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CS 112 Introduction to Programming

CS 112 Introduction to Programming. Graphics; Animation Yang (Richard) Yang Computer Science Department Yale University 308A Watson, Phone: 432-6400 Email: yry@cs.yale.edu. Admin. Issues on PS3 NumberCoolness: printSpaces(n): prints n – 1 spaces, not n

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CS 112 Introduction to Programming

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  1. CS 112 Introduction to Programming Graphics; Animation Yang (Richard) Yang Computer Science Department Yale University 308A Watson, Phone: 432-6400 Email: yry@cs.yale.edu

  2. Admin • Issues on PS3 • NumberCoolness: • printSpaces(n): prints n – 1 spaces, not n • You can either change the method to print n or adapt your main • a walkthrough to be held by Fitz this evening from 8 to 9 pm • please suggest topics (e.g., nested loops, method parameters) that you want us to cover more

  3. Recap • Nested loops • If a loop is inside another loop, we say the two form a nested loop • The #loop times of the inner loop can depend on the outer loop variable • Method with parameters • Specify parameters to control the behavior of a method • Redundancy removal/abstraction through generalization

  4. Example: Nested Loop int N = 5; for (int line = 1; line <= N; line++) { for (int j = 1; j <= (-1 * line + N); j++) { System.out.print("."); } System.out.print(line); for (int j = 1; j <= (line - 1); j++) { System.out.print("."); } System.out.println(); } ....1 ...2. ..3.. .4... 5....

  5. Example: Rewrite public static void dots(int n) { for (inti = 1; i <= n; i++) { System.out.print("."); } } public static void main(String[] args) { int N = 5; for (int line = 1; line <= N; line++) { dots( -1 * line + N ); System.out.print(line); dots( line – 1 ); System.out.println(); } // end of outer for loop }

  6. Out-Class Read: Another Way to Motivate Method With Parameters

  7. Example: Reuse Matrix • Print two copies of inverse diagonal matrix ..1 .2. 3.. ....1 ...2. ..3.. .4... 5.... int N = 5; for (int line = 1; line <= N; line++) { // initial dots for (int j = 1; j <= (-1 * line + N); j++) { System.out.print("."); } // the number System.out.print(line); // the second set of dots for (int j = 1; j <= (line - 1); j++) { System.out.print("."); } System.out.println(); }

  8. Solution 1 • This code is redundant. public class DrawMatrix1 { public static void main(String[] args) { drawMatrix3(); drawMatrix5(); } public static void drawMatrix3() { int N = 3; for (int line = 1; line <= N; line++) { for (int j = 1; j <= (-1 * line + N); j++) { // initial dots System.out.print("."); } System.out.print(line); // the number for (int j = 1; j <= (line - 1); j++) { // the second set of dots System.out.print("."); } System.out.println(); } } // end of drawMatrix3 public static void drawMatrix5() { int N = 5; for (int line = 1; line <= N; line++) { for (int j = 1; j <= (-1 * line + N); j++) { // initial dots System.out.print("."); } System.out.print(line); // the number for (int j = 1; j <= (line - 1); j++) { // the second set of dots System.out.print("."); } System.out.println(); } } // end of drawMatrix5 }

  9. Remove Redundancy public class DrawMatrix2 { } ERROR:N is out of scope public static void main(String[] args) { drawMatrix(); // should print 3 drawMatrix(); // should print 5 } // end of main int N = 3; N = 5; public static void drawMatrix() { for (int line = 1; line <= N; line++) { for (int j = 1; j <= (-1 * line + N); j++) { // initial dots System.out.print("."); } System.out.print(line); // the number for (int j = 1; j <= (line - 1); j++) { // the second set of dots System.out.print("."); } System.out.println(); } // end of for line } // end of drawMatrix

  10. Solution 2 public class DrawMatrix2 { } static int N; // introduce a class scope variable public static void main(String[] args) { drawMatrix(); // should print 3 drawMatrix(); // should print 5 } // end of main N = 3; N = 5; public static void drawMatrix() { for (int line = 1; line <= N; line++) { for (int j = 1; j <= (-1 * line + N); j++) { // initial dots System.out.print("."); } System.out.print(line); // the number for (int j = 1; j <= (line - 1); j++) { // the second set of dots System.out.print("."); } System.out.println(); } // end of for line } // end of drawMatrix

  11. Problem of Class Variable Indirection public class DrawMatrix2 { } static int N; // introduce a class scope variable public static void main(String[] args) { drawMatrix(); drawMatrix(); } // end of main Not self clear;Implicit, depends on context; N = 3; N = 5; public static void drawMatrix() { for (int line = 1; line <= N; line++) { for (int j = 1; j <= (-1 * line + N); j++) { // initial dots System.out.print("."); } System.out.print(line); // the number for (int j = 1; j <= (line - 1); j++) { // the second set of dots System.out.print("."); } System.out.println(); } // end of for line } // end of drawMatrix

  12. Solution 3: Parameterization • Specify a parameter to control the behavior of a method • Methods with parameters solve an entire class of similar problems • Redundancy removal/abstraction through generalization • The more general a building block, the easier to reuse it • We will learn more techniques on generalization/abstraction

  13. Method Design Heuristics • The main method should read as a concise summary of the overall set of tasks performed by the program. • Each method should have a clear set of responsibilities. • No method should do too large a share of the overall task. • Use method with parameters to remove redundancy with generalization, but do not over generalize • Data should be declared/used at the lowest level possible (localized data).

  14. Example: Draw Boxes • Consider the task of printing the following box figures using “*” and ““: ***** * * ***** ********** * * ********** ***** * * * * ***** • What method(s) and parameter(s)?

  15. Box Figures Param. Chaining ***** * * ***** ********** * * ********** ***** * * * * ***** public static void main(String[] args) { drawBox(5, 3); drawBox(10, 3); drawBox(5, 4); } // end of main public static void drawBox(int width, int height) { // step 1. print width of "*" repeat("*", width); System.out.println(); // step 2. loop a total of height - 2 times // for each row: // print * // print width -2 " " for (int i = 1; i <= height - 2; i++) { System.out.print("*"); repeat(" ", width - 2); System.out.println("*"); } // step 3. print width of "*" repeat("*", width); System.out.println(); } // end of drawBox public static void repeat(String s, int times) { for (int i = 1; i <= times; i ++) { System.out.print( s ); } } // end of repeat

  16. Method Correctness Analysis/Testing • Q: How do you analyze/test the correctness of a method? • Basic idea • A method is correct if for each input (parameters), the output is what you expect for the input • An issue: • Some input may not make sense. • Two approaches • Detect such bad input and report • Assume that the input is in the valid range: called precondition of the method

  17. Method Correctness Analysis/Testing ***** * * ***** ********** * * ********** ***** * * * * ***** public static void drawBox(int width, int height) { // step 1. print width of "*" repeat("*", width); System.out.println(); // step 2. loop a total of height - 2 times // for each row: // print * // print width -2 " " for (int i = 1; i <= height - 2; i++) { System.out.print("*"); repeat(" ", width - 2); System.out.println("*"); } // step 3. print width of "*" repeat("*", width); System.out.println(); } // end of drawBox public static void repeat(String s, int times) { for (int i = 1; i <= times; i ++) { System.out.print( s ); } } // end of repeat

  18. Recap: Graphics using StdDraw • To access a static method or class variable defined in another class, use <class-name>.<method-name>(…), for example, • StdDraw.setCanvasSize(100, 100); • Color.BLACK; • Method designers may design different methods for the same function StdDraw: rectangle(double x, double y, double halfWidth, double halfHeight); DrawingPanel (Java): rect (int x0, int y0, int width, int height);

  19. Example: Parameterized Drawing Center:(x0 + 100 /2, y0 + 50/2) Size: 50, 25 • Write method drawCar: 100 30 50 20 Center:(x0 + 100-30/2, y0+50/2) Size: 15, 10 15 (x0, y0) r=10 Center:(x0 + 15 + 10, y0) Radius: 10 Center:(x0 + 100-15-10, y0) Radius: 10

  20. drawCar Criticism of the method? publicstaticvoid drawCar(int x0, int y0) { StdDraw.setPenColor(Color.BLACK); StdDraw.filledRectangle(x0 + 100/2, y0 + 50/2, 100/2, 50/2); // draw wheels StdDraw.setPenColor(Color.RED); StdDraw.filledCircle(x0 + 15 + 10, y0, 10); StdDraw.filledCircle(x0 + 100 - 15 - 10, y0, 10); // draw window StdDraw.setPenColor(Color.CYAN); StdDraw.filledRectangle(x0 + 100 - 30 / 2, y0 + 25, 15, 10); } See Car.java for the final version.

  21. Example: Drawing using Loops • You may use loop (nested loop) to producemore “complex” patterns: public static void main(String[] args) { init(); // set x, y scale to 512 int x0 = 56, y0 = 56; int size = 50; for (int i = 0; i < 8; i++) { for (int j = 0; j < 8; j++) { StdDraw.square(x0 + i * size + size / 2, y0 + j * size + size / 2, size/2); } } // end of main What is the display?

  22. Example: Drawing using Loops

  23. Example: Book Cover • White 500x600 drawing panel • Three components at • (20, 415), (165, 330), (220, 85) with sizes 150, 120, and 240 • Each component • Yale blue background • white “CS112" text left @ 1/2 horizontal, 4/5 vertical • 10 brown (red=192, green=128, blue=64) “bricks” • 1 pixel between two adjacent bricks BookCover.java

  24. Summary of Progress So Far • Primitive data types, expressions and variables • for loops • Methods with parameters

  25. Motivating Example: Car Launch • Two cars launching from different heights, which lands further? • Car 1: initial height: 600m, horizontal speed: 30 m/sec, vertical speed: 20 m/sec • Car 2: initial height: 500m, horizontal speed: 40 m/sec, vertical speed: 30 m/sec • Write a program to display their trajectories in 10 sec

  26. Background: Physics • In physics, for each dimension (x or y), given initial velocity v0 in m/s, acceleration a in m/s2, and elapsed time t in s • the speed of the moving body at time t: • V = v0 + a t • The displacement of the moving body at time t: • Displacement = v0 t + ½ at 2 • The position of the moving body at time t: • Pos = Initial position + displacement

  27. Position • P0 + v0 t + ½ a t2 • Horizontal (x): • vx0 * t • Vertical (y): • g ≅ 9.81 m/s2, downward • y0 + vy0 t - ½ g t 2

  28. Example • See CarLaunchv1.java

  29. Backup Slides

  30. Exercise: Generalization • Consider the task of printing the following lines/boxes: ************* //13 ******* // 7 *********************************** // 35 ********** * * ********** ***** * * * * *****

  31. ************************* ********** * * ********** Design I • Provide a new method drawLine() by calling repeat() public static void drawLine(int width) { repeat(“*”, width); } // end of drawLine

  32. ************************* ********** * * ********** Design II • Consider a line as a special box with height • draw first line of stars • if (height >= 2) // a real box • draw rest public static void drawBox(int width, int height)

  33. The if statement Executes a block of statements only if a test is true if (test) { statement; ... statement; } • Example: if (gpa >= 2.0) { System.out.println("Application accepted."); }

  34. The if-else Statement • An else clause can be added to an if statement to make it an if-else statement: if ( test ) { statement1;} else { statement2; } • If the condition is true, statement1 is executed; if the condition is false, statement2 is executed • One or the other will be executed, but not both

  35. The if/else Statement • Example: if (gpa >= 2.0) { System.out.println("Welcome to Mars University!"); } else { System.out.println("Application denied."); }

  36. ************************* ********** * * ********** Design II • Consider a line as a special box // super elastic drawBox public static void drawBox(int width, int height) { // step 1. print width of "*" repeat("*", width); System.out.println(); for (int i = 1; i <= height - 2; i++) { System.out.print("*"); repeat(" ", width - 2); System.out.println("*"); } if (height >= 2) { // a real box repeat("*", width); System.out.println(); } // end of if } // end of drawBox public static void main(String[] args) { drawBox(13, -1); drawBox(10, 3); } // end of main

  37. ************************* ********** * * ********** Comparison public static void drawLine(int width) { repeat(“*”, width); } // end of drawLine // super elastic drawBox public static void drawBox(int width, int height) { // step 1. print width of "*" repeat("*", width); System.out.println(); for (inti = 1; i <= height - 2; i++) { System.out.print("*"); repeat(" ", width - 2); System.out.println("*"); } if (height >= 2) { // a real box repeat("*", width); System.out.println(); } // end of if } // end of drawBox public static void drawBox(int width, int height) { repeat("*", width); System.out.println(); for (int i = 1; i <= height - 2; i++) { System.out.print("*"); repeat(" ", width - 2); System.out.println("*"); } } // end of drawBox

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