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Introduction to Programming

Lecturer: Steve Maybank

Department of Computer Science and Information Systems

sjmaybank@dcs.bbk.ac.ukSpring 2013

Week 9: Arrays

Java Lab 8, Ex 1: RepeatString

Implement a class which contains the method main and the method public static String repeat(String str, int n)which returns the string str repeated n times.

Call the method repeat from main.

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Code for Exercise 2 (1)/** * Program to test the method repeat which prints out a * given string, repeated a given number of times. * @author S.J. Maybank * @version 22 February 2011 */public class RepeatString{ // main goes here // repeat goes here}

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Code for Exercise 2 (2)

public static void main(String[] args)

{ String str1 = repeat("Hello World", 3); String str2 = repeat("Hello World", 0); String str3 = repeat("H", 10); System.out.println("Examples of the output from the method

repeat"); System.out.println(str1); System.out.println(str2); System.out.println(str3); }

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Code for Exercise 2 (3)public static String repeat(String str, int n){ String result = ""; for(int i = 1; i <= n; i++) { result = result+str; } return result;}

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Java Lab 8, Ex 2: ReadDouble

Implement the method 

public static double readDouble(String prompt)

which displays the string prompt, followed by a space, then reads in a floating point number and returns it.

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Code for Exercise 3 (1)import java.util.Scanner;/** * Display a prompt string and then read in a number of type

double. * @author S.J. Maybank * @version 22 February 2011 */public class ReadDouble{ // main goes here // readDouble goes here}

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Code for Exercise 3 (2)

public static void main(String[] args){ double r = readDouble(ꞌꞌInput a number of type

double:"); System.out.println("The input number is "+r);}

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Code for Exercise 3 (3)

public static double readDouble(String prompt)

{Scanner in = new Scanner(System.in);System.out.print(prompt+" ");return in.nextDouble();

}

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Overview Nature of an array Declaration of an array Array indexing Partially filled arrays Array algorithms See Java for Everyone, Ch. 6

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Arrays

A mechanism for collecting together multiple values.

A way of allocating names to multiple variables.

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Array Declaration and Initialisation

double[] data = {32, 54, 67, 5};

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type of arrayvariable

name of arrayvariable

list of initial values

Names of variables: data[0], data[1], data[2], data[3]The numbers 0, 1, 2, 3 are indices

Example

data[3] = 35;System.out.println(data[3]); // prints 35/*Element: data[3]Value of the index: 3Value of the element data[3]: 35Each element can be used like any variable

oftype double. */

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Array Indexing Array indexing begins with 0 The length of the array data is

data.length Array indexing ends with data.length-1

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

System.out.println(data[i]);}

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Bounds Error

data[data.length] = 4; //errordata[-1] = 3; // error

/* A bounds error is automatically detected at run time. It is not detectedat compile time.

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Array References

int[] scores = {10, 9, 7, 4, 5};int[] values = scores;scores[3] = 10;System.out.println(values[3]); // prints 10!

/* The array variable is a pointer to the place in memory where the array is stored. */

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The Array Variable as a Pointer

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array stored in memoryscores values

10

9 7 4 5

The value of the variable score is a pointer to the array.The value of the variable values is also a pointer to the array

Array Declaration and Initialisation (2)

double[] data = new double[4];

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type ofarrayvariable

name ofarrayvariable

elementtype

length

The elements of the array data are automaticallyassigned the value 0, but do not rely on this!

Declaring Arrays

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int[] numbers = new int[10]; An array of 10 integers. All elements are initialised with 0

final int LENGTH = 10;int[] numbers = new int[LENGTH];

Use a named constant

int[] squares = {0, 1, 4, 9, 16};

String[] friends = {"Em", "Bob", "Sue"};

An array of three strings

double[] data = new int[10]; Error: a variable of type double[] cannot be assigned a value of type int[]

Partially Filled Arrays

In a partially filled array, only part of the array is used.

A companion variable keeps count of the number of array elements that have been used.

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Example of a Partially Filled Array

int currentSize = 0; // companion variableScanner in = new Scanner(System.in);while(in.hasNextDouble()) // check for further input{

if(currentSize < data.length){

data[currentSize] = in.nextDouble();currentSize++;

}}

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Printing a Partially Filled Array

for(int i = 0; i < currentSize; i++){

System.out.println(data[i]);}

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Array Algorithm 1/* find the sum and the average value of the

elements in the array data. */double total = 0;for(i = 0; i < data.length; i++){

total = total + data[i];}double average = 0;if(data.length>0){average = total/data.length;}

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Array Algorithm 2

/* Find the maximum and the minimum of the elements in the non-empty array data. */

double largest = data[0], smallest = data[0];for(int i = 0; i < data.length; i++){

if(data[i] > largest){largest = data[i];}if(data[i] < smallest){smallest = data[i];}

}

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Array Algorithm 3/* Linear search. */int searchedValue = 100, pos = 0;boolean found = false;while (pos < values.length && !found){

if(values[pos] == searchedValue){

found = true;}else{

pos++;}

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Array Algorithm 3 (Coda)

if(found){

System.out.println(ꞌꞌFound at positionꞌꞌ+pos);

}else{

System.out.println(ꞌꞌNot foundꞌꞌ);}

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