Frequently Asked Questions (general)

What are the main goals of this assignment? Use a symbol table, learn about natural language processing, and learn about Markov models.

What preparation do I need before beginning this assignment? Review the beginning of Section 4.4 (pages 608–617) on using symbol tables in client code. For reference, here is a partial API of the ST data type. Also review the material on the textbook on parameterized data type and generics (pages 566–570). The Key type can be any comparable type, such as String or Integer.

What is the origin of the Markov text generator? It was first described by Claude Shannon in 1948. The first computer version was apparently written by Don P. Mitchell, adapted by Bruce Ellis, and popularized by A. K. Dewdney in the Computing Recreations section of Scientific American. Brian Kernighan and Rob Pike revived the program in a University setting and described it as an example of design in The Practice of Programming. The program is also described in Jon Bentley's Programming Pearls.

Do I have to implement the prescribed API? Yes, or you will lose a substantial number of points.

Frequently Asked Questions (MarkovModel)

My program won't compile because the compiler can't find the symbol "ST." How can I fix that? The ST data type is not part of Java or stdlib.jar. You need to download ST.java—it's part of markov.zip—and put it in the same directory as MarkovModel.java.

Can I use java.util.TreeMap or java.util.HashMap for my symbol table instead of ST? No, please use ST.

I want to refer to the magic number 128 in more than one method. Any advice? You can define a global constant, as follows:

private static final int ASCII = 128;

How do I extract a substring of a given string? The method call s.substring(i, i+k) returns the the k-character substring of s, starting at i. Note that it includes the left endpoint but excludes the right endpoint.

How do I emulate the behavior of a circular string? There are a number of approaches. One way is to append the first k characters of the input text to the input text.

How do I convert a char to an int? A char is a 16-bit (unsigned) integer. Java will automatically promote a char to an int if you use it as an index into an array.

How do I specify the newline character? Use either the character literal '\n' or the string literal "\n".

How do I use StdRandom.discrete()? There are two overloaded methods named StdRandom.discrete().

• One takes a floating-point array probabilities[] and returns index i with probability equal to probabilities[i]. The array entries must be non-negative and sum to 1.

• The other takes an integer array frequencies[] and returns index i with probability proportional to frequencies[i]. The array entries must be non-negative and not all zero.

Frequently Asked Questions (TextGenerator)

Should my program generate a different output each time I run it? Yes.

How can I read in the input text from standard input? Use StdIn.readAll(). Do not remove whitespace.

My random text ends in the middle of a sentence. Is that OK? Yes, that's to be expected.

After executing the program, the command prompt appears on the same line as the random text. Is that OK? Yes. It's because the random text does not end with a newline character. If you want to add a call to StdOut.println() at the end of your program, that's fine—we promise not to deduct.

For which values of k should my program work? It should work for all well defined values of k, from 0 up to, and including, the length of the input text. As k gets larger, your program will use more memory and take longer.

My program is slow when T is large because I am concatenating the T characters, one at a time, to form string of length T. What else can I do? Do you need to form the entire string? Why not print the characters, one at a time, as you generate them?

I get an OutOfMemoryException. How do I tell Java to use more of my computer's memory? Depending on your operating system, you may have to ask the Java Virtual Machine for more main memory beyond the default.

% java-introcs -Xmx500m TextGenerator 7 1000 < input.txt

Testing (MarkovModel)

Thoroughly test your MarkovModel. We provide a main() as a start to your testing.

    public static void main(String[] args) {
        String text1 = "banana";
        MarkovModel model1 = new MarkovModel(text1, 2);
        StdOut.println("freq(\"an\", 'a')    = " + model1.freq("an", 'a'));
        StdOut.println("freq(\"na\", 'b')    = " + model1.freq("na", 'b'));
        StdOut.println("freq(\"na\", 'a')    = " + model1.freq("na", 'a'));
        StdOut.println("freq(\"na\")         = " + model1.freq("na"));
        StdOut.println();

        String text3 = "one fish two fish red fish blue fish"; 
        MarkovModel model3 = new MarkovModel(text3, 4);
        StdOut.println("freq(\"ish \", 'r') = " + model3.freq("ish ", 'r'));
        StdOut.println("freq(\"ish \", 'x') = " + model3.freq("ish ", 'x'));
        StdOut.println("freq(\"ish \")      = " + model3.freq("ish "));
        StdOut.println("freq(\"tuna\")      = " + model3.freq("tuna"));
    }

% java-introcs MarkovModel
freq("an", 'a')    = 2
freq("na", 'b')    = 1
freq("na", 'a')    = 0
freq("na")         = 2

freq("ish ", 'r') = 1
freq("ish ", 'x') = 0
freq("ish ")      = 3
freq("tuna")      = 0

To test random(), write a loop that calls random() repeatedly, and count how many times a particular character is returned. For example, with model1 above, random("na") should return 'b' about one-half of the time; with model2 above, random("fish") should return 'o' about one-quarter of the time.

Testing (TextGenerator)

Be sure to test TextGenerator with different values of k.

• An order-0 Markov model generates a random sequence of letters where each letter appears with probability proportional to its frequency in the input text. For input17.txt there are 9 g's, 7 a's, and 1 c. So we want the probability of generating a 'g' to be 9/17, an 'a' to be 7/17, and a 'c' to be be 1/17. In a sequence of 100 characters, we'd therefore expect on average about 53 g's, 41 a's, and 6 c's.

  % java-introcs TextGenerator 0 100 < input17.txt
  gaaagaacagcagacgacggaagaaggaggaaaaggaggggaggggggaggaggaagggagaaaggagacagcggaggggacgggaggagaggaggagag
  

• As documented in the assignment specification, in an order 2 model for input17.txt, the next character after "ga" is 'a' with probability 1/5 and 'g' with probability 4/5. So, if you run the following command 10 times, you should expect (on average) to see "gag" approximately 8 times and "gaa" approximately 2 times.

  % java-introcs TextGenerator 2 3 < input17.txt
  gag
  

Possible Progress Steps (MarkovModel)

These are purely suggestions for how you might make progress. You do not have to follow these steps.

1. Download and unzip markov.zip. Make sure that ST.java and the testing .txt files are in the same directory as the .java files you are writing.

2. Review the material in the textbook on symbol tables as well as IntegerSort.java.

3. Create one or more instance variables to support the two freq() methods. One strategy is to maintain two symbol tables—one for the one-argument freq() method and one for the two-argument freq() method.
   • For each k-gram (a string), the first symbol table tells you how many times it appears in the text (an integer).
   • For each k-gram (a string), the second symbol table tells you how many times each ASCII character succeeds the k-gram in the text (an array of 128 integers).

4. Write the constructor to create the circular version of the input text. Then initialize and populate your symbol tables, using the symbol-table methods contains(), get(), and put(). This will be a substantial amount of code, relative to the other methods in this class.

   Do not save the original text (or the circular text) as an instance variable because no instance method will need this information after the symbol table is initialized.

5. Test: In the main(), try creating some MarkovModel objects. For example:

           String text1 = "banana";
           MarkovModel model1 = new MarkovModel(text1, 2);
           ...
           String text2 = "gagggagaggcgagaaa";
           MarkovModel model2 = new MarkovModel(text2, 2);
   

6. Write the toString() method. Use the enhanced for loop to access each key–value pair in your symbol table:

           // st2 is the second symbol table (corresponding to the two-argument freq() method)
           String result = "";
           for (String key : st2) {
               result += key + ": ";
        
               // get the character frequency array
               int[] frequency = st2.get(key);
   
               // for each non-zero entry, append the character and the frequency
               ...          
   
               // append a newline character
               result += "\n";
            }
   

7. Test the constructor and toString() method: This can help you debug small test cases. In the main(), print some MarkovModel objects. For example:

           String text1 = "banana";
           MarkovModel model1 = new MarkovModel(text1, 2);
           StdOut.println(model1);
           ...
           String text2 = "gagggagaggcgagaaa";
           MarkovModel model2 = new MarkovModel(text2, 2);
           StdOut.println(model2);
   

   Sample output for model1:

   ab: a 1 
   an: a 2 
   ba: n 1 
   na: b 1 n 1 
   

   Sample output for model2:

   aa: a 1 g 1 
   ag: a 3 g 2 
   cg: a 1 
   ga: a 1 g 4 
   gc: g 1 
   gg: a 1 c 1 g 1 
   

8. Write the order() method. This should be a one-liner.

9. Using the symbol-table instance variables, write the two freq() methods.

10. Use the main() provided above to test your code before continuing. You should add more testing of the constructor, order(), and freq() methods.

11. Write the random() method. To generate a random character with probability proportional to its frequency you may call either of the two StdRandom.discrete() methods.

12. It may not be obvious from your final results whether random() is working as intended, so be sure to test it thoroughly. Next, test your complete MarkovModel data type before continuing.

Possible Progress Steps (TextGenerator)

These are purely suggestions for how you might make progress. You do not have to follow these steps.

1. Read in k and T from the command line; read the input text from standard input.

2. Create a MarkovModel object of order k using the input text.

3. To generate a trajectory of length T, use the first k characters in the input text as the initial k-gram and print the initial k-gram. Then, repeatedly generate and print a new random character according to the Markov model and update the k-gram to store the last k characters printed.

4. Make sure to test your program on large inputs (we provide several), large outputs, and different values of k.

Enrichment

• Here's a website that generate pseudo-random computer science papers. It uses something called a context-free grammar instead of a Markov chain, but otherwise is similar in spirit to what you are doing on this assignment.

• Here are Garfield comics generated by a Markov chain.

• What else can I do with a random text generator? One former COS 126 student recited its output during the Frist filibuster.

• We are implementing the model Shannon described in his landmark paper. But the "one reads until this letter is encountered" method in his quote on the assignment page is, ironically, not a statistically accurate example of his model. If we run your program with input "wawawaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaawy" then 'w' should be followed by 'a' 75% of the time, while the "read until" model will follow 'w' with 'a' only 15% of the time. What would be involved in simulating this other model? Which one do you think gives more realistic text?