Chapter 8 Arrays

Programming and Problem SolvingWith Java

Copyright 1999, James M. Slack

Chapter 8ArraysOne-dimensional ArraysTwo-dimensional ArraysComputer Graphics

Programming and Problem Solving With Java 2

One-dimensional ArraysSo far, most of our programs input few values

Program stores these values in a few variablesProgram does some calculation, then displays output

Value 1

Value 2

Value 3

Output

Program

Variable 1

Variable 2

Variable 3


One-dimensional ArraysProgram may read lots of values

But reads and processes one at a time

Value 1

Value 2

Value 3

Output

Program

Variable 1Value 4

Value 5

Value n

...


One-dimensional ArraysSome programs need to store many values

Example: read many values, display in reverse orderTedious to create a variable for each value

Value 1

Value 2

Value 3

Output

Program

Value 4

Value 5

Value n

...

Variable 1

Variable 2

Variable 3

Variable 4

Variable 5

Variable n

...


One-dimensional ArraysAlternative: use an array

Define one array variableStore many values in that single variableAccess the values in any order in the array

Value 1

Value 2

Value 3

Output

Program

Array variableValue 4

Value 5

Value n

...


One-dimensional ArraysDifference between simple and array variables

Simple variable holds a single value

Array variable holds several values

intNum(integer variable)

437

intNum[0]

437

intNum[1]

12

intNum[2]

94


Arrays: DefiningTo define an array variable

int[] intNum;

Must then create an array objectintNum = new int[3];

Creates an array object with 3 integers

Can do in one stepint[] intNum = new int[3];

intNum[0]

0

intNum[1]

0

intNum[2]

0


Arrays: DefiningAfter defining the array, access elements by

subscriptint[] intNum = new int[3];

intNum[0] = 437;intNum[2] = intNum[0] - 343;intNum[1] = (intNum[2] - 10) / 7;

intNum[0]

437

intNum[1]

12

intNum[2]

94


Arrays: DefiningExample program

import Keyboard;

public class ThreeIntegers{ static final int MAX_NUMBERS = 3;

public static void main(String[] args) throws java.io.IOException { // Define the array variable and allocate space for an // array object int[] intNum = new int[MAX_NUMBERS];

// Read in three numbers intNum[0] = Keyboard.readInt("First number: "); intNum[1] = Keyboard.readInt("Second number: "); intNum[2] = Keyboard.readInt("Third number: ");

// Display the array System.out.println(); System.out.println("The numbers are:"); for (int i = 0; i < intNum.length; i++) { System.out.println(intNum[i]); } }}

First number: 425Second number: 39Third number: -125

The numbers are:42539-125

intNum.length is size of array


Arrays: DefiningExample: Store rainfall data

Array must store 18 floating-point valuesdouble[] rainFall = new double[18];

Better approach -- use a constantstatic final int NUM_YEARS = 18;...double[] rainFall = new double[NUM_YEARS];

Can now assign values to positions in arrayrainFall[0] = 30.55;rainFall[1] = 23.94;rainFall[2] = 18.32;...

Year Rainfall Year Rainfall Year Rainfall

1982 30.55 1988 25.67 1994 28.76

1983 23.94 1989 29.45 1995 27.47

1984 18.32 1990 31.14 1996 25.43

1985 32.28 1991 23.52 1997 26.64

1986 27.87 1992 32.29 1998 29.37

1987 26.58 1993 21.23 1999 28.56


Arrays: DefiningArray of String objects

Define arraystatic final int NUM_NAMES = 5;

String[] name = new String[NUM_NAMES];

Assign valuesname[0] = "Smith";name[1] = "Jones";name[2] = "Miller";name[3] = "Lui";name[4] = "Gonzales";

Display valuesfor (int nameNumber = 0; nameNumber < name.length; nameNumber++){ System.out.println(nameNumber + ": " + name[nameNumber]);}

Output0: Smith1: Jones2: Miller3: Lui4: Gonzales


Arrays: DefiningArray of Money objects

Define arrayMoney[] quarterlySales = new Money[4];

Assign valuesquarterlySales[0] = new Money(400, 00); // first quarterquarterlySales[1] = new Money(450, 00); // second quarterquarterlySales[2] = new Money(375, 00); // third quarterquarterlySales[3] = new Money(425, 00); // fourth quarter

Display valuesfor (int quarter = 0; quarter < 4; quarter++){ System.out.println("Sales for quarter " + (quarter + 1) + ": " + quarterlySales[quarter]);}

OutputSales for quarter 1: 400.00Sales for quarter 2: 450.00Sales for quarter 3: 375.00Sales for quarter 4: 425.00


Arrays: DefiningInitial value of numeric array is zeros

static final int NUM_YEARS = 18;...double[] rainFall = new double[NUM_YEARS];

for (int year = 0; year < rainFall.length; year++){ System.out.print(rainFall[year] + " ");}System.out.println();

Output0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

Initial value of object array (String, Money, ...) is nullstatic final int NUM_NAMES = 5;

String[] name = new String[NUM_NAMES];

for (int nameNumber = 0; nameNumber < name.length; nameNumber++){ System.out.print(name[nameNumber] + " ");}System.out.println();

Outputnull null null null null


Arrays: UsingArray variable + subscript = single element

static final int NUM_YEARS = 18;...double[] rainFall = new double[NUM_YEARS];double amount;

amount = 123.45;rainFall[0] = 30.55;rainFall[1] = 23.94;...

Treat rainFall[0] just like any double variableUsage Simple Variable Subscripted Array Variable

Assignment amount = 10.6; rainFall[3] = 10.6;

Input amount = Keyboard.readDouble();

rainFall[3] = Keyboard.readDouble();

Display System.out.println(amount); System.out.println(rainFall[3]);

Expression x = amount / 2; x = rainFall[3] / 2;

Comparison if (amount > 5.0) ... if (rainFall[3] > 5.0) ...

Pass to method check(amount); check(rainFall[3]);


Arrays: UsingExample: Compute average rainfall for all 18 years

Set up the rainFall arraydouble[] rainFall = new double[18];

rainFall[0] = 30.55;rainFall[1] = 23.94;...

Sum all the amounts - first (long!) approachtotal = rainFall[0] + rainFall[1] + rainFall[2] + rainFall[3] + rainFall[4] + rainFall[5] + rainFall[6] + rainFall[7] + rainFall[8] + rainFall[9] + rainFall[10] + rainFall[11] + rainFall[12] + rainFall[13] + rainFall[14] + rainFall[15] + rainFall[16] + rainFall[17];

Sum all the amounts - second (better) approachdouble total = 0.0;for (int year = 0; year < rainFall.length; year++){ total = total + rainFall[year];}

Display the averageSystem.out.println("Average rainfall is " + (total / rainFall.length));


Arrays: UsingAccess outside the array

double[] rainFall = new double[18];

rainFall[19] = -5;

Run-time errorjava.lang.ArrayIndexOutOfBoundsException: 19 at Test.main(Test.java)

Make sure array access is within bounds

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 ??


Arrays: UsingDisplaying values of array

Must display individual valuesfor (int year = 0; year < rainFall.length; year++){ System.out.println(rainFall[year]);}

Reading values into an arrayMust read individual values

for (int year = 0; year < rainFall.length; year++){ rainFall[year] = Keyboard.readDouble("Enter rainfall for year " + year + ": ");}


Arrays: InitializingCan initialize values of array in definition

double[] rainFall = {30.55, 23.94, 18.32, 32.28, 27.87, 26.58, 25.67, 29.45, 31.14, 23.52, 32.29, 21.23, 28.76, 27.47, 25.43, 26.64, 29.37, 28.56};

Don't need to specify array size, or use new operatorInitialize array of objects

StringsString[] name = {"Smith", "Jones", "Miller"};

Other objectsMoney quarterlySales[4] = { new Money(400, 00), // first quarter new Money(450, 00), // second quarter new Money(375, 00), // third quarter new Money(425, 00) }; // fourth quarter


Arrays: Copying with Assignment// Copies one array to another using the// assignment operator

public class TestArrayAssignment{ public static void main(String[] args) { // Make an array int[] originalArray = { 7, 4, 5, 2 };

// Define another int[] aCopy;

// Copy using assignment aCopy = originalArray;

// Change value in original array originalArray[0] = 999;

// Display values in both arrays System.out.println(originalArray[0] + " " + aCopy[0]); }}

999 999

originalArray

null

aCopy

7

4

5

2

originalArray aCopy

7

4

5

2

Afterassignment

Beforeassignment


Arrays: CopyingTwo ways to make a copy of an array

Use for statement to copy individual elements// Make an arrayint[] originalArray = { 7, 4, 5, 2 };

// Define anotherint[] aCopy = new int[originalArray.length];

// Copy individual elementsfor (int element = 0; element < originalArray.length; element++){ aCopy[element] = originalArray[element];}

Use System.arraycopy() method// Make an arrayint[] originalArray = { 7, 4, 5, 2 };

// Define anotherint[] aCopy = new int[originalArray.length];

// Copy using arrayCopy()System.arraycopy(originalArray, // Source array 0, // Source array position aCopy, // Target array 0, // Target array position originalArray.length); // Number of elements

COPYCOPY


Arrays: Copying with arraycopy()// Copies one array to another using // System.arraycopy() public class TestArrayCopy{ public static void main(String[] args) { // Make an array int[] originalArray = { 7, 4, 5, 2 };

// Declare another and allocate space int[] aCopy = new int[originalArray.length];

// Copy using arrayCopy() System.arraycopy(originalArray, 0, aCopy, 0, originalArray.length);

// Change value in original array originalArray[0] = 999;

// Display value in copy System.out.println(originalArray[0] + " " + aCopy[0]); }}

999 7

originalArray aCopy

7

4

5

2

originalArray aCopy

7

4

5

2

Afterarraycopy()

Beforearraycopy()

7

4

5

2


Arrays: ParametersCan pass an array to a method

Array is an object, so it works like other object parameters

If method changes value in the array, will also change actual parameter

Method can store values in array for caller// Reads rain fall data from user into rainFall array parameterstatic void readRainFall(double[] rainFall)throws java.io.IOException{ for (int year = 0; year < rainFall.length; year++) { rainFall[year] = Keyboard.readDouble("Enter rainfall for year " + year + ": "); }}


Arrays: ParametersMethod can return array object

Useful when method allocates the array object// Reads rain fall data from user into rainFall array and// returns the arraystatic double[] readRainFall()throws java.io.IOException{ // Find out how large to make the array int numYears = Keyboard.readInt( "How many years of rainfall data? ");

// Allocate the array double[] rainFall = new double[numYears];

// Read data from user for (int year = 0; year < rainFall.length; year++) { rainFall[year] = Keyboard.readDouble("Enter rainfall for year " + year + ": "); }

// Return the array return rainFall;}


Arrays: ParametersWhat's wrong with this?

// Reads rain fall data from user into rainFall WRONG!!// array parameter (allocates space)static void readRainFall(double[] rainFall)throws java.io.IOException{ // Find out how large to make the array int numYears = Keyboard.readInt( "How many years of rainfall data? ");

// Allocate the array rainFall = new double[numYears];

// Read data from user for (int year = 0; year < rainFall.length; year++) { rainFall[year] = Keyboard.readDouble("Enter rainfall for year " + year + ": "); }}


Example: 1-column SpreadsheetUse of the program

--- One-column SpreadSheet ---

(D)isplay (E)nter (Q)uit: ePosition: 3Data: 59.5


(D)isplay (E)nter (Q)uit: d3 59.55 18.0Total 77.5


(D)isplay (E)nter (Q)uit: d3 18.65 18.0Total 36.6

(D)isplay (E)nter (Q)uit: q


Example: 1-column SpreadsheetCode

// This program stores a column of numbers. We can change// any of the numbers, and total them up. Then we can change the// numbers some more. User commands are://// D Display all nonzero entries in the column and the total// E Enter a new value for one of the entries// Q Quit the program

import Keyboard;

public class SpreadSheet{ static final int NUM_ENTRIES = 20;

public static void main(String[] args) throws java.io.IOException { double[] column = new double[NUM_ENTRIES]; char selection;

System.out.println("--- One-column SpreadSheet ---"); System.out.println();


Example: 1-column SpreadsheetCode (continued)

do { selection = Keyboard.readChar( "(D)isplay (E)nter (Q)uit: ", "deq"); switch (Character.toLowerCase(selection)) { case 'd': // Display the spreadsheet with total double total = 0.0; for (int loc = 0; loc < column.length; loc++) { if (column[loc] != 0.0) { System.out.println(loc + "\t" + column[loc]); total = total + column[loc]; } } System.out.println("Total\t" + total); break; case 'e': // Let user enter a new value in the spreadsheet int entry = Keyboard.readInt("Position: ", 0, column.length - 1); column[entry] = Keyboard.readDouble("Data: "); break;


Example: 1-column SpreadsheetCode (continued)

case 'q': // Do nothing, but could confirm whether to quit break;

default: System.out.println("Switch statement error"); break; } System.out.println(); } while (selection != 'q'); }}


Example: Letter CountingProgram to

count letter frequency in a text file

Program asks for input and output files

Input file name: frogs.txtOutput file name: frogs.out

Letter Frequency a 15 b 5 c 3 d 7 e 25 f 7 g 7 h 12 i 12 j 0 k 1 l 7 m 5 n 11 o 19 p 1 q 1 r 16 s 12 t 15 u 5 v 2 w 6 x 0 y 3 z 0

Letter FrequencyProgram

Output File: frogs.out

FROGSFrogs are amphibians, and arefound all over the world. Somerequire water, while others areat home on land. Frogs eatinsects and other food byshooting out their long, stickytongue. They have webbed feetwhich are suitable for swimming.

Input File: frogs.txt


Example: Letter CountingUse array of 26 integers to store count of each letter

Problem: How to map letter (character) to position in array (integer)?Program reads letter 'B' in fileTranslate 'B' into 1 Increment array at position 1

0 1 2 3 4 23 24 25

Count

of A's

Count

of B's

Count

of C's

Count

of D's

Count

of E's

Count

of X's

Count

of Y's

Count

of Z's

...


Example: Letter CountingTwo ways to map letters to subscripts

Use switch statement (long, tedious, error-prone)switch (inputChar){ case 'A': index = 0; break; case 'B': index = 1; break; ...}

Use static method!// letterToInt: Returns 0 for 'A' or 'a', 1 for 'B'// or 'b', etc.static int letterToInt(char letter){ return ((int) Character.toLowerCase(letter)) - (int) 'a';}

'B' 'B' 1 1


Example: Letter CountingExample: 'B' 1

Convert to lower case 'b'Character.toLowerCase('B')

Convert to Unicode 98(int) 'b'

Subtract 'a'98 - (int) 'a'= 98 - 97

Result is 1

Code Char Code Char Code Char Code Char Code Char

32 (space) 51 3 70 F 89 Y 108 l

33 ! 52 4 71 G 90 Z 109 m

34 " 53 5 72 H 91 [ 110 n

35 # 54 6 73 I 92 \ 111 o

36 $ 55 7 74 J 93 ] 112 p

37 % 56 8 75 K 94 ^ 113 q

38 & 57 9 76 L 95 _ 114 r

39 ' 58 : 77 M 96 ` 115 s

40 ( 59 ; 78 N 97 a 116 t

41 ) 60 < 79 O 98 b 117 u

42 * 61 = 80 P 99 c 118 v

43 + 62 > 81 Q 100 d 119 w

44 , 63 ? 82 R 101 e 120 x

45 - 64 @ 83 S 102 f 121 y

46 . 65 A 84 T 103 g 122 z

47 / 66 B 85 U 104 h 123 {

48 0 67 C 86 V 105 i 124 |

49 1 68 D 87 W 106 j 125 }

50 2 69 E 88 X 107 k 126 ~


Example: Letter CountingTo access array by letter

// letterToInt: Returns 0 for 'A' or 'a', 1 for 'B'// or 'b', etc.static int letterToInt(char letter){ return ((int) Character.toLowerCase(letter)) - (int) 'a';}

...

frequency[letterToInt('B')]++;

0 1 2 3 4 23 24 25

Count

of A's

Count

of B's

Count

of C's

Count

of D's

Count

of E's

Count

of X's

Count

of Y's

Count

of Z's

...


Example: Letter CountingSteps for letter-counting

program Ask user for name of text

document; open the file Ask user for name of output

file; and open it Read in characters from input

file. For each letter, increment corresponding location in frequency array

Write frequency array to output file as a table

Close input and output files

Divide into 4 static methods main(): Initializes streams,

controls other methods letterToInt(): Converts 'A' or

'a' to 0, 'B' or 'b' to 1, etc. countLetters(): Counts

frequency of each letter in input stream, returns frequencies in an array

writeFrequency()Displays letter frequencies on output stream


Example: Letter CountingStructure chart

countLetters writeFrequency

main

letterToInt


Example: Letter CountingMethod main()

public static void main(String[] args) throws java.io.IOException { int[] frequency; String inputFileName, outputFileName;

// Open the input file inputFileName = Keyboard.readString("Input file: "); outputFileName = Keyboard.readString("Output file: "); // Check the input file File inputFile = new File(inputFileName); if (inputFile.exists() && inputFile.canRead()) { // Check the output file File outputFile = new File(outputFileName); if (!outputFile.exists() || outputFile.canWrite()) { // Initialize the input and output streams BufferedReader input = new BufferedReader(new FileReader(inputFile)); PrintWriter output = new PrintWriter(new BufferedWriter( new FileWriter(outputFile)));


Example: Letter CountingMethod main() (continued)

// Count the letter frequencies from the input file frequency = countLetters(input);

// Write the frequencies to the output file writeFrequency(output, frequency);

// Close the files input.close(); output.close(); } else { System.out.println("Can't write to " + outputFileName); } } else { System.out.println("Can't read from " + inputFileName); } }


Example: Letter CountingMethod countLetters()

// countLetters: Returns count of the number of times each// letter appears in the input file. All// letters are converted to upper case. All// non-letters are ignored. The input stream// should be initialized.static int[] countLetters(BufferedReader input)throws java.io.IOException{ char inputChar; int[] frequency = new int[LETTERS_IN_ALPHABET]; int inputCharAsInt;

// Read the file; count letter frequencies while ((inputCharAsInt = input.read()) != -1) { inputChar = (char) inputCharAsInt; if (Character.isUpperCase(inputChar) || Character.isLowerCase(inputChar)) { frequency[letterToInt(inputChar)]++; } }

return frequency;}


Example: Letter CountingMethod writeFrequency()

// writeFrequency: Writes the frequency array to the// output file as a table. The output// stream should be initialized.static void writeFrequency(PrintWriter output, int[] frequency){ output.println("Letter Frequency"); for (char letter = 'a'; letter <= 'z'; letter++) { output.println(" " + letter + " " + frequency[letterToInt(letter)]); }}


Example: Statistical AnalysisProgram to find average, high, and low of a series

of numbersWill read list of numbers from a fileFile format:

3415.12590.329009.1

Store the numbers in an arrayProgram will make the array just large enough to hold the

valuesOnce loaded, can go through array to find statistics

number of data values

data values


Example: Statistical AnalysisSample run of program

--- Data Analysis Program ---

Input file name: data.dat

(A)verage (H)igh (L)ow (D)isplay (Q)uit: aAverage: 5.64444

(A)verage (H)igh (L)ow (D)isplay (Q)uit: dStart position: 0End position: 80 3.41 8.12 5.63 2.14 9.35 7.46 5.37 6.48 3.2

(A)verage (H)igh (L)ow (D)isplay (Q)uit: hHigh value: 9.3

(A)verage (H)igh (L)ow (D)isplay (Q)uit: q


Example: Statistical AnalysisStructure chart

main

readDataFromFile

findAverage findMin findMax

doAnalysis

displayData

data

datam

inim

um

data

average datamaxim

um

data

data


Example: Statistical AnalysisMethod main()

public static void main(String[] args) throws java.io.IOException, java.text.ParseException { System.out.println("--- Data Analysis Program ---"); System.out.println();

// Get file name from user, initialize File object String fileName = Keyboard.readString("Input file: "); File inputFile = new File(fileName);

// Check input file if (inputFile.exists() && inputFile.canRead()) { BufferedReader input = new BufferedReader(new FileReader(inputFile)); double[] data = readDataFromFile(input); char selection;

do { System.out.println(); selection = Character.toLowerCase(Keyboard.readChar( "(A)verage (H)igh (L)ow (D)isplay (Q)uit: ", "ahldq"));


Example: Statistical AnalysisMethod main() (continued)

switch (selection) { case 'a': System.out.println("Average: " + findAverage(data)); break; case 'h': System.out.println("High value: " + findMax(data)); break; case 'l': System.out.println("Low value: " + findMin(data)); break; case 'd': displayData(data); break; case 'q': // do nothing break; default: System.out.println("Switch statement error"); break; } } while (selection != 'q'); } }}


Example: Statistical AnalysisTechnique for loading

an array Starting at one end, put

each value in next available position

Use a variable to keep track of next available position

Empty array

0 1 2 3 4 5 6 7

Array is full

'W''V''Y''D'

0 1 2 3 4 5 6 7

Array has data

'T''S''M''L''W''V''Y''D'

0 1 2 3 4 5 6 7


Example: Statistical AnalysisMethod readDataFromFile()

// readDataFromFile: Reads data from the file into the data // array static double[] readDataFromFile (BufferedReader input) throws java.io.IOException, java.text.ParseException { // Get a NumberFormat object NumberFormat formatter = NumberFormat.getInstance();

// Read number of values from stream int numValues = formatter.parse(input.readLine()).intValue();

// Define the array of the correct size double[] data = new double[numValues];

// Read data from stream into array for (int i = 0; i < numValues; i++) { data[i] = formatter.parse(input.readLine()).doubleValue(); }

return data; }


Example: Statistical AnalysisMethod findAverage()

// findAverage: Returns the mean (average) of all numbers in // the array, or zero if the array is empty. static double findAverage(double[] data) { double total = 0.0;

if (data.length == 0) { return 0.0; }

for (int i = 0; i < data.length; i++) { total = total + data[i]; } return total / data.length; }


Example: Statistical AnalysisHow to find the minimum value in an array

Scan the array, remembering the smallest value we've seen so far

At the beginning of the scan, the smallest we've seen so far is the first element

At the end of the scan, we'll know the smallest

intNum[0]

437

intNum[1]

12

intNum[2]

94


Example: Statistical AnalysisMethod findMin()

// findMin: Returns the minimum value of all numbers in the // array, or zero if the array is empty. static double findMin(double[] data) { double min;

if (data.length == 0) { return 0.0; }

min = data[0];

// Find min, where min is the minimum value in data[k] // for all k such that 0 £ k < n for (int i = 1; i < data.length; i++) { if (data[i] < min) { min = data[i]; } }

return min; }


Example: Statistical AnalysisMethod displayData()

// displayData: Lists the data in the array between locations // entered by the user. static void displayData(double[] data) throws java.io.IOException { int start = Keyboard.readInt("Start position: ", 0, data.length - 1); int finish = Keyboard.readInt("End position: ", start, data.length - 1);

for (int i = start; i <= finish; i++) { System.out.println(i + "\t" + data[i]); } }


Changing an Array's SizeCan't change array's size directlyIndirect way to do it:

Allocate a new array of the desired sizeCopy the elements from the old array to the new oneAssign the new array to the old array's variable

The standard Java class Vector (java.util.Vector) works this wayWorks like an arrayGrows automatically when it runs out of room


Changing an Array's SizeExample

// Allocate an array of 5 doublesdouble[] anArray = new double[5];

Let's put some data in the array.// Put 5 double values into the arrayanArray[0] = 1.2;anArray[1] = 3.2;anArray[2] = 6.5;anArray[3] = 4.1;anArray[4] = 8.3; // FULL!

Allocate a new array of 10 elements// Allocate an array of 10 doublesdouble[] newArray = new double[10];

Copy elements// Copy elements from the original array to the new oneSystem.arraycopy(anArray,0, newArray, 0, anArray.length);

Assign new array object to old array variable// Assign new array object to anArrayanArray = newArray;

Easy to write a method to do this (try it!)


Two-dimensional ArraysOne-dimensional

array has rowsTwo-dimensional

array has rowsand columns 0 1 2 3 4 5

0 1 2 3 4 5

0

1

2

3

One-dimensional array

Two-dimensional array


Two-dimensional Arrays: DefiningDefining a two-dimensional array

Very similar toone-dimensional

ExampleStore status of

traffic lights

N

S

EW

5th Ave

4th Ave

2nd Ave

1st Ave

1st S

t

2nd

St

3rd

St

5th

St

6th

St

7th

St

8th

St

3rd Ave

4th

St

1/10 mile

Megaville

final static int NUM_STREETS = 8;final static int NUM_AVENUES = 5;

final static int LIGHT_RED = 1;final static int LIGHT_GREEN = 2;final static int LIGHT_YELLOW = 3;final static int LIGHT_BROKEN = 4;

int[][] signal = new int[NUM_AVENUES][NUM_STREETS];


Two-dimensional Arrays: DefiningConceptual view of two-dimensional array

int[][] signal = new int[NUM_AVENUES][NUM_STREETS];

0 1 2 3 4 5

0

1

2

3

6 7

4

Columns

Row

s


Two-dimensional Arrays: UsingUsing a two-dimensional array

Access location with 2 subscripts: row# and column#

signal[1][4] = LIGHT_RED;

0 1 2 3 4 5

1

0

1

2

3

6 7

4

Columns

Row

s


Two-dimensional Arrays: UsingExamining all rows in a single column

How many traffic signals on Fourth St are green?int numberGreenLights = 0;for (int avenue = 0; avenue < NUM_AVENUES; avenue++){ if (signal[avenue][3] == LIGHT_GREEN) { numberGreenLights++; }}System.out.println("Green lights on Fourth Street: " + numberGreenLights);

0 1 2 3 4 5

0

1

2

3

6 7

4

Columns

Row

s


Two-dimensional Arrays: UsingExamining all columns in a single row

Which traffic signals on Second Ave are broken?for (int street = 0; street < NUM_STREETS; street++){

if (signal[1][street] == LIGHT_BROKEN) { System.out.println("Broken light: Second Ave and " + street + " Street"); }}

0 1 2 3 4 5

0

1

2

3

6 7

4

Columns

Row

s


Two-dimensional Arrays: AccessTo go through the entire array row by row

Hold the row number fixed at 0. Go through all the columns in row 0.

Hold the row number fixed at 1. Go through all the columns in row 1.

Continue for all other rows in the array.To go through the entire array column by column

Hold the column number fixed at 0. Go through all the rows in column 0.

Hold the column number fixed at 1. Go through all the rows in column 1.

Continue for all other columns in the array.


Two-dimensional Arrays: AccessCount all broken lights in city (process by rows)

int numberBrokenLights = 0;for (int avenue = 0; avenue < NUM_AVENUES; avenue++){ for (int street = 0; street < NUM_STREETS; street++) { if (signal[avenue][street] == LIGHT_BROKEN) { numberBrokenLights++; } }}System.out.println("Total broken lights: " + numberBrokenLights);

0 1 2 3 4 5

0

1

2

3

6 7

4

Columns

Row

s


Two-dimensional Arrays: AccessCount all broken lights in city (process by columns)

int numberBrokenLights = 0;for (int street = 0; street < NUM_STREETS; street++){ for (int avenue = 0; avenue < NUM_AVENUES; avenue++) { if (signal[avenue][street] == LIGHT_BROKEN) { numberBrokenLights++; } }}System.out.println("Total broken lights: " + numberBrokenLights);

0 1 2 3 4 5

0

1

2

3

6 7

4

Columns

Row

s

Reverse these


Two-dimensional Arrays: InitializeCan initialize two-dimensional array in definition

Example - define array to hold this

Syntaxdouble[][] x = { {12.2, 43.1, 95.4}, {59.3, 28.4, 64.3} };

Put each row within its own bracesThe number of columns can vary from row to row

Compiler defines each row with the given elements

Column 0 Column 1 Column 2

Row 1 12.2 43.1 95.3

Row 2 59.3 28.4 64.3


Two-dimensional Arrays: ParametersCan pass two-dimensional array to method

// countBrokenLights: Returns the number of brokenlights in// Megavillestatic int countBrokenLights(int[][] signal){ int numberBrokenLights = 0; for (int avenue = 0; avenue < signal.length; avenue++) { for (int street = 0; street < signal[avenue].length; street++) { if (signal[avenue][street] == LIGHT_BROKEN) { numberBrokenLights++; } } } return numberBrokenLights;}

Arrays are objects - so passed as an objectChanges made to array contents are persistentChanges made to array object are not persistent


Two-dimensional Arrays: SpreadsheetTwo-dimensional array example: spreadsheetFeatures

14 rows and 4 columns for numbersUser may

enter row and column headings

Program will compute total for each column after every change

January(0) February(1) March(2) April(3) Sales( 0) 112000 115100 117600 121300 Other inc( 1) 5000 5500 4500 3800 ( 2) 0 0 0 0 Materials( 3) -45000 -46500 -48000 -51000 Labor( 4) -48000 -48000 -48000 -48000 Administ( 5) -4000 -4000 -4000 -4000 Utilities( 6) -3500 -4200 -4400 -5100 Depreciat( 7) -2100 -2100 -2100 -2100 Mainten( 8) -3500 -3500 -3500 -3500 Other exp( 9) -2500 -2350 -4700 -2840 Taxes(10) -4200 -4200 -4200 -4200 (11) 0 0 0 0 (12) 0 0 0 0 (13) 0 0 0 0 TOTALS 4200 5750 3200 4360

+--- SpreadSheet ---| N) Number R) Row title C) Column title| H) Help D) Display Q) Quit| Enter selection:


Two-dimensional Arrays: SpreadsheetThree arrays needed

Row titles (1-dimensional)Column titles (1-dimensional)Rows and columns of

numbers (2-dimensional) 0 1 2 3

0123456789

1011121314

columnTitle

cell

0 1 2 3

0

0

0123456789

1011121314

rowTitle


Two-dimensional Arrays: Spreadsheet// This is a simple spreadsheet program. It has row// titles, column titles, and a 14 row by 4 column space for// numbers. When the user displays the spread sheet, totals are// computed for each column. User commands are selected by typing// their first letter. They are://// (N)umber - Enter a number into the spreadsheet// (R)ow title - Enter a row title// (C)olumn title - Enter a column title// (H)elp - Display help for commands// (Q)uit - Quit the program

import Keyboard;import TextMenu;import Format;

public class SpreadSheet extends TextMenu{ // Array size constants final static int MAX_ROWS = 14; final static int MAX_COLS = 4;

// Event codes final static int NUMBER_CMD = 1; final static int ROW_TITLE_CMD = 2; final static int COLUMN_TITLE_CMD = 3; final static int HELP_CMD = 4; final static int DISPLAY_CMD = 5;


Two-dimensional Arrays: SpreadsheetConstructor

// Constructor public SpreadSheet() { // Set menu title super("SpreadSheet");

// Initialize menu choices addSelection("Number", 'N', NUMBER_CMD); addSelection("Row title", 'R', ROW_TITLE_CMD); addSelection("Column title", 'C', COLUMN_TITLE_CMD); addSelection("Help", 'H', HELP_CMD); addSelection("Display", 'D', DISPLAY_CMD); addSelection("Quit", 'Q', TextMenu.QUIT);

// Initialize the spreadsheet clearSpreadSheet(); }


Two-dimensional Arrays: SpreadsheethandleEvent()

// handleEvent: If the given event code is defined, invokes // the action associated with that given event // code. Otherwise, does nothing public int handleEvent(int event) throws java.io.IOException { switch (event) { case NUMBER_CMD: enterNumber(); break; case ROW_TITLE_CMD: enterRowTitle(); break; case COLUMN_TITLE_CMD: enterColTitle(); break; case HELP_CMD: displayHelpInfo(); break; case DISPLAY_CMD: displaySpreadSheet(); break; } return event; }


Two-dimensional Arrays: SpreadsheetclearSpreadSheet()

// clearSpreadSheet: Puts zeros in all cells, sets all titles // to empty private void clearSpreadSheet() { int row, col;

// Clear row titles for (row = 0; row < MAX_ROWS; row++) { rowTitle[row] = ""; }

// Clear column titles for (col = 0; col < MAX_COLS; col++) { colTitle[col] = ""; }

// Clear cells for (row = 0; row < MAX_ROWS; row++) { for (col = 0; col < MAX_COLS; col++) { cell[row][col] = 0.0; } } }


Two-dimensional Arrays: SpreadsheetUser data entry

// enterNumber: Cell contains number user entered. private void enterNumber() throws java.io.IOException { int row = Keyboard.readInt("Row number: ", 0, MAX_ROWS - 1); int col = Keyboard.readInt("Column number: ", 0, MAX_COLS - 1); cell[row][col] = Keyboard.readDouble("Cell value: "); }

// enterRowTitle: Row title contains title user entered private void enterRowTitle() throws java.io.IOException { int row = Keyboard.readInt("Row number: ", 0, MAX_ROWS - 1); rowTitle[row] = Keyboard.readString("Row title: "); }

// enterColTitle: Column title contains title user entered private void enterColTitle() throws java.io.IOException { int col = Keyboard.readInt("Column number: ", 0, MAX_COLS - 1); colTitle[col] = Keyboard.readString("Column title: "); }


Two-dimensional Arrays: SpreadsheetwriteColumnTitles()

// writeColumnTitles: Displays the column titles across the top // of the spreadsheet private void writeColumnTitles() { System.out.print(Format.pad("", 14));

for (int col = 0; col < MAX_COLS; col++) { System.out.print(Format.padRight(colTitle[col], 11) + "(" + col + ")"); } System.out.println(); }


Two-dimensional Arrays: SpreadsheetdisplayColumnTotals()

// displayColumnTotals: Displays column totals across the bottom // of the spreadsheet private void displayColumnTotals() { double total;

System.out.print(Format.padRight("TOTALS", 14));

for (int col = 0; col < MAX_COLS; col++) { total = 0.0; for (int row = 0; row < MAX_ROWS; row++) { total = total + cell[row][col]; } System.out.print(Format.pad(total, 14, 2)); } System.out.println(); }


Two-dimensional Arrays: SpreadsheetdisplaySpreadSheet()

// displaySpreadSheet: Displays the spreadsheet, with row and // column titles, and column totals private void displaySpreadSheet() { System.out.println(); writeColumnTitles();

for (int row = 0; row < MAX_ROWS; row++) { System.out.print(Format.pad(rowTitle[row], 10) + "(" + Format.pad(row, 2) + ")"); for (int col = 0; col < MAX_COLS; col++) { System.out.print(Format.pad(cell[row][col], 14, 2)); } System.out.println(); } displayColumnTotals(); }


Two-dimensional Arrays: SpreadsheetdisplayHelpInfo()

// displayHelpInfo: Displays a brief description of all the // commands void displayHelpInfo() { System.out.println(" (N)umber - Enter a number"); System.out.println(" (R)ow title - Enter a row title"); System.out.println(" (C)olumn title - Enter a column title"); System.out.println(" (H)elp - Display help for " + "commands"); System.out.println(" (Q)uit - Quit the program"); }

// Instance variables private double[][] cell = new double [MAX_ROWS][MAX_COLS]; private String[] rowTitle = new String[MAX_ROWS]; private String[] colTitle = new String[MAX_COLS];

public static void main(String[] args) throws java.io.IOException { (new SpreadSheet()).run(); }}


Computer GraphicsComputers more graphical today than ever

Graphical user interfaces (GUI)Multimedia applications

2D graphics are simplest (and quite common)Problems with two dimensions

ClippingReal-world vs. screen coordinatesViewportsHidden lines

Problems with three dimensionsLightColorSurface contour


Computer GraphicsWill examine 2D graphicsWill use Java Frame object

Coordinates are x,y values (column, row)Coordinate 0,0 is at top left cornerCoordinate of bottom right corner depends on resolution

Draw with drawLine() method in java.awt.Graphics “Draws a line between the coordinates (x1,y1) and

(x2,y2) using the current colorParameters

x1 - the x coordinate of the start of the line y1 - the y coordinate of the start of the line x2 - the x coordinate of the end of the line y2 - the y coordinate of the end of the line” (JDK help)


Computer GraphicsCan store a shape as

a list of vertices (30, 0), (90, 0), (90, 80),

(120, 80), (60, 120), (0, 80), (30, 80), (30, 0)

Closed shape: first andlast vertices are same

Store vertices in 2-Darray

int[][] arrow = { {30, 0}, {90, 0}, {90, 80}, {120, 80}, {60, 120}, {0, 80}, {30, 80}, {30, 0}, {-1, -1} };

-1, -1 marks the end

00 30,0 90,0

90,80120,80

60,120

0,8030,80


Computer GraphicsProgram to draw an arrow

// Draws an arrow on a frame.

import java.awt.*;

public class DrawArrow extends Frame{ int[][] arrow = { {30, 0}, {90, 0}, {90, 80}, {120, 80}, {60, 120}, {0, 80}, {30, 80}, {30, 0}, {-1, -1} };

public DrawArrow() { super("Draw Arrow"); setSize(200, 200); show(); }

public void paint(Graphics g) { drawShape(g, arrow); }


Computer GraphicsProgram to draw an arrow (cont’d)

// drawShape: Draws the shape line by line // Note: -1, -1 is the end marker, and there must be at least // one x, y coordinate before the end marker. private void drawShape(Graphics g, int[][] shape) { // Move to the first position int currentX = shape[0][0]; int currentY = shape[0][1];

for (int i = 1; shape[i][0] != -1; i++) { g.drawLine(currentX, currentY, shape[i][0], shape[i][1]); currentX = shape[i][0]; currentY = shape[i][1]; } } public static void main(String[] args) { new DrawArrow(); }}


Computer Graphics: ManipulationShape manipulations

Translate (move)Scale (change size)Rotate

Translating a shapeTo move r horizontally: add r to each x-valueTo move s vertically, add s to each y-value

00

r

s


Computer Graphics: ManipulationScaling a shape

Relative to origin (0, 0)Multiply x values by

horizontal scaling factorMultiply y values by

vertical scaling factorIf shape not at origin

Translate to originScale shapeTranslate back

00

1.5

1.25


Computer Graphics: ManipulationRotating a shape degrees

is degrees in radiansConverting between degrees and radians

Compute new verticesnewX = x cos - y sin new Y = y cos + x sin

Assumes shape at origin

00

20°= 0.35 radians

dr

360

2

rd

2

360


Computer Graphics: MatricesScaling and rotation require shape to be at origin

Doing separate rotation, scaling, and translation (2 times) steps is inefficient

Better to do all calculations once: use matricesMatrix

2D array of numbersExamples

1 2

3 4181 5 2 4

0 6

0 25

7

. .

.

.


Computer Graphics: MatricesMatrix multiplication A x B

# columns in A == # rows in BResult has # rows in A, # columns in B

Example

7 56 41 2

3 2 4 17 1 5 3

3 x 2 2 x 4

=

Must match

Size of result

56 19 53 2246 16 44 1817 4 14 7

3 x 4

C A B A B A Bi j i j i j i n n j, , , , , , ,... 1 1 2 2


Computer Graphics: MatricesTranslation matrix

r units horizontal s units vertical

Scaling matrixhorizontal factor f vertical factor g

Rotation matrixangle radians

newX newY x y

f

g1 1

0 0

0 0

0 0 1

newX newY x y

r s

1 1

1 0 0

0 1 0

1

newX newY x y1 1

0

0

0 0 1

cos sin

sin cos


Computer Graphics: Demo ProgramDemonstration of transformation, scaling, rotation


Computer Graphics: Demo Program// Demonstration of some simple graphics routines - SLOW APPROACH.

import java.awt.*;

public class Draw2Arrows extends Frame{ int[][] arrow = { {30, 0}, {90, 0}, {90, 80}, {120, 80}, {60,120}, {0, 80}, {30, 80}, {30, 0}, {-1, -1} }; int[][] bigArrow;

public Draw2Arrows() { super("Draw 2 Arrows"); setSize(500, 400); // Copy arrow to bigArrow bigArrow = new int[arrow.length][2]; for (int row = 0; row < arrow.length; row++) { for (int col = 0; col < 2; col++) { bigArrow[row][col] = arrow[row][col]; } } // Translate bigArrow scale(bigArrow, 2, 3); rotate(bigArrow, 1); // 1 radian is about 57 degrees translate(bigArrow, 350, 20); show(); }


Computer Graphics: Demo Program public void paint(Graphics g) { drawShape(g, arrow); drawShape(g, bigArrow); } // drawShape: Draws the shape line by line // Note: -1, -1 is the end marker, and there must be at least // one x, y coordinate before the end marker. private void drawShape(Graphics g, int[][] shape) { // Move to the first position int currentX = shape[0][0]; int currentY = shape[0][1];

for (int i = 1; shape[i][0] != -1; i++) { g.drawLine(currentX, currentY, shape[i][0], shape[i][1]); currentX = shape[i][0]; currentY = shape[i][1]; } }


Computer Graphics: Demo Program // matrixMult: Multiplies an array of vertices by a 3x3 matrix, // giving a new array of vertices private void matrixMult(double[] a, double[][] b, double[] result) { for (int col = 0; col < 3; col++) { result[col] = 0.0; for (int k = 0; k < 3; k++) { result[col] = result[col] + a[k] * b[k][col]; } } }


Computer Graphics: Demo Program // transformShape: Transforms the shape by the given transform // matrix private void transformShape(int[][] shape, double[][] transformMatrix) { double[] oldPosition = { 0, 0, 1 }; double[] newPosition = { 0, 0, 1 };

for (int i = 0; shape[i][0] != -1; i++) { oldPosition[0] = shape[i][0]; oldPosition[1] = shape[i][1]; matrixMult(oldPosition, transformMatrix, newPosition); shape[i][0] = (int) Math.round(newPosition[0]); shape[i][1] = (int) Math.round(newPosition[1]); } }

// scale: Scales the shape by the given factors private void scale(int[][] shape, double xFactor, double yFactor) { double[][] scaleMat = { { xFactor, 0, 0 }, { 0, yFactor, 0 }, { 0, 0, 1 } }; transformShape(shape, scaleMat); }


Computer Graphics: Demo Program // rotate: Rotates the shape counterclockwise by the given // radians private void rotate(int[][] shape, double radians) { double[][] rotateMat = { { Math.cos(radians), Math.sin(radians), 0 }, { -Math.sin(radians), Math.cos(radians), 0 }, { 0, 0, 1 } }; transformShape(shape, rotateMat); }

// translate: Moves shape to the right and up by the given // amounts private void translate(int[][] shape, int moveX, int moveY) { double[][] translateMat = { { 1, 0, 0 }, { 0, 1, 0 }, { moveX, moveY, 1 } }; transformShape(shape, translateMat); }

public static void main(String[] args) { new Draw2Arrows(); }}


Computer Graphics: Demo ProgramSlow approach: program does this for each vertex

Scale by 2 horizontally, 5 vertically

Rotate by 1 radian

Translate by 500 horizontally, 20 vertically

newX newY x y1 1

2 0 0

0 5 0

0 0 1

newX newY x y1 1

1 1 0

1 1 0

0 0 1

cos( ) sin( )

sin( ) cos( )

newX newY x y1 1

1 0 0

0 1 0

500 20 1

Documents

Chapter 8 Arrays