CS 170 Java Programming 1. Week 9: Learning about Loops

Transcription

1 CS 170 Java Programming 1 Week 9: Learning about Loops

2 What s the Plan? Topic 1: A Little Review ACM GUI Apps, Buttons, Text and Events Topic 2: Learning about Loops Different kinds of loops Using loops to modify Picture objects Writing loop functions that work with String objects Topic 3: Introducing Animation Using loops to animate your GUI objects

3 Your "IC" or "Lab" Document Use Word or OpenOffice to create a new document Save the file as IC09.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 9" Start DrJava and we'll start with a little GUI review Open TurtleWorld.java in your week09 folder

4 Topic 1 (Review) ACM GUI Apps, Widgets and Events

5 ACM GraphicsProgram Review Notice how ACM GUI apps are different Three different import statements Inheritance: extends GraphicsProgram Two different methods you will override The acm.graphics package contains graphical shapes Exercise 9.1 create a GTurtle variable Create the variable outside of your methods Put the word private in front of it This is called an instance variable

6 Creating Graphical Objects Exercise 9.2 create a GTurtle object Put inside init() method (means initialization) Initialize GTurtle instance variable (NOT NEW VAR) Look up GTurtle docs on class Web site Add it to the center of your program's canvas Find out the width and height of canvas Use width / 2 half the width of the turtle Run and shoot a picture

7 Using Interactors (aka Widgets) The javax.swing package contains interactors JButton: push buttons JLabel: labels JTextField: text input areas Exercise 9.3 create three JButton objects Create them inside the init() method "Turn Left", "Turn Right", "Move" Add them to the SOUTH panel of your program Run and snap a screen-shot

8 Hooking Up Events Buttons don't do anything without an event handler Event handling is in the java.awt.event package For ACM GUI apps, "activate" all buttons in program addactionlisteners(); // bottom of init() Respond to Action events with this method public void actionperformed(actionevent e) { // Your code goes here }

9 Responding to Events First, you need to know which button was clicked ActionEvent variable has two methods e.getactioncommand(): String on button e.getsource(): reference to button Exercise 9.4 Activate your turtle Get the text on the button Use sequential if statements to decide what to do Call the methods in your GTurtle variable Run, move your turtle around and snap a picture

10 Other Event Listeners We can listen for mouse events as well In ACM programs, just add this to your init() method addmouselisteners(); // bottom of init() Respond to different mouse events with these handlers: mouseclicked(), mousemoved(), etc. All are void methods that take a MouseEvent parameter Exercise 9.5 hook up mouse events for your program When clicked, use setlocation() to move the turtle

11 Topic 2 Introducing Counted Loops

12 Iteration and Loops A CS term that means repeating a set of actions Also called repetition or looping Statements used in iteration are called loops Loops are flow-of-control statements similar to if Like the if statement, a loop evaluates a condition If the condition is true, a group of actions are performed If the condition is false, the actions are skipped

13 Iteration and Selection Iteration is not the same as selection, though A loop returns to its test after performing its actions If the test is still true, then the actions are performed again A selection statement only performs its actions once You cannot replace a loop with a selection statement

14 Java and Iteration Java has three loops: while, do-while, and for Each is designed to work best in a specific situation One difference is where the test is made Test at the bottom { } // // Loop Body Loop Test Test at the top Loop Test { } // // Loop Body

15 Loop Classification Not the most useful method of classification Doesn't give you any empirical guidance for designing Better: ask how each loop is controlled Two basic methods of loop control Simplest method relies on a counter Called counting or counted loops "Repeat these actions 10 times" More complex loops test the occurrence of a particular event, or combination of events. These are called indefinite or indeterminate loops

16 For Loops A counted loop is simplest kind of loop The loop is controlled by a or counter variable Also known as the loop index Use it to for "perform this action 10 times" Java's for loop specializes in this for (int idx = 0; idx < 10; idx++) { System.out.println("idx = " + idx); }

17 The for Loop Syntax The for loop has both a header and body (like if) Header has 3 expressions: initialize, test, and update

18 Your Turn Exercise 9.6 Try these in the Interactions pane Write a loop that prints 40 asterisks to the screen Write a method that prints n asterisks to the screen Call your method with 5 and 10 Write a loop that adds 5 numbers input by the user

19 Modifying Pictures Let's use some loops to modify some Picture objects Open your own (640 x 480) picture in the Interactions Pane String fname = FileChooser.pickAFile(); Picture p = new Picture(fname); p.explore(); Now, find out how large the Picture is and save the info int width = p.getwidth(); int height = p.getheight();

20 The Pixel class The Pixel class models individual pixels in your picture Each object of the Pixel class has An (x, y) position in a picture (x = horizontal, y = vertical) A red, green, and blue value (between 0 and 255) We can get a pixel at a specific location in a picture by using the getpixel() method of the Picture class Pixel pix1 = p.getpixel(0,0);

21 Pixel Methods Pixels "know" how to do certain things You can ask a Pixel for its color values, or change them int red = pix.getred(); pix.setred(red / 2); // half as much You can also ask a Pixel for the entire Color Color pc = pix.getcolor(); pix.setcolor(pc.darker()); You can ask a Pixel to tell you where it is located int x = pix.getx();

22 Your Turn Exercise 9.7 Let's continue modifying our Picture Start by drawing a green line vertically down the center import java.awt.* so we can use Color Write a loop that starts at 0 and goes until height Inside, grab a Pixel from your Picture, using width/2for x and the loop index for y Set the PixelColor to green Show the picture again When it works, draw a horizontal red line and snap a pic

23 Nested Loops Nested loops let us process all of the pixels in a picture Write one loop that processes all rows Inside that loop, write another loop that visits the columns for (int y = 0; y < height; y++) { for (int x = 0; x < width; x++) { Pixel pix = p.getpixel(x, y); Color col = pix.getcolor(); col = col.darker().darker(); pix.setcolor(col); } }

24 Your Turn Exercise 9.8 Let's use some nested loops Process the upper-left quadrant: make all pixels darker Process the lower-right quadrant: make all pixels brighter Process the upper-right quadrant: add vertical stripes Only process every other x position in your loop Process the lower-left quadrant: add a checkerboard If the row is even, process the odd pixels If the row is odd, process the even pixels Save as a new file name and paste into your IC doc

25 Just An Example

26 For Loop Functions

27 Step 1: The Skeleton Same steps for all function problems 1. Write the header 2. Create a result variable 3. Return the result 4. Run your tests

28 Step 2: Loop Through Strings Normally String problems require you to visit every character (or set of characters) in the String You can write a "skeleton" for this part too int len = str.length() for (int i = 0; i < len; i++) { // do your processing here }

29 Step 3: Processing Inside the loop, process characters or substrings Use the charat() or substring() method to extract int len = str.length(); for (int i = 0; i < len; i++) { // Use only one of these below char ch = str.charat(i); String sch = str.substring(i, i+1); }

30 Introducing the while Loop Counted loops with for are easy All of the pieces are in one place The initializer for your counter The test expression The update expression The simpler, general-purpose while loop can also be used to create counted loops It's just not quite as convenient as for

31 Counted while Loops To use the while statement to build a counted loop, you must do three things: 1. Create a counter variable, and initialize it before the loop is encountered. 2. Test the counter variable inside the while loop's boolean condition expression 3. Update your counter at the end of the loop body. This takes the place of the for loop's update expression.

32 A Counted while Skeleton Here is a skeleton for building counted while loops: int counter = 0; while (counter < 10) { // Loop body statements counter++; } Use this basic structure for many of exercises this week

33 While Loop Functions Exercise 9.10 mixstring in WhileLoopProblems Always start with the basic function skeleton Notice that these problems require a while loop Add basic while loop pattern (skeleton) to code Use shorter string for length of variable Add character from a to result, then from b After loop, see if index is < length Add remaining characters to end Shoot a screenshot of tests passing

34 Topic 3 Loops and Animation

35 Animation Appearance of motion by changing the shape or position of objects as a fixed, but rapid rate Provides the illusion of motion, similar to movies Similar (but different) techniques for applets and ACM Four steps: 1. Create objects to be animated as instance vars 2. Paint object at current location, shape, color 3. Call mutator methods to update state of object 4. Advance to next frame of animation

36 Frame-Rate and Skeleton Normally want to redisplay from frames per sec Create constant in terms of milliseconds inside run() final int FRAME_RATE = 1000 / 30; ACM animation: endless loop inside run() method public void run() { advanceoneframe(); // update objects pause(frame_rate); // wait for next }

37 Your Turn Let's animate our GTurtle so that it moves automatically It's up to you to keep it "on the table"; if it crawls off, it dies Add the advanceoneframe() method Inside, move your turtle forward 5 spaces Start by adding the skeleton animation loop to run() Add the constant FRAME_RATE Add a loop that continues while the turtle is visible Have to approximate using getx() and gety() When the loop ends, display a message box

38 Finishing Up To complete your IC "lab" document: Complete Exercise 9.11 in the For Loop lesson, Exercise 9.12 in the While Loop lesson, and Exercises 9.13 in the Loop Functions lesson, and Exercises in the Introducing Animation Lesson. Submit to Blackboard when done Reading for next week: Mediacomp: read Chapter 4 for modifying Pictures Gaddis: finish Chapter 3, start Chapter 5

39 Homework and Quiz Homework, Quiz and Assignments Deadline is Monday at noon 11 homework problems 5 for loop functions (similar to exam) 5 while loop functions (similar to exam) 1 animation, GUI program Upload to Assignment Dropbox, press Submit Proficiency Quiz 6: Simple for and while loop functions Practice on assignments You will have ½ hour, assigned seating

40 Additional Credits Some material adapted from instructor materials created by B. Ericson to accompany Introduction to Computing and Programming with Java: A Multimedia Approach by M. Guzdial and B. Ericson, released under a Creative Commons 3.0 Attribution License.