Transcript 05_inout_for

File Input and Graphics
• An extract of slides from set 05_inout.ppt
• Designed to get you ready for the HW3.
Standard Input and Output
Command-line Input vs. Standard Input
Command line inputs.
Use command line inputs to read in a few user values.
Not practical for many user inputs.
Input entered before program begins execution.
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Standard input.
Flexible OS abstraction for input.
By default, stdin is received from Terminal window.
Input entered while program is executing.
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Standard Input and Output
Standard input. Book authors provide library StdIn to read text input.
Standard output. Book authors provide library StdOut to write text
output. (But versions of Java ≥ 5.0 have printf() -- so we can use
System.out just like StdOut!)
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Reading Data from a File
Recall how we read from the standard input?
StdIn: Princeton provides a library to simplify this
Methods called on StdIn include:
• readInt(), readDouble(), readString(), …, isEmpty()
Here’s a good idea:
Let’s use the same interface to read from a data file!
(Interface here means same methods.)
Issues:
Need to “connect to” or “open” the data file.
There’s just one standard input, but we could read from multiple files.
The library In
In: Another Princeton provided library that simplifies the standard
Java ways to do this.
Use In in a very similar way to StdIn -- but you must:
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declare a new In object and create it by opening a file.
use your new In object by name instead of StdIn.
In fileIn = new In(“mydatafile.txt”);
int k = fileIn.readInt();
double[] data = new double[100];
int i = 0;
while ( ! fileIn.isEmpty() ) data[i++] = fileIn.readDouble();
1.This opens a file for reading. Looks for mydatafile.txt in the current
working directory where your program is running.
2.Reads on int value from the file into k.
3.Then reads double after double into the array. Stops when there are
no more values in the file to be read.
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The library In
Declaring an In object:
In fileIn = new In(“mydatafile.txt”);
Remember what defining a new array looks like?
double[] data = new double[100];
In general we’ll see later that in Java we often do things like:
Type variable = new Type(some initialization info);
Questions:
What could go wrong when you open a file?
What do you think happens?
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Getting the Princeton Libraries
Java’s standard libraries (e.g. Math, System, etc.)
Automatically included when you build a Java program
Other libraries (your’s, our’s, Princton’s):
•You must add them to your project, folder, etc. using your IDE.
•These can be in source files, e.g. StdIn.java
•Or, in an archive file called a Jar file
We provide a Jar file with all the Princeton libraries: stdlib.jar
Downlaod stdlib.jar and then:
In DrJava: from Preference menu, choose Resource Locations
Then click Add by Extra Classpath and find and select stdlib.jar
Standard Drawing
Read: page 136-146
Try it out!
Used in HW3.
The Princeton Library StdDraw is very very nice!
Standard Draw
Standard drawing. We provide library StdDraw to plot graphics.
To use. Add Jar file, or ownload StdDraw.java and put in working
directory.
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);
}
}
(½, ½3)
% java Triangle
(0, 0)
(1, 0)
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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);
rescale coordinate
system
while (!StdIn.isEmpty()) {
double x = StdIn.readDouble();
double y = StdIn.readDouble();
StdDraw.point(x, y);
}
read in points,
and plot them
}
}
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Data Visualization
If using re-direction, looks like this slide. But see our demo Java code that reads from the file.
bounding box
% more < USA.txt
669905.0 247205.0 1244962.0 490000.0
1097038.8890 245552.7780
1103961.1110 247133.3330
1104677.7780 247205.5560
...
coordinates of
13,509 US cities
% java PlotFilter < USA.txt
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Plotting a Function
double[] a = new double[N+1];
for (int i = 0; i <= N; i++)
a[i] = Math.sin(4*Math.PI*i/N) + Math.sin(20*Math.PI*i/N);
StdDraw.setXscale(0, N);
StdDraw.setYscale(-2.0, +2.0);
for (int i = 0; i < N; i++)
StdDraw.line(i, a[i], i+1, a[i+1]);
y  sin 4x  sin20x
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Animation
Animation loop. Repeat the following:
Clear the screen.
Move the object.
Draw the object.
Display and pause for a short while.
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Ex. Bouncing ball.
Ball has position (rx, ry) and constant velocity (vx, vy).
Detect collision with wall and reverse velocity.
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(+1, +1)
(vx, vy)
(rx, ry)
(-1, -1)
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Bouncing Ball
public class BouncingBall {
public static void main(String[] args) {
double rx = .480, ry = .860;
position
double vx = .015, vy = .023;
constant velocity
radius
double radius = .05;
StdDraw.setXscale(-1.0, +1.0);
StdDraw.setYscale(-1.0, +1.0);
rescale coordinates
while(true) {
if (Math.abs(rx + vx) > 1.0) vx = -vx;
if (Math.abs(ry + vy) > 1.0) vy = -vy;
rx = rx + vx;
ry = ry + vy;
update position
StdDraw.clear(StdDraw.GRAY);
StdDraw.setPenColor(StdDraw.BLACK);
StdDraw.filledCircle(rx, ry, radius);
StdDraw.show(50);
}
}
}
bounce
clear background
draw the ball
turn on animation mode:
display and pause for 50ms
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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:
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StdDraw.picture(rx, ry, "earth.gif");
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Add following code upon collision with wall:
StdAudio.play("boing.wav");
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Computer Animation
Computer animation. Display a sequence
of closely related images in rapid succession
to produce the illusion of movement.
Frame rate. Use 15-70 frames per second
to "trick" human eye and brain into seeing
smooth motion.
Ex 1. Television and motion pictures.
Ex 2. Java mascot Duke cart-wheeling.
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http://java.sun.com/docs/books/tutorial
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Java Implementation
public class Duke {
public static void main(String[] args) {
int images = 17;
int WIDTH = 130, HEIGHT = 80;
StdDraw.setCanvasSize(WIDTH, HEIGHT);
for (int t = 0; true; t++) {
int i = 1 + (t % images);
String file = "T" + i + ".gif";
StdDraw.picture(0.5, 0.5, file);
StdDraw.show(100);
T1.gif }
T17.gif
}
}
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