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1.5 Input and Output
Introduction to Programming in Java: An Interdisciplinary Approach · Robert Sedgewick and Kevin Wayne · Copyright © 2008 · February 19, 2009 7:44 AM 2
Input and Output
Input devices.
Output devices.
Goal. Java programs that interact with the outside world.
Display (^) Speakers Printer MP3 Player Keyboard Mouse Hard drive Network Digital camera Microphone Hard drive Network 3
Input and Output
Input devices.
Output devices.
Our approach.
! Define Java libraries of functions for input and output.
! Use operating system (OS) to connect Java programs to:
file system, each other, keyboard, mouse, display, speakers.
Display (^) Speakers Printer MP3 Player Keyboard Mouse Hard drive Network Digital camera Microphone Hard drive Network 4
Terminal. Application where you can type commands to control the
operating system.
Terminal
Mac OS X Microsoft Windows
5
Command-Line Input and Standard Output
Command-line input. Read an integer N as command-line argument.
Standard output.
! Flexible OS abstraction for output.
! In Java, output from System.out.println() goes to standard output.
! By default, standard output is sent to Terminal.
public class RandomSeq { public static void main(String[] args) { int N = Integer.parseInt(args[ 0 ]); for (int i = 0 ; i < N; i++) { System.out.println(Math.random()); } } } %^ java^ RandomSeq^4
6
Old Bird's Eye View
7
New Bird's Eye View
Standard Input and Output
Redirection and Piping
14 % java RandomSeq 1000 > data.txt redirect stdout
Redirecting Standard Output
Redirecting standard output. Use OS directive to send standard
output to a file for permanent storage (instead of terminal window).
15 % more < data.txt
… % java Average < data.txt
redirect stdin
Redirecting Standard Input
Redirecting standard input. Use OS directive to read standard input
from a file (instead of terminal window).
16
Connecting Programs
Piping. Use OS directive to make the standard output of one program
become the standard input of another.
% java RandomSeq 1000000 | java Average
% java RandomSeq 1000000 | java Average
Standard Drawing
18
Standard Draw
Standard drawing. We provide library StdDraw to plot graphics.
To use. Download StdDraw.java and put in working directory.
(0, 0) (1, 0) (!, !! 3 ) public class Triangle { public static void main(String[] args) { double t = Math.sqrt(3.0) / 2.0; StdDraw.line(0.0, 0.0, 1.0, 0.0); StdDraw.line(1.0, 0.0, 0.5, t); StdDraw.line(0.5, t, 0.0, 0.0); StdDraw.point(0.5, t/3.0); } } % java Triangle 19
Data Visualization
Plot filter. Read in a sequence of (x, y) coordinates from standard
input, and plot using standard drawing.
public class PlotFilter { public static void main(String[] args) { double xmin = StdIn.readDouble(); double ymin = StdIn.readDouble(); double xmax = StdIn.readDouble(); double ymax = StdIn.readDouble(); StdDraw.setXscale(xmin, xmax); StdDraw.setYscale(ymin, ymax); while (!StdIn.isEmpty()) { double x = StdIn.readDouble(); double y = StdIn.readDouble(); StdDraw.point(x, y); } } } rescale coordinate system read in points, and plot them 20
Data Visualization
% more < USA.txt 669905.0 247205.0 1244962.0 490000. 1097038.8890 245552. 1103961.1110 247133. 1104677.7780 247205. ... % java PlotFilter < USA.txt coordinates of 13,509 US cities bounding box
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Barnsley Fern
Barnsley fern. Play chaos game with different rules.
Q. What does computation tell us about nature?
Q. What does nature tell us about computation?
20 th^ century sciences. Formulas.
21 st^ century sciences. Algorithms?
2% .50^ .27y probability new x new y 15% -.14x^ +^ .26y^ +^ .57^ .25x^ +^ .22y^ -^. 13% .17x^ -^ .21y^ +^ .41^ .22x^ +^ .18y^ +^. 70% .78x^ +^ .03y^ +^ .11^ -.03x^ +^ .74y^ +^. 28
Animation
Animation loop. Repeat the following:
! Clear the screen.
! Move the object.
! Draw the object.
! Display and pause for a short while.
Ex. Bouncing ball.
! Ball has position (rx, ry) and constant velocity (vx, vy).
! Detect collision with wall and reverse velocity.
(rx, ry) (vx, vy) (-1, -1) (+1, +1) 29
Bouncing Ball
public class BouncingBall { public static void main(String[] args) { double rx = .480, ry = .860; double vx = .015, vy = .023; double radius = .05; StdDraw.setXscale(-1.0, +1.0); StdDraw.setYscale(-1.0, +1.0); while(true) { if (Math.abs(rx + vx) + radius > 1.0) vx = - vx; if (Math.abs(ry + vy) + radius > 1.0) vy = - vy; rx = rx + vx; ry = ry + vy; StdDraw.setPenColor(StdDraw.GRAY); StdDraw.filledSquare(0.0, 0.0. 1.0); StdDraw.setPenColor(StdDraw.BLACK); StdDraw.filledCircle(rx, ry, radius); StdDraw.show( 20 ); } } } bounce position constant velocity update position clear background draw the ball turn on animation mode: display and pause for 50ms rescale coordinates radius 30
Bouncing Ball Demo
% java BouncingBall
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Special Effects
Images. Put .gif, .png, or .jpg file in the working directory and
use StdDraw.picture() to draw it.
Sound effects. Put .wav, .mid, or .au file in the working directory and
use StdAudio.play() to play it.
Ex. Modify BouncingBall to display image and play sound upon collision.
! Replace StdDraw.filledCircle() with:
! Add following code upon collision with wall:
StdAudio.play("boing.wav"); StdDraw.picture(rx, ry, "earth.gif"); 32
Deluxe Bouncing Ball Demo
% java DeluxeBouncingBall 33
Bouncing Ball Challenge
Q. What happens if you call StdDraw.filledSquare() once before loop
(instead of inside)?
% java DeluxeBouncingBall
