COMP102: 181. Menu. More while loops. Admin: Test. Peter Andreae

Transcription

1 Menu COMP102: 181 More while loops Admin: Test

2 Designing loops with numbers COMP102: 182 When the number of steps is known at the beginning of the loop: int count = 0; int num = 1; while ( count < number) { OR while ( num <= number) { ádo actions ádo actions count = count + 1; num = num + 1; Can count from 0 or from 1 If counting from 0, loop while count is less than target: (count is the number of iterations that have been completed) If counting from 1, loop while num is less than or equal to target: (num is the iteration it is about to do)

3 Designing nested loops with numbers COMP102: 183 2D structures, eg table of rows and columns: Can do rows in the outside loop and columns in the inside loop, or vice versa int row = 0; int col = 0; while ( row < numberofrows ) { while ( col < numberofcols ) { int col = 0; int row = 0; while ( col < numberofcols ) { while ( row < numberofrows ) { ádo actions for row, col ádo actions for row, col col++; row++; row++; col++

4 General while loops COMP102: 184 /** Practice times-tables until got 5 answers correct */ public void playarithmeticgame (){ int score = 0; while ( score < 5) { // ask an arithmetic question int a = this.randominteger(10); int b = this.randominteger(10); int ans = UI.askInteger("What is " + a + " times " + b + "?" ); if ( ans == a * b ) { score = score +1; UI.println("You got 5 right answers" ); public int randominteger(int max) { return (int) (Math.random() * max ) + 1;

5 General while loops COMP102: 185 /** Ask a multiplication problem until got it right */ public void practicearithmetic (){ int a = this.randominteger(10); int b = this.randominteger(10); String question = "What is " + a + " times " + b + "?"; boolean correct = false; while (! correct) { int ans = UI.askInteger(question); if ( ans == a * b ) { correct = true; UI.println("You got it right!" ); This seems unnecessarily complex!! public int randominteger(int max) { return (int) (Math.random() * max ) + 1;

6 Loops with the test "in the middle" COMP102: 186 If the condition for exiting the loop depends on the actions, need to exit in the middle! Common with loops asking for user input. break allows you to exit a loop (while, or for) Must be inside a loop Ignores any if 's Does not exit the method ( return does that ) while ( true ) { actions to set up for the test if ( exit-test ) { break; additional actions

7 General while loops with break COMP102: 187 /** Ask a multiplication problem until got it right */ public void practicearithmetic (){ int a = this.randominteger(10); int b = this.randominteger(10); String question = "What is " + a + " times " + b + "?"; boolean correct = false; while (! true correct) { int ans = UI.askInteger(question); if ( ans == a * b ) { correct break; = true; Setting up for test Test and break no additional actions UI.println("You got it right!" ); Only use break when the exit is not at the beginning of the loop.

8 More loops with user input COMP102: 188 Make user guess a magic word: public void playguessinggame(string magicword){ UI.println("Guess the magic word:"); while (true) { String guess = UI.askString("your guess: "); if ( guess.equalsignorecase(magicword) ) { UI.println("You guessed it!"); break; Setting up for test Test and break UI.println("No, that wasn't right. Try again!"); Additional actions

9 Testing your program COMP102: 189 A) Need to try out your program on sample input while removing the "easy" bugs. Can be a pain if need lots of input (eg TemperatureAnalyser) UI window has a menu item "set input" to get input from a text file instead of user typing it. Þ don't have to type lots of data each time Create the text file, eg in Notepad Select file using menu before the program has started asking for input. File can contain multiple sequences of data. B) Need to test your program on a range of inputs Easy, "ordinary", inputs Boundary cases values that are only just in range, or just out of range Need to check that your if conditions are right Invalid data does your program handle invalid input correctly? Creating test cases involves creativity have to try to come up with ways to break your program.

10 Text Input: reading multiple values COMP102: 190 If there is no input yet, the UI.next () methods will just wait. Always print a prompt to the user before you try to read! It is not safe to call UI.nextInt() or UI.nextDouble() unless you can be certain the next token is an integer / double! How can you tell? UI.hasNextInt() à boolean true if next token is an integer UI.hasNextDouble() à boolean true if next token is a double UI.hasNext() à boolean true if there is a next token (always true for text pane)

11 next vs. nextline() COMP102: 191 next(), nextint(), nextdouble() picks up any spaces, discards them, picks up characters to make next token (until it reaches a space), returns the token next() returns it as a String nextint() returns it as an int, nextdouble() returns it as a double. nextline() Picks up all the characters (including spaces) until it reaches end-of-line character, throws away end-of-line, and returns all the characters (including spaces) as a String.

12 Input with "next" methods Method What it does Returns next() Read and return next token of user s input String hasnext() nextint() nextdouble() hasnextint() hasnextdouble() nextboolean() nextline() Returns true if there is another token in the user input. Waits for the user to type something if necessary. Read the next token of the user's input. Return it as a integer if it is a number. Throws an exception if it is not a number. Returns true if next token in the input is an int / double. Waits for user to type something if necessary. Read the next token of the user's input. Return true if it is "yes", "y", or "true", return false if it is "no", "n", or "false" (case insensitive). Throws an exception if it is anything else. Read the remaining characters of the user's input up to (but not including) the next end-of-line and return them as a string. Reads and throws away the end-of-line character. If there are no characters on the line, then it returns an empty string (""). boolean int double boolean boolean String COMP102: 192

13 Using next methods COMP102: 193 /** sum up all numbers entered by user */ : UI.print( Enter numbers: end with done : ); double sum = 0; while (UI.hasNextDouble() ) { //peeking at next value or token double amt = UI.nextDouble(); //getting the next value and move pointer sum = sum + amt; UI.nextLine(); // throw away the done UI.printf( Total of all numbers entered: %.2f %n, sum); * MENU * Enter numbers: end with done : done Total of all numbers entered:

14 Menu COMP102: 194 Files Admin assignments you are important to people!

15 Files COMP102: 195 The UI text pane window is transient: Typing large amounts of input into the text pane is a pain! It would be nice to be able to save the output of the program easily. Large amounts of text belong in files How can your program read from a file and write to a file? Writing to files is like writing to the UI text pane! Use print, println, printf methods But, need extra objects: Reading from files is a bit different Doesn't use "ask " methods File and PrintStream objects Need to use "next " methods And need extra objects: File and Scanner objects

16 Text with the text pane COMP102: 196 UI Window red: 40 green: 60 blue: 30 all done UI.askInteger(); UI.println(); My Program : int r =UI.askInteger("red"); int g =UI.askInteger("green"); int b =UI.askInteger("blue"); UI.setColor(new Color(r,g,b); : UI.println("all done");

17 Text with Files COMP102: 197 Needs several objects: Need File object to talk to the actual file on the disk. Need Scanner/PrintStream object to talk to the File object Program talks to the Scanner or PrintStream object. A real file: myfile.txt My Program : int r =scan.nextint(); int g =scan.nextint(); int b =scan.nextint(); UI.setColor(new Color(r,g,b); : outfile.println("all done"); nextint(); Scanner object println(); PrintStream object File object

18 Using a Scanner COMP102: 198 Scanner: a class in Java that allows a program to read input from a file (or any other source of characters such as a String, a socket, ) File: a class in Java that connects to an actual file on disk and get characters in and out of the file Program needs to make a File object an get the next token, or the next line String filename = "My File.txt"; File infile = new File(fileName); Scanner scan = new Scanner(inFile); int r = scan.nextint(); My File.txt

19 Scanner COMP102: 199 A Scanner breaks up the source into a sequence of chunks that the program can get, one at a time. lines, (separated by the end-of-line characters) tokens (separated by spaces, tabs, or end-of-line's) Program can read the next token (or the next line) Scanner scan = new Scanner ( new File("My File.txt") ); while ( scan.hasnext() ){ double radius = scan.nextdouble(); UI.drawOval(X-radius, Y-radius, radius*2, radius*2); My File.txt

20 Scanner "next" methods COMP102: 200 Method What it does Returns next() Read and return next token String nextint() nextdouble() nextboolean() Read the next token. Return it as a number, if it is a number. Throws an exception if it is not a number. Read the next token. Return true if it is "true"; return false if it is "false". Throws an exception if it is anything else. int double boolean hasnext() Returns true if there is another token boolean hasnextint() hasnextdouble() hasnextboolean() nextline() close() Returns true if there is another token AND the next token is an int / double / Boolean Read characters up to the next end-of-line and return them as a string. Reads and throws away the end-of-line character. If the first character is an end-of-line, then it returns an empty string (""). close the file boolean String

21 Scanner methods. COMP102: 201 Scanner has a cursor that keeps track of where it is up to in the file. ACCY308 Lecture Tue GBLT2 ACCY308 Lecture Fri GBLT3 ACCY308 Lecture Tue GBLT3 ACCY330 Lecture Fri RHLT2 ACCY330 Lecture Wed RHLT2 ACCY401 Comp-Lab Mon RWW402 ACCY401 Lecture Mon RWW220 ACCY402 Lecture Wed RWW311 ACCY412 Lecture Wed RWW311 ACCY421 Lecture Thu RWW311 ALIN201 Lecture Mon KK204 ALIN201 Lecture Wed KK204 ALIN201 Tutorial Wed AM102 ALIN301 Lecture Tue KK105 ALIN301 Lecture Thu KK105 ALIN301 Tutorial Thu MY103 ANTH101 Lecture Mon KKLT303 ANTH101 Lecture Tue KKLT303

22 Reading lines using Scanner: COMP102: 202 /** Read lines from a file and print them to UI text pane. */ public void readfile(){ File myfile = new File( input.txt ); Scanner scan = new Scanner(myfile); UI.println( input.txt ); while (scan.hasnext() == true){ String line= scan.nextline(); UI.println(line); UI.println( end of input.txt ); Missing bits to handle exceptions!! Almost right, but compiler complains!!! Dealing with files may raise exceptions

23 Files: handling exceptions COMP102: 203 If a piece of code might raise an exception: Have to enclose it in a try { catch (IOException e) { what to do what to do if it goes wrong public void readfile(){ File myfile = new File( input.txt ); try { Scanner scan = new Scanner(myfile); while (scan.hasnext()){ String line = scan.nextline(); UI.println(line); UI.println( end of input.txt ); catch (IOException e) { UI.println( File failure: + e);

24 Reading from files: example COMP102: 204 /** Finds oldest person in file of ages and names. */ public void printoldest(string filename){ try { Scanner scan = new Scanner(new File(filename)); String oldest = ""; int maxage = 0; while (scan.hasnext()){ int age = scan.nextint(); Read a token, String name = scan.nextline(); then read rest of line if (age > maxage) { maxage = age; oldest = name; UI.printf( Oldest is %s (%d)%n, oldest, maxage); catch (IOException e) { UI.println("File failure: " + e); 66 Marie Curie 48 James Clerk Maxwell 84 Isaac Newton 62 Aristotle

25 Reading data from a file COMP102: 205 public void drawshapes(string filename){ try { Scanner scan = new Scanner( new File(fileName) ); Stop at end of file while ( scan.hasnext() ){ double left = scan.nextdouble(); double top = scan.nextdouble(); String shape = scan.next(); int r = scan.nextint(); int g = scan.nextint(); int b = scan.nextint(); UI.setColor( new Color (r, g, b) ); if (shape.equals("oval") ){ UI.fillOval(left, top, WIDTH, HEIGHT); else { UI.fillRect(left, top, WIDTH, HEIGHT); catch (IOException e) { UI.println( File failure: + e); Oval Rect Oval Reading all the values on the line Do something with all the values