Programming and Problem Solving With Java Copyright 1999, James M. Slack Chapter 8 Arrays One-dimensional Arrays Two-dimensional Arrays Computer Graphics

Programming and Problem Solving With Java 2 One-dimensional Arrays  So far, most of our programs input few values  Program stores these values in a few variables  Program does some calculation, then displays output Value 1 Value 2 Value 3 Output Program Variable 1 Variable 2 Variable 3

Programming and Problem Solving With Java 3 One-dimensional Arrays  Program may read lots of values  But reads and processes one at a time Value 1 Value 2 Value 3 Output Program Variable 1 Value 4 Value 5 Value n...

Programming and Problem Solving With Java 4 One-dimensional Arrays  Some programs need to store many values  Example: read many values, display in reverse order  Tedious 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...

Programming and Problem Solving With Java 5 One-dimensional Arrays  Alternative: use an array  Define one array variable  Store many values in that single variable  Access the values in any order in the array Value 1 Value 2 Value 3 Output Program Array variable Value 4 Value 5 Value n...

Programming and Problem Solving With Java 6 One-dimensional Arrays  Difference between simple and array variables  Simple variable holds a single value  Array variable holds several values

Programming and Problem Solving With Java 7 Arrays: Defining  To define an array variable int[] intNum;  Must then create an array object intNum = new int[3];  Creates an array object with 3 integers  Can do in one step int[] intNum = new int[3];

Programming and Problem Solving With Java 8 Arrays: Defining  After defining the array, access elements by subscript int[] intNum = new int[3]; intNum[0] = 437; intNum[2] = intNum[0] - 343; intNum[1] = (intNum[2] - 10) / 7;

Programming and Problem Solving With Java 9 Arrays: Defining  Example 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: 425 Second number: 39 Third number: -125 The numbers are: intNum.length is size of array

Programming and Problem Solving With Java 10 Arrays: Defining  Example: Store rainfall data  Array must store 18 floating-point values double[] rainFall = new double[18];  Better approach -- use a constant static final int NUM_YEARS = 18;... double[] rainFall = new double[NUM_YEARS];  Can now assign values to positions in array rainFall[0] = 30.55; rainFall[1] = 23.94; rainFall[2] = 18.32;...

Programming and Problem Solving With Java 11 Arrays: Defining  Array of String objects  Define array static final int NUM_NAMES = 5; String[] name = new String[NUM_NAMES];  Assign values name[0] = "Smith"; name[1] = "Jones"; name[2] = "Miller"; name[3] = "Lui"; name[4] = "Gonzales";  Display values for (int nameNumber = 0; nameNumber < name.length; nameNumber++) { System.out.println(nameNumber + ": " + name[nameNumber]); }  Output 0: Smith 1: Jones 2: Miller 3: Lui 4: Gonzales

Programming and Problem Solving With Java 12 Arrays: Defining  Array of Money objects  Define array Money[] quarterlySales = new Money[4];  Assign values quarterlySales[0] = new Money(400, 00); // first quarter quarterlySales[1] = new Money(450, 00); // second quarter quarterlySales[2] = new Money(375, 00); // third quarter quarterlySales[3] = new Money(425, 00); // fourth quarter  Display values for (int quarter = 0; quarter < 4; quarter++) { System.out.println("Sales for quarter " + (quarter + 1) + ": " + quarterlySales[quarter]); }  Output Sales for quarter 1: Sales for quarter 2: Sales for quarter 3: Sales for quarter 4:

Programming and Problem Solving With Java 13 Arrays: Defining  Initial 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();  Output  Initial value of object array (String, Money,...) is null static 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();  Output null null null null null

Programming and Problem Solving With Java 14 Arrays: Using  Array variable + subscript = single element static final int NUM_YEARS = 18;... double[] rainFall = new double[NUM_YEARS]; double amount; amount = ; rainFall[0] = 30.55; rainFall[1] = 23.94;...  Treat rainFall[0] just like any double variable

Programming and Problem Solving With Java 15 Arrays: Using  Example: Compute average rainfall for all 18 years  Set up the rainFall array double[] rainFall = new double[18]; rainFall[0] = 30.55; rainFall[1] = 23.94;...  Sum all the amounts - first (long!) approach total = 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) approach double total = 0.0; for (int year = 0; year < rainFall.length; year++) { total = total + rainFall[year]; }  Display the average System.out.println("Average rainfall is " + (total / rainFall.length));

Programming and Problem Solving With Java 16 Arrays: Using  Access outside the array double[] rainFall = new double[18]; rainFall[19] = -5;  Run-time error java.lang.ArrayIndexOutOfBoundsException: 19 at Test.main(Test.java)  Make sure array access is within bounds ??

Programming and Problem Solving With Java 17 Arrays: Using  Displaying values of array  Must display individual values for (int year = 0; year < rainFall.length; year++) { System.out.println(rainFall[year]); }  Reading values into an array  Must read individual values for (int year = 0; year < rainFall.length; year++) { rainFall[year] = Keyboard.readDouble("Enter rainfall for year " + year + ": "); }

Programming and Problem Solving With Java 18 Arrays: Initializing  Can 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 operator  Initialize array of objects  Strings String[] name = {"Smith", "Jones", "Miller"};  Other objects Money 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

Programming and Problem Solving With Java 19 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

Programming and Problem Solving With Java 20 Arrays: Copying  Two ways to make a copy of an array  Use for statement to copy individual elements // Make an array int[] originalArray = { 7, 4, 5, 2 }; // Define another int[] aCopy = new int[originalArray.length]; // Copy individual elements for (int element = 0; element < originalArray.length; element++) { aCopy[element] = originalArray[element]; }  Use System.arraycopy() method // Make an array int[] originalArray = { 7, 4, 5, 2 }; // Define another int[] 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 COPY

Programming and Problem Solving With Java 21 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

Programming and Problem Solving With Java 22 Arrays: Parameters  Can 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 parameter static 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 + ": "); }

Programming and Problem Solving With Java 23 Arrays: Parameters  Method can return array object  Useful when method allocates the array object // Reads rain fall data from user into rainFall array and // returns the array static 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; }

Programming and Problem Solving With Java 24 Arrays: Parameters  What'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 + ": "); }

Programming and Problem Solving With Java 25 Example: 1-column Spreadsheet  Use of the program --- One-column SpreadSheet --- (D)isplay (E)nter (Q)uit: e Position: 3 Data: 59.5 (D)isplay (E)nter (Q)uit: e Position: 5 Data: 18.0 (D)isplay (E)nter (Q)uit: d Total 77.5 (D)isplay (E)nter (Q)uit: e Position: 3 Data: 18.6 (D)isplay (E)nter (Q)uit: d Total 36.6 (D)isplay (E)nter (Q)uit: q

Programming and Problem Solving With Java 26 Example: 1-column Spreadsheet  Code // 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();

Programming and Problem Solving With Java 27 Example: 1-column Spreadsheet  Code (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;

Programming and Problem Solving With Java 28 Example: 1-column Spreadsheet  Code (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'); }

Programming and Problem Solving With Java 29 Example: Letter Counting  Program to count letter frequency in a text file  Program asks for input and output files Input file name: frogs.txt Output file name: frogs.out

Programming and Problem Solving With Java 30 Example: Letter Counting  Use 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 file  Translate 'B' into 1  Increment array at position 1

Programming and Problem Solving With Java 31 Example: Letter Counting  Two 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'; }

Programming and Problem Solving With Java 32 Example: Letter Counting  Example: 'B'  1  Convert to lower case 'b' Character.toLowerCase('B')  Convert to Unicode 98 (int) 'b'  Subtract 'a' 98 - (int) 'a' =  Result is 1

Programming and Problem Solving With Java 33 Example: Letter Counting  To 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')]++;

Programming and Problem Solving With Java 34 Example: Letter Counting  Steps 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

Programming and Problem Solving With Java 35 Example: Letter Counting  Structure chart

Programming and Problem Solving With Java 36 Example: Letter Counting  Method 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)));

Programming and Problem Solving With Java 37 Example: Letter Counting  Method 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); }

Programming and Problem Solving With Java 38 Example: Letter Counting  Method 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; }

Programming and Problem Solving With Java 39 Example: Letter Counting  Method 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)]); }

Programming and Problem Solving With Java 40 Example: Statistical Analysis  Program to find average, high, and low of a series of numbers  Will read list of numbers from a file  File format:  Store the numbers in an array  Program will make the array just large enough to hold the values  Once loaded, can go through array to find statistics number of data values data values

Programming and Problem Solving With Java 41 Example: Statistical Analysis  Sample run of program --- Data Analysis Program --- Input file name: data.dat (A)verage (H)igh (L)ow (D)isplay (Q)uit: a Average: (A)verage (H)igh (L)ow (D)isplay (Q)uit: d Start position: 0 End position: (A)verage (H)igh (L)ow (D)isplay (Q)uit: h High value: 9.3 (A)verage (H)igh (L)ow (D)isplay (Q)uit: q

Programming and Problem Solving With Java 42 Example: Statistical Analysis  Structure chart

Programming and Problem Solving With Java 43 Example: Statistical Analysis  Method 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"));

Programming and Problem Solving With Java 44 Example: Statistical Analysis  Method 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'); }

Programming and Problem Solving With Java 45 Example: Statistical Analysis  Technique 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

Programming and Problem Solving With Java 46 Example: Statistical Analysis  Method 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; }

Programming and Problem Solving With Java 47 Example: Statistical Analysis  Method 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; }

Programming and Problem Solving With Java 48 Example: Statistical Analysis  How 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

Programming and Problem Solving With Java 49 Example: Statistical Analysis  Method 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; }

Programming and Problem Solving With Java 50 Example: Statistical Analysis  Method 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]); }

Programming and Problem Solving With Java 51 Changing an Array's Size  Can't change array's size directly  Indirect way to do it:  Allocate a new array of the desired size  Copy the elements from the old array to the new one  Assign the new array to the old array's variable  The standard Java class Vector (java.util.Vector) works this way  Works like an array  Grows automatically when it runs out of room

Programming and Problem Solving With Java 52 Changing an Array's Size  Example // Allocate an array of 5 doubles double[] anArray = new double[5]; Let's put some data in the array. // Put 5 double values into the array anArray[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 doubles double[] newArray = new double[10];  Copy elements // Copy elements from the original array to the new one System.arraycopy(anArray,0, newArray, 0, anArray.length);  Assign new array object to old array variable // Assign new array object to anArray anArray = newArray;  Easy to write a method to do this (try it!)

Programming and Problem Solving With Java 53 Two-dimensional Arrays  One-dimensional array has rows  Two-dimensional array has rows and columns

Programming and Problem Solving With Java 54 Two-dimensional Arrays: Defining  Defining a two-dimensional array  Very similar to one-dimensional  Example  Store status of traffic lights 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];

Programming and Problem Solving With Java 55 Two-dimensional Arrays: Defining  Conceptual view of two-dimensional array int[][] signal = new int[NUM_AVENUES][NUM_STREETS];

Programming and Problem Solving With Java 56 Two-dimensional Arrays: Using  Using a two-dimensional array  Access location with 2 subscripts: row# and column# signal[1][4] = LIGHT_RED;

Programming and Problem Solving With Java 57 Two-dimensional Arrays: Using  Examining 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);

Programming and Problem Solving With Java 58 Two-dimensional Arrays: Using  Examining 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"); }

Programming and Problem Solving With Java 59 Two-dimensional Arrays: Access  To 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.

Programming and Problem Solving With Java 60 Two-dimensional Arrays: Access  Count 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);

Programming and Problem Solving With Java 61 Two-dimensional Arrays: Access  Count 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); Reverse these

Programming and Problem Solving With Java 62 Two-dimensional Arrays: Initialize  Can initialize two-dimensional array in definition  Example - define array to hold this  Syntax double[][] x = { {12.2, 43.1, 95.4}, {59.3, 28.4, 64.3} };  Put each row within its own braces  The number of columns can vary from row to row  Compiler defines each row with the given elements

Programming and Problem Solving With Java 63 Two-dimensional Arrays: Parameters  Can pass two-dimensional array to method // countBrokenLights: Returns the number of brokenlights in // Megaville static 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 object  Changes made to array contents are persistent  Changes made to array object are not persistent

Programming and Problem Solving With Java 64 Two-dimensional Arrays: Spreadsheet  Two-dimensional array example: spreadsheet  Features  14 rows and 4 columns for numbers  User may enter row and column headings  Program will compute total for each column after every change

Programming and Problem Solving With Java 65 Two-dimensional Arrays: Spreadsheet  Three arrays needed  Row titles (1-dimensional)  Column titles (1-dimensional)  Rows and columns of numbers (2-dimensional)

Programming and Problem Solving With Java 66 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;

Programming and Problem Solving With Java 67 Two-dimensional Arrays: Spreadsheet  Constructor // 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(); }

Programming and Problem Solving With Java 68 Two-dimensional Arrays: Spreadsheet  handleEvent() // 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; }

Programming and Problem Solving With Java 69 Two-dimensional Arrays: Spreadsheet  clearSpreadSheet() // 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; }

Programming and Problem Solving With Java 70 Two-dimensional Arrays: Spreadsheet  User 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: "); }

Programming and Problem Solving With Java 71 Two-dimensional Arrays: Spreadsheet  writeColumnTitles() // 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(); }

Programming and Problem Solving With Java 72 Two-dimensional Arrays: Spreadsheet  displayColumnTotals() // 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(); }

Programming and Problem Solving With Java 73 Two-dimensional Arrays: Spreadsheet  displaySpreadSheet() // 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(); }

Programming and Problem Solving With Java 74 Two-dimensional Arrays: Spreadsheet  displayHelpInfo() // 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(); }

Programming and Problem Solving With Java 75 Computer Graphics  Computers more graphical today than ever  Graphical user interfaces (GUI)  Multimedia applications  2D graphics are simplest (and quite common)  Problems with two dimensions  Clipping  Real-world vs. screen coordinates  Viewports  Hidden lines  Problems with three dimensions  Light  Color  Surface contour

Programming and Problem Solving With Java 76 Computer Graphics  Will examine 2D graphics  Will use Java Frame object  Coordinates are x,y values (column, row)  Coordinate 0,0 is at top left corner  Coordinate 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 color  Parameters 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)

Programming and Problem Solving With Java 77 Computer Graphics  Can 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 and last vertices are same  Store vertices in 2-D array 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

Programming and Problem Solving With Java 78 Computer Graphics  Program 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); }

Programming and Problem Solving With Java 79 Computer Graphics  Program 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(); }

Programming and Problem Solving With Java 80 Computer Graphics: Manipulation  Shape manipulations  Translate (move)  Scale (change size)  Rotate  Translating a shape  To move r horizontally: add r to each x-value  To move s vertically, add s to each y-value

Programming and Problem Solving With Java 81 Computer Graphics: Manipulation  Scaling a shape  Relative to origin (0, 0)  Multiply x values by horizontal scaling factor  Multiply y values by vertical scaling factor  If shape not at origin  Translate to origin  Scale shape  Translate back

Programming and Problem Solving With Java 82 Computer Graphics: Manipulation  Rotating a shape  degrees   is degrees in radians  Converting between degrees and radians  Compute new vertices  newX = x cos  - y sin   new Y = y cos  + x sin   Assumes shape at origin

Programming and Problem Solving With Java 83 Computer Graphics: Matrices  Scaling 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 matrices  Matrix  2D array of numbers  Examples

Programming and Problem Solving With Java 84 Computer Graphics: Matrices  Matrix multiplication A x B  # columns in A == # rows in B  Result has # rows in A, # columns in B  Example

Programming and Problem Solving With Java 85 Computer Graphics: Matrices  Translation matrix  r units horizontal  s units vertical  Scaling matrix  horizontal factor f  vertical factor g  Rotation matrix  angle  radians

Programming and Problem Solving With Java 86 Computer Graphics: Demo Program  Demonstration of transformation, scaling, rotation

Programming and Problem Solving With Java 87 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(); }

Programming and Problem Solving With Java 88 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]; }

Programming and Problem Solving With Java 89 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]; }

Programming and Problem Solving With Java 90 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); }

Programming and Problem Solving With Java 91 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(); }

Programming and Problem Solving With Java 92 Computer Graphics: Demo Program  Slow approach: program does this for each vertex  Scale by 2 horizontally, 5 vertically  Rotate by 1 radian  Translate by 500 horizontally, 20 vertically