Admin. CS 112 Introduction to Programming. Recap. Example: Nested Loop. Example: Rewrite

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1 Admin CS 112 Introduction to Programming Graphics; Animation Yang (Richard) Yang Computer Science Department Yale University 308A Watson, Phone: q 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 2 Recap Example: Nested Loop q Nested loops o If a loop is inside another loop, we say the two form a nested loop o The #loop times of the inner loop can depend on the outer loop variable q Method with parameters o Specify parameters to control the behavior of a method o Redundancy removal/abstraction through generalization for (int j = 1; j <= (-1 * line + N); j++) { for (int j = 1; j <= (line - 1); j++) { Example: Rewrite public static void dots(int n) { for (int i = 1; i <= n; i++) { Out-Class Read: Another Way to Motivate Method With Parameters dots( -1 * line + N ); dots( line 1 ); // end of outer for loop 6 1

2 Example: Reuse Matrix q Print two copies of inverse diagonal matrix // initial dots for (int j = 1; j <= (-1 * line + N); j++) { // the second set of dots for (int j = 1; j <= (line - 1); j++) { Solution 1 public class DrawMatrix1 { drawmatrix3(); drawmatrix5(); public static void drawmatrix3() { int N = 3; // end of drawmatrix3 public static void drawmatrix5() { // end of drawmatrix5 This code is redundant. 8 Remove Redundancy public class DrawMatrix2 { int N = 3; drawmatrix(); // should print 3 N = 5; drawmatrix(); // should print 5 // end of main ERROR: N is out of scope public static void drawmatrix() { // end of for line // end of drawmatrix Solution 2 public class DrawMatrix2 { static int N; // introduce a class scope variable N = 3; drawmatrix(); // should print 3 N = 5; drawmatrix(); // should print 5 // end of main public static void drawmatrix() { // end of for line // end of drawmatrix 9 10 Problem of Class Variable Indirection public class DrawMatrix2 { static int N; // introduce a class scope variable N = 3; drawmatrix(); N = 5; drawmatrix(); // end of main Not self clear; Implicit, depends on context; public static void drawmatrix() { // end of for line // end of drawmatrix Solution 3: Parameterization q Specify a parameter to control the behavior of a method q Methods with parameters solve an entire class of similar problems q Redundancy removal/abstraction through generalization The more general a building block, the easier to reuse it We will learn more techniques on generalization/abstraction

3 Method Design Heuristics Example: Draw Boxes 1. The main method should read as a concise summary of the overall set of tasks performed by the program. 2. Each method should have a clear set of responsibilities. 3. No method should do too large a share of the overall task. 4. Use method with parameters to remove redundancy with generalization, but do not over generalize 5. Data should be declared/used at the lowest level possible (localized data). q Consider the task of printing the following box figures using * and : What method(s) and parameter(s)? Box Figures Param. Chaining drawbox(5, 3); // end of main drawbox(10, 3); drawbox(5, 4); public static void repeat(string s, int times) { for (int i = 1; i <= times; i ++) { System.out.print( s ); // end of repeat // step 2. loop a total of height - 2 times // for each row: // print * // print width -2 " " // step 3. print width of "*" // end of drawbox Method Correctness Analysis/Testing q Q: How do you analyze/test the correctness of a method? q Basic idea A method is correct if for each input (parameters), the output is what you expect for the input q 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 Method Correctness Analysis/Testing Recap: Graphics using StdDraw public static void repeat(string s, int times) { for (int i = 1; i <= times; i ++) { System.out.print( s ); // end of repeat // step 2. loop a total of height - 2 times // for each row: // print * // print width -2 " " q To access a static method or class variable defined in another class, use <classname>.<method-name>( ), for example, StdDraw.setCanvasSize(100, 100); Color.BLACK; q Method designers may design different methods for the same function StdDraw: rectangle(double x, double y, double halfwidth, double halfheight); // step 3. print width of "*" // end of drawbox DrawingPanel (Java): rect (int x0, int y0, int width, int height); 18 3

4 Example: Parameterized Drawing drawcar Criticism of the method? q Write method drawcar: 50 (x0, y0) (x , y0) 30 r=10 20 (x /2, y0 + 50/2) Size: 50, 25 (x , y0) (x /2, y0+50/2) Size: 15, 10 public static void drawcar(int x0, int y0) { StdDraw.setPenColor(Color.BLACK); StdDraw.filledRectangle(x /2, y0 + 50/2, 100/2, 50/2); // draw wheels StdDraw.setPenColor(Color.RED); StdDraw.filledCircle(x , y0, 10); StdDraw.filledCircle(x , y0, 10); // draw window StdDraw.setPenColor(Color.CYAN); StdDraw.filledRectangle(x / 2, y0 + 25, 15, 10); Radius: 10 Radius: 10 See Car.java for the final version. 20 Example: Drawing using Loops Example: Drawing using Loops q You may use loop (nested loop) to produce more complex patterns: 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? Example: Book Cover q White 500x600 drawing panel q Three components at (20, 415), (165, 330), (220, 85) with sizes 150, 120, and 240 Each component Yale blue background white CS112" text 1/2 horizontal, 4/5 vertical 10 brown (red=192, green=128, blue=64) bricks 1 pixel between two adjacent bricks Summary of Progress So Far q Primitive data types, expressions and variables q for loops q Methods with parameters BookCover.java 24 4

5 Motivating Example: Car Launch Background: Physics q 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 q In physics, for each dimension (x or y), given initial velocity v 0 in m/s, acceleration a in m/s 2, 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 = v 0 t + ½ a t 2 The position of the moving body at time t: Pos = Initial position + displacement q Write a program to display their trajectories in 10 sec Position Example q P 0 + v 0 t + ½ a t 2 q See CarLaunchv1.java q Horizontal (x): v x0 * t q Vertical (y): g 9.81 m/s 2, downward y 0 + v y0 t - ½ g t 2 Exercise: Generalization q Consider the task of printing the following lines/ boxes: Backup Slides *** //13 ** // 7 // 35 5

6 Design I Design II q Provide a new method drawline() by calling repeat() public static void drawline(int width) { repeat( *, width); // end of drawline public static void drawbox(int width, int height) q Consider a line as a special box with height draw first line of stars if (height >= 2) // a real box draw rest The if statement Executes a block of statements only if a test is true if (test) { statement;... statement; q Example: if (gpa >= 2.0) { System.out.println("Application accepted."); The if-else Statement q An else clause can be added to an if statement to make it an if-else statement: if ( test ) { statement1; else { statement2; q If the condition is true, statement1 is executed; if the condition is false, statement2 is executed q One or the other will be executed, but not both 34 The if/else Statement Design II q Example: if (gpa >= 2.0) { System.out.println("Welcome to Mars University!"); else { System.out.println("Application denied."); q Consider a line as a special box // super elastic drawbox if (height >= 2) { // a real box // end of if // end of drawbox drawbox(13, -1); drawbox(10, 3); // end of main 6

7 Comparison public static void drawline(int width) { repeat( *, width); // end of drawline // end of drawbox // super elastic drawbox if (height >= 2) { // a real box // end of if // end of drawbox 7