| Frequently Asked Questions |
What are the goals of this assignment? To introduce you to object-oriented programming and to reinforce the message of Lecture 1.
What's the best way to convert the char values in the string to int values for the array? The number zero is char '0'. The char data type is a java primitive, so you can compare two of them, or do arithmetic with them to strip off the code part and just leave the integer part.
In the initial register, fill.charAt(0) is the leftmost bit. But in the Lecture 1 slides, bit 0 is the rightmost bit. Does the index 0 refer to the leftmost bit or the rightmost bit? How should I arrange the bits in my register array? 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. However, to keep your code easy to follow (and easy to debug), you may want to assign each element in your register array the same index that the corresponding 1 or 0 has in the initial String fill argument. This will also make it easier to write your toString() method. If you choose to use the traditional convention of index 0 referring to the least or rightmost bit, then be aware that you will need to compensate for the left to right nature of the initial String. This is a minor issue, but part of your challenge is to avoid getting confused by it.
Whichever indexing convention you use, be aware that the tap number which is used as a constructor argument for LFSR and as a command-line input for PhotoMagic will refer to the distance away from the leftmost bit (our fixed tap). (e.g., a tap number of 2 refers to the digit 2 slots away from the left end.) The beauty of Object Oriented Programming is that the details of your implementation are up to you, as long as you adhere to the details of the API.
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.
I notice that toString() is not explicitly called in the test client code. What is the explanation?
LFSR test1 = new LFSR("01101000010", 8);
StdOut.println(test1);
When a Java object is passed to any of the print methods, a call is made to that object's toString() method if such a method exists.
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 2, but when I try generate()with the 20 bit fill and the tap at position 3, 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 2 in your LFSR code. The LFSR constructor arguments should be used to set the size of the register and the tap position.
I get an ArrayOutOfBounds or NullPointerException error. What could cause this? Do your constructors initialize all of the data members (N, reg, tap, and rightmostbit)? 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 read in the .png files? Use our Picture.java data type, described in Section 3.1 of the textbook. You will only need the constructors and methods of the Picture API summarized on page 330. 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. If you download Grayscale.java, you will also want to download Luminance.java. Look at Luminance.java to see how to retrieve the r, g, and b values for the Color.
| Testing |
Be sure to thoroughly test each piece of your code as you write it. We offer some suggestions below.
public static void main(String[] args) { LFSR test = new LFSR("01101000010", 2); StdOut.println(test); }
With this test client, once your implementation is working, you will get this output, which matches the example given in lecture:
public static void main(String[] args) { LFSR test = new LFSR("01101000010", 2); StdOut.println(test); for (int i = 0; i < 10; i++) { test.step(); StdOut.println(test); } }
0 1 1 0 1 0 0 0 0 1 0 1 1 0 1 0 0 0 0 1 0 1 1 0 1 0 0 0 0 1 0 1 1 0 1 0 0 0 0 1 0 1 1 0 1 0 0 0 0 1 0 1 1 0 0 0 0 0 0 1 0 1 1 0 0 1 0 0 0 1 0 1 1 0 0 1 0 0 0 1 0 1 1 0 0 1 0 0 0 1 0 1 1 0 0 1 0 0 1 1 0 1 1 0 0 1 0 0 1 0 0 1 1 0 0 1 0 0 1 0 0
as your test client, which should result in the following sequence:
public static void main(String[] args) { LFSR test = new LFSR("01101000010", 2); for (int i = 0; i < 10; i++) StdOut.print(test.generate(8) + " "); StdOut.println(); }
201 61 185 107 152 191 72 77 47 50
as your test client, which should result in the following sequence:
public static void main(String[] args) { LFSR test = new LFSR("01101000010100010000", 3); for (int i = 0; i < 10; i++) StdOut.print(test.generate(8) + " "); StdOut.println(); }
42 217 23 193 26 156 156 231 135 189
java PhotoMagic pipe.png 01101000010100010000 3
java PhotoMagic Xpipe.png 01101000010100010000 3
| Submission |
Your programs must be named LFSR.java and PhotoMagic.java. Use the following readme file template. Don't forget to hit the "Run Script" button on the submission system to test that it compiles cleanly.
| Possible Progress Steps |
These are purely suggestions for how you might make progress. You do not have to follow these steps.
public class LFSR {
private int N; // number of fill bits provided
private int[] reg; // array of integers
// representing the register bits
private int tap; // index of the tap bit
private int rightmostbit; // index of the rightmost bit
}
As you fill in the methods for LFSR, use the test clients described in the test section above.
When implementing the step() method, the tap positions are "moved" simply by using modular arithmetic. The code here is intended as a strong hint:
int N = 11;
for (int i = 0; i < N; i++)
StdOut.print((i + 5) % N);
StdOut.println();
It produces the output
5 6 7 8 9 10 0 1 2 3 4
import java.awt.Color;
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