CS 170 Java Programming 1. Week 10: Loops and Arrays

Transcription

1 CS 170 Java Programming 1 Week 10: Loops and Arrays

2 What s the Plan? Topic 1: A Little Review Use a counted loop to create graphical objects Write programs that use events and animation Topic 2: Advanced Loop Concepts More on indefinite, sentinel and flag-controlled loops How to use Java's jump statements to control loop flow How to use iterators to read and write files Topic 3: Array Basics Create primitive arrays and process them with methods

3 Topic 1 Events, Loops and Animation Review

4 Your "IC" or "Lab" Document Use Word or OpenOffice to create a new document Save the file as IC10.doc (Office compatible) Place on your network U: drive or on a thumb drive Put your name and today's date at the top of the sheet Title it "CS 170 Lab Exercises Week 10" Start DrJava and open CardDemo.java in week10 We'll use this to review a few concepts

5 Week 9 Review Here are some things to note in the starter code The program extends the ACM GraphicsProgram A cards folder containing images already added Instance variable for card names already defined Each card represented by rank and suit (ac) Find cards by their position in the String Instance variable for Random generator defined Instance variables for table and animation variables init() and run() methods already defined 5

6 Review: Loop and Substrings Let's start with a loop that processes the instance variable deck and creates GImage objects for each name, and then adds the new objects to the center of the screen. How many repetitions? (each name is 2 characters long) How do we get the name of the card? How do we create the full card name? How do we add the card to the center of the canvas Exercise 10.1 Once you've added all the cards, compile, run and shoot a picture of your init() method.

7 Review: GObjects and Mouse Events Exercise 10.2 listen for mouse events, and, when a button is clicked, remove the object under mouse cursor if the object is a GImage object How do you listen for mouse events in an ACM program? How do you respond to a mouse click? How do you find the position of the mouse click? How do you find out if the object is agimage? How do you remove an object? When you remove a card, then add 1 to points Shoot a screenshot and paste it into your IC document

8 Animation Review Appearance of motion by changing the shape or position of objects as a fixed, but rapid rate: similar to movies A basic animation skeleton inside the run() method final int FRAME_RATE = 1000 / 30; do { advanceoneframe(); // update pause(frame_rate); } while (whatever???);

9 Animation Review Exercise 10.3 add basic animation skeleton to app Use FRAME_RATE of 1000 / 30; do while getelementcount() > 1 Add advanceoneframe() method Inside advanceoneframe(), move card by dx, dy Top item: getelement(getelementcount()-1); If it moves off table, subtract 1 from points, remove card, and re-initialize dx, dy Show score after loop ends 9

10 Review: Events, Starting and Stopping Create two new JButtons: startbtn and stopbtn Create an instance variable, running = false Inside actionperformed() If start button is clicked, set running true If stop button is clicked, set running false Inside run() Only advance frame if running Exercise 10.4 run and snap a picture

11 Topic 2 More Loop Concepts

12 Loop Concepts this Week Loops involving indefinite conditions Usually using the while or do-while loop syntax Sentinel loops look for a particular value in input Flag-controlled loops set a boolean variable EOF or data loops use iterators to process data Limit loops measure the results of a calculation Jump statements "fine-tune" loop actions Iterators: objects that can be used to control a loop

13 Sentinel Loops Exercise 10-5 Let's look at one common way to write a sentinel loop. Open CurrencyConverter program. Basic structure already in place. Add a loop that asks the user for a dollar amount, and converts that to Euro values. Stop when user enters Q The value "Q" is the sentinel that controls the loop This version is called priming read since we read the value one time before the loop "starts pumping"

14 Flag-Controlled Loop Another way to write a sentinel loop is to use a Boolean flag in conjunction with an if statement. Exercise 10.6 Convert program to flag-controlled loop Add a boolean variable done before loop Change condition to while (! done) Prompt and read at start of loop If input is "Q", then set done to true Otherwise convert and process Notice that this requires only one read statement

15 Jump Statements Jumps are "low-level" flow of control statements In contrast to "high-level" statements like if and loops A jump statement is sometimes called a goto Although goto is a reserved word in Java, its not used Java does have two "controlled" jump statements break : used to exit a switch or a loop (but not an if) continue : used to prematurely "restart" a loop Java also has "labeled" versions of each of these Used to exit or restart a particular nested loop

16 The break Statement break jumps out of a switch or loop (but not an if) In a loop, break jumps to the first statement following the loop body as shown here. Can make your code clearer when used to construct a loop-and-a-half

17 A Loop-and-a-Half Exercise 10.7 Convert program to loop and a half Remove boolean variable done before loop Change condition to while (true) Prompt and read at start of loop If input is "Q", then break Can remove else block for convert and process Snap a picture Notice code is shorter and a little easier to read May want to read Eric Roberts article on why this is so

18 Iterators and Scanners An iterator is an object that allows you to loop over a collection of values. You use iterators with while loops Iterators can be used to implement data loops Keep reading until we are out of information The Scanner class is actually an iterator Let's use it to read the text in Alice in Wonderland Create a new console program named AskAlice Add throws IOException to the main() header Add import java.io.* as well

19 Iterators and Files Instead of reading from the console (System.in), we want to read from a text file. Do that with the File class. File text = new File("alice.txt"); Scanner alice = new Scanner(text); How do we know when to stop? Ask alice! while (alice.hasnext()) { // read from the scanner here }

20 Your Turn to Ask Alice You can read from a file using any of the Scanner methods that you already know how to use nextline() reads an entire line next() reads one word Exercise 10.8 Read the file and print on the console each line that starts with "CHAPTER" the total number of lines in the file at the end Shoot me a screen-shot

21 Writing to Files You can create text files using the PrintWriter class PrintWriter pw = new PrintWriter("alice.stats"); Must have permission to write in directory Automatically creates the file and opens it Use same methods you used with System.out Use pw.close() when you are done writing. Exercise 10.9 instead of printing to the screen, have AskAlice save the data in a text file. Open the file to make sure your program works and shoot a screen shot.

22 The continue Statement The continue statement only works inside loops Instead of leaving the loop, it starts the next iteration while and do-while continue jumps to the loop test expression In for, it jumps to the update expression

23 Continue at Work Exercise Open AdvancedLoopProblems Find sumdigits() problem. Similar to homework. How do you sum the digits in a String? Look at each character If it is not a digit, then try the next character If it is a digit, convert to a number and add to sum Step 1: Write the function/loop skeletons Step 2: Use Character.isDigit() to see if digit Step 3: Convert using character arithmetic

24 Topic 3 Introducing Arrays

25 What Are Arrays? Think of an array like a collection variable Holds multiple values instead of a single value Just like a String holds multiple characters An array can hold values of any type Can store both objects and primitive values Each object or value must be of the same type

26 Array Elements The entire collection shares a single name Individual objects (values) are called elements Elements are accessed by means of their position You'll use a subscript or index to specify position First element starts at 0, next is 1 and so on Elements are contiguous in memory

27 Array Variables Array variables store references (like object variables) Variables refer to the actual values

28 Creating Array Variables To declare an array variable you specify: The type of elements you ll store in the array This can be any type, object or primitive A name for the entire collection Following standard naming rules A set of empty brackets These following either the name or the type "Java style" puts brackets after type Associates brackets with type rather than with variable

29 Declaration Examples Here are some examples char[ ] grades; Creates an array variable that refers to a group of char In this example, the brackets appears after the type int pieces[ ]; Creates an array variable that refers to a group of int Note brackets can appear after the variable name You really shouldn't use this style of declaration To store data you have to create the actual array

30 Instantiating Array Elements The actual array (the elements) are created with new This is called instantiating or allocating the array Use name of class followed by brackets Put the number of array elements inside brackets Number can be a constant, expression or variable grades = new char[5]; // literal // This uses an expression pieces = new int[ntiles * ntiles];

31 Array Initialization It is common to define the array variable and to create the array all in one statement like this: double[] scores = new double[5]; When you create an array like this, the values are set to: 0 if the array holds numeric or char values false if the array holds boolean values null if the array holds object references (like String)

32 Initializer Lists An initializer list is a set of values, separated by commas, enclosed in braces char[] grades = {'A', 'B', 'C', 'D', 'F'}; When using an initializer list, you: do not provide a size for the array Java counts the number of elements do not use the new operator

33 Accessing Array Elements To access an individual array element you use the element's subscripted name The array name along with a bracketed index value Store (or retrieve) a value in an array like this: grades[0] = 'A'; // storing a value char second = grades[1]; // retrieving a value int pos = 2; char third = grades[pos]; // a variable index Subscript can be constant or variable

34 Storage Errors Two kinds of errors you can make when storing or retrieving values in an array: Store the wrong kind of thing in an element grades[0] = 3.2; This is a compile-time error You can't store a double in a char array element

35 Index Errors Use an invalid subscript to access an element grades[5] = 'C'; grades has only 5 elements The subscripts go from 0 to 4, not 0 to 5 Creates a runtime error

36 Array Length Every array has a public field, length, that stores the number of elements in the array When using the length field, remember : Strings have a length() method, arrays have a field String s = "Abara"; int lens = s.length(); int lena = grades.length; The last element in any array is at index a.length-1

37 Finishing Up To complete your IC "lab" document: Complete Exercises (GuessOMatic) in the More Indefinite Loops lesson, Exercise in the Java Jumps lesson, Exercise in the Iterators and Limits lesson, Exercises in the Primitive Arrays lesson and Exercises in the Arrays and Methods lesson Submit to Blackboard when done Reading for this week: Gaddis: Finish Chapter 3, first part of Chapter 5

38 Homework and Quiz Homework, Quiz and Assignments (Noon, Monday) Homework Problems 5 Advanced Loop Problems (1 point each) 5 Array Method Problems (1 points each) 1 Iterator problem Upload to Assignment Dropbox. Make sure to press Submit Proficiency Quiz 7: More String Loops, Array Methods Practice on homework assignments You will have ½ hour, assigned seating