CS177 RECITATION WEEK 7

1. CS177 RECITATION WEEK 7 Input and Output (Text & Graphical)

2. ANNOUNCEMENTS • Project 3 Milestone due • Thursday, October 22 by 9:00 pm • Project3 due • Thursday, October 29 by 9:00 pm

3. Exam 1 Questions The answer is B

4. Exam 1 Questions The answer is D

5. Exam 1 Questions The answer is A

6. Exam 1 Questions Do you remember pre-increment from recitations? First increment and then add. Added j values: 1, 3, 5, 7, 9 and 11 The answer is C

7. QUESTIONS?

8. OUTPUT • We have been using • System.out.print() • System.out.println() to print to the console window • Things printed on the console are lost when we exit the program • We can keep the output of our program after program termination using redirection to a file with ‘>’

9. OUTPUT REDIRECTION EXAMPLE Task: Generate 100 random numbers and print them to a file random.txt: public class RandomSeq { public static void main( String [] args ) { for ( int i = 0; i < 100; i++ ) { System.out.println( Math.random() ); } } } Run the program using the command: java RandomSeq > random.txt

10. INPUT • We have been using methods of the StdIn library to get input from the console • When performing tasks with many inputs, we would not like to type them one by one on the console  need to get input from file • Then we use input redirection from file with ‘<‘

11. INPUT REDIRECTION EXAMPLE Task: Read 100 numbers from the file numbers.txt and print their sum public class Sum{ public static void main(String [] args) { double sum = 0; for ( int i = 0; i < 100; i++ ) { double value = StdIn.readDouble(); sum += value; } System.out.println( “Sum is ” + sum ); } } Run the program using the command: java Sum < numbers.txt

12. PIPING • The output from one program can be the input to another program using piping • To send the output of program Writer to the program Reader use the following command: • java Writer | java Reader • Example: • To print the sum of 100 random numbers use: • java RandomSeq | java Sum • instead of the two commands: • java RandomSeq > numbers.txt • java Sum < numbers.txt

13. GRAPHICAL OUTPUT • We focused on text output until now • The real world is full of graphics: • Imagine having games with text-based interface? Or websites? • System.out.println (print)  text output • StdDraw methods  graphical output

14. StdDraw • Imagine as a device capable of drawing shapes on a two-dimensional canvas • Implemented by authors of the book: • Need to download StdDraw.java in the directory of your program (just as we did for StdIn) • Use methods to draw points, lines, rectangles, circles… as well as format the drawing area (set color, resize etc.) • !!!Always use StdDraw.x whether x is the method name, color name, etc.

15. BASIC DRAWING: LINES & POINTS !!! Canvas we are drawing on is like Quadrant I of Cartesian plane

16. LINES & POINTS . StdDraw.line( 0, 0, 1, 1 ); x0 y0 x1 y1 StdDraw.point(0.5, 0.5)

17. SHAPE METHODS: Q: How can we draw a square? Idea: Draw four lines using StdDraw.line Better idea: Use StdDraw.square Methods for drawing shapes:

18. COLORS • You can draw shapes of different colors using StdDraw.COLOR_NAME where available pen colors are • Need to set color before drawing, • i.e. to draw a pink line use • StdDraw.setPenColor(StdDraw.PINK); • StdDraw.line(0,0,1,1); • instead of • StdDraw.line(0,0,1,1); • StdDraw.setPenColor(StdDraw.PINK);

19. CONTROLLING THE DRAW AREA StdDraw also has methods to provide more control over the display

20. Let’s mark cities having population > 500 on the US map: public class PlotFilter { public static void main(String[] args) { double x0 = StdIn.readDouble(); double y0 = StdIn.readDouble(); double x1 = StdIn.readDouble(); double y1 = StdIn.readDouble(); StdDraw.setXscale(x0, x1); StdDraw.setYscale(y0, y1); while (!StdIn.isEmpty()) { double x = StdIn.readDouble(); double y = StdIn.readDouble(); StdDraw.point(x, y); } } }

21. SHAPE EXAMPLES public class Shapes { public static void main( String args[] ) { StdDraw.setPenColor( StdDraw.RED); StdDraw.square(.2, .8, .1); StdDraw.filledSquare(.8, .8, .2); StdDraw.setPenColor( StdDraw.BLACK); StdDraw.circle(.8, .2, .2); double[] xd = {.1, .2, .3, .2}; double[] yd = {.2, .3, .2, .1}; StdDraw.filledPolygon(xd, yd); } }

22. Let’s simulate the movement of a bouncing ball in the unit box: public class BouncingBall { public static void main(String[] args) { StdDraw.setXscale(-1.0, 1.0); StdDraw.setYscale(-1.0, 1.0); double rx = 0.480, ry = 0.860; // position double vx = 0.015, vy = 0.023; //velocity double radius = 0.05; while (true) {// main animation loop // bounce off wall according to law of elastic collision if (Math.abs(rx + vx) > 1.0 - radius) vx = -vx; if (Math.abs(ry + vy) > 1.0 - radius) vy = -vy; rx = rx + vx; //update position ry = ry + vy; StdDraw.setPenColor(StdDraw.GRAY); // clear the background StdDraw.filledSquare(0, 0, 1.0); StdDraw.setPenColor(StdDraw.BLACK); StdDraw.filledCircle(rx, ry, radius); // draw ball on the screen StdDraw.show(20); // display and pause for 20 ms } } } ANIMATIONS set the scale of the coordinate system

23. PLOTTING FUNCTION GRAPHS public class FunctionGraph { public static void main(String[] args) { int N = Integer.parseInt(args[0]); double[] x = new double[N+1]; double[] y = new double[N+1]; for (int i = 0; i <= N; i++) { x[i] = Math.PI * i / N; y[i] = Math.sin(4*x[i]) + Math.sin(20*x[i]); } StdDraw.setXscale(0, Math.PI); StdDraw.setYscale(-2.0, +2.0); for (int i = 0; i < N; i++) { StdDraw.line(x[i], y[i], x[i+1], y[i+1]); } } } the function y = sin(4x) + sin(20x), sampled at N points between x = 0 and x = pi

24. FINAL QUESTIONS?