various numbers of years– columns for interest rates– rows for years

• This two-dimensional table calls for a two-dimensional array.

Introduction to Multidimensional Arrays, cont.

• An array with n indices is called an n-dimensional array.

• The arrays we considered previously, which had one index, were one-dimensional arrays.

Introduction to Multidimensional Arrays, cont.

Multidimensional-Array Basics

• example declarationint[][] table = new int [10][6];

orint[][] table;

table = new int[10][6];

• The number of bracket pairs in the declaration is the same as the number of indices.

Multidimensional-Array Basics, cont.

• syntaxBase_Type[]…[] Array_Name =

new Base_Type[Length_1]…[Length_n];

• exampleschar[][] page = new char [100][80];

double[][][] threeDPicture =

new double[10][20][30];

SomeClass[][] entry =

new SomeClass[100][80];

Multidimensional-Array Basics, cont.

Multidimensional-Array Basics, cont.

Multidimensional-Array Parameters

• Methods may have multidimensional-array parameters and return a multidimensional array.

Implementation of Multidimensional Arrays

• Multidimensional arrays are implemented in Java using one-dimensional arrays.

• Considerint[][] table = new int[10][6];

– The array table is a one-dimensional array of length 10.

– Its base type is int[].– Therefore, it is an array of arrays.

Implementation of Multidimensional Arrays, cont.

• This permits us to use the length instance variable, instead of an integer literal, to control a for loop used to initialize or print the values of the elements of an array.

• example for (r = 0; r < table.length; r++)

for (c = 0; c < table[r].length; c++)

(optional) Ragged Arrays

• Since a two-dimensional array in Java is an array of arrays, each row can have a different number of elements (columns).

• Arrays in which rows have different numbers of elements are called ragged arrays.

• Exampleint[][] b = new int[3][];

b[0] = new int[5];

b[1] = new int[7];

b[2] = new int[4];

Summary

• You have learned about arrays and how to use them in Java programs.

• You have learned how to use array parameters and how to define methods that return an array.

• You have learned the proper use of an array as an instance variable.

• You have learned about multidimensional arrays.