2. Arrays and Input/Output

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In this assignment, you will write array-based programs that read input with StdIn, print results with StdOut, draw graphics with StdDraw, play sound using StdAudio. You will also redirect standard input to read from a file.

Getting Started

Read Sections 1.4 and 1.5 of the textbook and review the corresponding lectures and precepts.
Download and unzip arrays.zip , then open the arrays project in IntelliJ.
Take the Getting Help in COS 126 Quiz.
Consult the Programming Assignments FAQ as needed.
Refer to the relevant textbook library APIs in the Java Cheatsheet.
Reminder: You may use only Java features introduced so far.

Dice and the Gaussian Distribution

Write a program DiceHistogram.java that takes two command-line arguments: $n$ (the number of fair six-sided dice) and $trials$ (the number of trials). Each trial consists of rolling all $n$ dice, summing the results, and recording the total. Use an integer array to tabulate how many times each possible total occurs (from $n$ to $6n$) over all trials. Then print a text histogram of the results.

Print one line per total. Each line contains:

the total, right justified in a field of width 3,
a colon,
one space,
a sequence of * characters whose length equals the number of times that total occurred.

Here are a few sample executions. Because the rolls are random, your output may vary. However, when the number of trials is large, the histogram should resemble the examples below.

~/Desktop/arrays> java-introcs DiceHistogram 2 500
  2: **************
  3: ***************************
  4: *********************************
  5: *************************************************************
  6: **********************************************************************
  7: ******************************************************************************
  8: ***********************************************************************
  9: **********************************************************
 10: **********************************************
 11: ****************************
 12: **************

~/Desktop/arrays> java-introcs DiceHistogram 10 1000
 10:
 11:
 12:
 13:
 14:
 15:
 16:
 17:
 18: *
 19: ****
 20:
 21: ***
 22: ******
 23: ********
 24: ****************
 25: *************
 26: **********
 27: *********************************
 28: ****************************************
 29: *********************************
 30: ***************************************************
 31: *****************************************************************
 32: ********************************************************
 33: **************************************************************************************
 34: ***********************************************************
 35: *********************************************************************
 36: ***********************************************************************************
 37: **************************************************************
 38: *****************************************************************
 39: ***************************************
 40: *****************************************************
 41: ************************************
 42: ****************************
 43: ************************
 44: ************************
 45: *********
 46: ***********
 47: *******
 48: ***
 49: **
 50:
 51:
 52: *
 53:
 54:
 55:
 56:
 57:
 58:
 59:
 60:

Context: The central limit theorem, a key result in probability and statistics, asserts that when the number of dice and trials is large, the histogram of the sums approaches the familiar bell curve (Gaussian distribution).

Q.What values should my program handle for n and trials?

Assume that $0 \leq n < 166$ (fits in a field of width 3) and $trials \geq 0$.

Q.How do I print an integer right-justified in a field of width 3?

Use formatted printing. For example,

StdOut.printf("%3d", value);

Q.I have the magic number 6 sprinkled through DiceHistogram.java. Is there a better way?

Yes. Define a named constant and use it throughout. For example:

int SIDES = 6;

Echo Filter

Consider what happens if you superpose a sound file with a delayed copy of itself. You will hear the same sound twice, with the second instance slightly delayed. If the delay is relatively short (say, 15,000 samples or about 1/3 of a second) and you reduce the loudness of the delayed copy by scaling its samples (e.g, multiply by 0.5), the result is an echo effect, similar to what you might hear in a tunnel or large empty room.

A sound wave is a mechanical wave and is subject to the principle of superposition: to combine two sound waves, add the corresponding sample values. Sample shifted and rescaled

Write a program EchoFilter.java that takes three command-line arguments: a string filename, an integer delay, and a real-valued decay factor. It applies an echo filter to the given sound file and plays the resulting sound.

The length of the resulting sound (in samples) is the original length plus the delay.

Here are some sample executions:

~/Desktop/arrays> java-introcs EchoFilter IHaveADream.wav 0 0.0


~/Desktop/arrays> java-introcs EchoFilter IHaveADream.wav 15000 0.5


~/Desktop/arrays> java-introcs EchoFilter PearlHarbor.wav 15000 0.5


~/Desktop/arrays> java-introcs EchoFilter TheHillWeClimb.wav 10000 0.5


~/Desktop/arrays> java-introcs EchoFilter Crow.wav 10000 0.5


~/Desktop/arrays> java-introcs EchoFilter HelloWorld.wav 5000 1.0


~/Desktop/arrays> java-introcs EchoFilter RowYourBoat.wav 117600 1.0

Context: An echo filter is a popular voice effect that you can apply on social media platforms such as TikTok.

Q.Is delay measured in samples?

Yes. The echo begins delay samples after the original sound.

Q.Can I assume that delay is a non-negative integer?

Yes. Negative delays correspond to reverse echo effects, but we will not test those cases.

Q.Can I assume that decay is between 0 and 1?

Yes. We will test your program only with decay values between 0 and 1.

Q.Is it possible for delay to be longer than the number of samples in the original array?

Yes. Think about an echo from a far-away mountain: the echo might not start until after the original sound has ended.

Q.How do I test my program?

Start with a small example. For example, use decay = 0.5, delay = 3, and

double[] original = { 1.0, 0.0, 0.5, 0.0, -1.0, -0.5, 1.0, 0.25 };

to match the example shown in the diagram above. While it would not make sense to play this tiny example, you can print the resulting array to verify that your calculations are correct.

Q.How do I print the contents of an array for debugging?

If a[] is an array, System.out.println(a) prints a reference to the array (not its elements). To print the elements, use a for loop.

Q.How many times should my program call StdAudio.play()?

Only once. Call StdAudio.play() with a double[] array as argument.

Q.The echo filter repeats the sound only once. How could I get a repeating, decaying echo?

Superpose several delayed copies of the original sound, with each successive copy shifted by a larger delay and scaled by a smaller decay factor. Feel free to experiment with this idea.

World Maps

Write a program WorldMap.java that reads boundary data for a country (or other geographic entity) from standard input (using StdIn) and draws it to standard drawing (using StdDraw). A country consists of a set of regions (e.g., states or provinces). Each region is described by a polygon. Represent a polygon using two floating-point arrays: one containing the $x$-coordinates and the other containing the $y$-coordinates of the polygon’s vertices.

Input format. The first line contains two integers: width and height. The remainder of the input is divided into regions.

The first token in each region is the region name. For simplicity, names contain no spaces.
The next token is an integer specifying the number of vertices in the region’s polygon.
The remaining tokens in the region are the $x$- and $y$-coordinates of the vertices.

Output format. Draw the polygons to standard drawing, using the following guidelines:

Call StdDraw.setCanvasSize() to set the size of the canvas to width-by-height pixels.
Call StdDraw.setXscale() and StdDraw.setYscale() so that the $x$-coordinates range from 0 to width and the $y$-coordinates range from 0 to height.
Call StdDraw.polygon() to draw each polygon. This method takes two arrays as arguments: the array of $x$-coordinates and the array of $y$-coordinates. It draws the polygon whose vertices are given by those $x$- and $y$-coordinates.

Data files. The project folder includes the following data files:

Here are some sample executions:

~/Desktop/arrays> java-introcs WorldMap < shapes.txt



~/Desktop/arrays> java-introcs WorldMap < usa.txt



~/Desktop/arrays> java-introcs WorldMap < iceland.txt



~/Desktop/arrays> java-introcs WorldMap < world.txt

Q.Can I assume the input file is properly formatted?

Yes. You may assume it matches the specified format (the expected number, order, and types of tokens).

Q.What if a region has multiple boundary polygons?

In the input file, it appears as multiple regions, one polygon per region. So, you don’t need to handle multi-polygon regions.

Q.Can I draw the boundary of a region as a sequence of line segments instead of a polygon?

No. You must draw each region using StdDraw.polygon(), as specified.

Q.My program runs slowly for inputs with a large number of regions. Any way to speed it up?

Yes. By default, as soon as you call a drawing method such as StdDraw.polygon(), the result appears on screen. Updating the screen is relatively expensive (in terms of CPU time). If there are thousands of geometric objects to draw, this can slow down your program considerably. To avoid this slowdown, use the double-buffering feature of standard draw:

Call StdDraw.enableDoubleBuffering() once before any drawing code. Now all drawing is done on an offscreen canvas.
Call the drawing commands such as StdDraw.polygon(). Nothing will appear on screen yet.
Call StdDraw.show(). This transfers the offscreen canvas to the onscreen canvas.

You can think of double buffering as collecting all drawing commands and then displaying them all at once.

Q.My favorite country is not supplied as one of the data files. Why not?

Some of the source data files we used had technical discrepancies, which made converting them into our format more challenging. If you wish to add a country, please send us an input file in the specified format.

Q.The displayed map distorts the areas of regions. How can I fix this?

Every map projection that represents a sphere on a plane introduces some distortion. For example, with the Mercator projection, Greenland appears to be about the same size as Africa; in reality, Africa is 14 times larger by area. The Gall–Peters projection reduces area distortion but distorts shapes instead.

Submission

Upload all required files to TigerFile :

DiceHistogram.java
EchoFilter.java
WorldMap.java
readme.txt
acknowledgments.txt

Grading Breakdown

Files	Points
DiceHistogram.java	12
EchoFilter.java	12
WorldMap.java	12
readme.txt	2
Getting Help Quiz	2
Total	40