Programming Assignment Checklist: LFSR
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Frequently Asked Questions
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What are the goals of this assignment?
To introduce you to object-oriented programming and to reinforce the
message of Lecture 1.
Where can I review the concepts involved?
Review the
Lecture 1 slides
to reacquaint yourself with the basic ideas behind LFSRs.
Read Sections 3.1 and 3.2 of the textbook
to learn the basics of object-oriented programming.
What optional files are available for this assignment?
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If you want some help getting started with LFSR, we
provide a template. It saves you from copying and pasting
the signatures of the API.
Additionally,
it includes one possible choice of internal data representation
(private instance variables/fields) that can be used to
implement the behaviour of an LFSR. (Using different representations
is allowed too.)
The template is here:
LFSR.java. You are welcome to use the whole template
as a starting point, or you can just peek at it when you see fit.
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The archive file lfsr-pngs.zip includes a number of sample .png files
including original images and encrypted versions. They are used by the examples below. You should place the .png files in
the exact same directory
as your .java files for this week.
- You can browse the individual files in the LFSR FTP directory.
What are the APIs that I need to use as a client? How can I use them?
Your code will be using Picture and Color.
The back of the textbook has abbreviated APIs for them; see the Using Color and Picture section for more details.
Do the APIs that I define for LFSR and PhotoMagic
need to be exactly the same as the prescribed ones?
Yes, we will be testing the methods in the API directly.
If your method has a different signature or does not behave as specified,
you will lose a substantial number of points.
You may not add public methods to the API; however, you may add private methods
(which are only accessible in the class in which they are declared).
How should I represent the LFSR?
There are several possible approaches. The approach outlined in the template
is to use an int[] array of size N, each entry of which is either 0 or 1.
If you follow this approach, the constructor amounts to creating the array and
converting the char values in the string to int values for the
array (and initializing your other instance variables). However, you are allowed to
use any efficient representation that you like.
How do I create an instance variable which is an array if I don't know its
length at compile time?
Declare it as an instance variable, but initialize it in constructor (where you will
know its length).
For example, see Program 3.2.3 in the textbook.
What extra comments should I include when writing object-oriented programs?
You must comment the purpose of every method, using the names of the argument
variables in your description.
Additionally, you must comment the purpose of each instance variable.
In the initial register, fill.charAt(0) is the
leftmost bit. But in the Lecture 1 slides and the assignment page, bit 0
is the rightmost bit. Do I have to arrange the bits in my register array
that way?
Strings are always indexed from left to right (the way we read English).
Traditionally, binary digits are indexed from right to left, with bit 0 as the lowest order or
rightmost bit.
You are welcome to use either approach here—just be careful to do it
correctly and consistently.
How do I convert the char values in the string
to int values for the array?
The number zero is represented by the char value '0'.
The char data type is a Java primitive type, so you can compare two
of them with if (c == '0').
My step() method is producing 19 for the binary number 11001. What am
I doing wrong?
You are calculating the bits in reverse order. 19 is the decimal value of the
binary number 10011.
My generate() works with the 11 bit fill and the tap at position 8, but
when I try generate()with the 20 bit fill and the tap at position 16, I get
a different answer from the one shown in the example. What am I doing wrong?
Make sure you are not hardwiring 11 or 8 in your LFSR code. The LFSR
constructor arguments should set the size of the register and the tap position.
The toString() is not explicitly called in the test
client code, but it still gets called. How does this work?
LFSR lfsr = new LFSR("01101000010", 8);
StdOut.println(lfsr);
Any object in Java can be promoted (converted) to String type.
This is what is happening here. The way that we can tell Java
how to do this promotion for an arbitrary user-defined type is
by defining a
.toString() method for its class. When Java wants
to do a promotion, it uses the String value returned by
.toString() as the converted/promoted value.
I get an ArrayOutOfBounds or NullPointerException error. What could cause this?
Do your constructors initialize all of the instance variables
(e.g., N, reg, and tap)?
Did you allocate memory for your array with new?
Did you inadvertently redeclare int N or int[] reg
in a method or constructor, thereby hiding the instance variable with the
same name?
How do I do exclusive or (XOR) in Java? Use the ^
Java symbol.
The operation a ^ b, where a and b are int values,
does a bit-by-bit exclusive or of the values. It also can be applied to
boolean values.
Why I am I getting 48s and 49s when I print out
values for debugging?
Internally, characters are represented as integers. The code
for '0' is 48, and the code for '1' is 49. (Read about
ASCII
or
Unicode for more information.)
When you perform arithmetic operations on characters, they
are promoted to int.
So '0' + 1 and '1' ^ '1' ^ '1' are two examples
of expressions that evaluate to the int value 49. (But (char)('0'+1) is '1'.)
Rather than subtracting 48s in your code, can you
write your code
in such a way that arithmetic is never performed directly on char
values?
In .toString(), I loop the right number of times, but the returned String looks too short.
What can cause this? This can be the reverse of the problem above. If you take the integer 0 or the integer 1 and
cast it to a char, you end up with a special control character that looks blank when printed.
Why are we using the .png format instead of .jpg?
It is a lossless format that preserves the precise bits in the colors.
How do I read in the .png files?
Use the Picture
data type, described in Section 3.1 of the textbook.
You will only need the constructors and methods of the Picture API.
To see it in action, look at the program
Grayscale.java
which takes the name of a picture file as a command line argument, converts
all pixels to gray, and displays all those pixels.
See Luminance.java
to see how to retrieve the r, g, and b
values for the Color.
Sometimes my encrypted picture looks like a shadowy version of the original.
How do I pick a good tap number? Here are suggested tap numbers for
maximizing the cycle of
different size linear feedback shift registers:
5-bit (tap 2), 6-bit (tap 4), 9-bit (tap 4),
10-bit (tap 6), 11-bits (tap 8), 20-bit (tap 16), 30-bit (tap 22),
36-bit (tap 24), 48-bit (tap 42), 60 bits (tap 58),
100 bits (tap 62), and 150 bits (tap 96).
What is a java.awt.HeadlessException and how did I cause it?
The assignment only asks you to call show() once, in PhotoMagic.main().
Calling it in other places can give this error.
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The Color class comes built in to Java, in the "Abstract Window Toolkit" package. Write
import java.awt.Color;
at the top of your program, then you can create Color variables
such as
Color princetonOrange = new Color(244, 111, 27); // constructor args are R, G, B
There is an abbreviated API for Color on the booksite cheatsheet.
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The Picture class was created for this course.
Normally, Picture will already have been installed with the
rest of the course materials.
The
import statement is not used with Picture —
you can use it immediately in your code, for example
// internet connection required for this to work
Picture pic = new Picture("http://www.cs.princeton.edu/~cos126/java.png");
pic.show(); // shows Java
One way to check if Picture is installed is to run the
following command
at the command line, which calls Picture.main():
java-introcs Picture http://www.cs.princeton.edu/~cos126/java.png
If it's working
correctly, you'll see an image of Java and the text output 400-by-136 at the command line.
There is an abbreviated API for Picture on the booksite cheatsheet.
- If Picture does not seem to be installed correctly, one solution
is to copy Picture.java to the same directory as your other files for
this assignment.
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Want more detail? (This is optional!) We don't recommend reading the full,
technical,
source code of Picture.java. (You don't need to know
how a class was implemented just to use it.) Instead, read
Java's automatically-built
documentation pages (Javadocs).
Here are the Javadoc pages
for
Color and
Picture.
Search these pages with Ctrl-F/Command-F — they have too much information
for reading straight through to be useful.
Be sure to thoroughly test each piece of your code as you write it.
We offer some suggestions in the assignment specification.
You should also test your data type with other parameters.
These are purely suggestions for how you might make progress. You do
not have to follow these steps.
% java-introcs PhotoMagic Xpipe.png 01101000010100010000 16
% java-introcs PhotoMagic Xbaboon-red.png 01101000010100010000 16
% java-introcs PhotoMagic Xbaboon-green.png 01101000010100010000 16
% java-introcs PhotoMagic Xbaboon-blue.png 01101000010100010000 16
% java-introcs PhotoMagic Xshield.png 010101010101010101010101010101 22
% java-introcs PhotoMagic Xscarpet-cookies.png 01101000010100010000 16 [ picture of something delicious ] % java-introcs PhotoMagic Xjava.png 001110001111000100001101010010000100010010000000001100000100 58 [ picture of something you could drink with cookies ]