1. Programming Practice - Markov Chain Algorithm in Java

Transcription

1 1. Programming Practice - Markov Chain Algorithm in Java Java classes of Markov chain algorithm Class Prefix Class Chain Class Markov Prefix Class class Prefix public Vector pref; // NPREF adjacent words from input static final int MULTIPLIER = 31; // for hashcode() // Prefix constructor: duplicate existing prefix Prefix(Prefix p) pref = (Vector) p.pref.clone(); // Prefix constructor: n copies of str Prefix(int n, String str) pref = new Vector(); for (int i = 0; i < n; i++) pref.addelement(str); // Prefix hashcode: generate hash from all prefix words public int hashcode() int h = 0; for (int i = 0; i < pref.size(); i++) h = MULTIPLIER * h + pref.elementat(i).hashcode(); return h; // Prefix equals: compare two prefixes for equal words public boolean equals(object o) Prefix p = (Prefix) o; for (int i = 0; i < pref.size(); i++) if (!pref.elementat(i).equals(p.pref.elementat(i))) return false; return true;

2 Chain Class class Chain static final int NPREF = 2; // size of prefix static final String NONWORD = "\n"; // "word" that can't appear Hashtable statetab = new Hashtable(); // key = Prefix, value = suffix Vector Prefix prefix = new Prefix(NPREF, NONWORD); // initial prefix Random rand = new Random(); // Chain build: build State table from input stream void build(inputstream in) throws IOException StreamTokenizer st = new StreamTokenizer(in); st.resetsyntax(); // remove default rules st.wordchars(0, Character.MAX_VALUE); // turn on all chars st.whitespacechars(0, ' '); // except up to blank while (st.nexttoken()!= st.tt_eof) add(st.sval); add(nonword); // Chain add: add word to suffix list, update prefix void add(string word) Vector suf = (Vector) statetab.get(prefix); if (suf == null) suf = new Vector(); statetab.put(new Prefix(prefix), suf); suf.addelement(word); prefix.pref.removeelementat(0); prefix.pref.addelement(word); // Chain generate: generate output words void generate(int nwords) prefix = new Prefix(NPREF, NONWORD); for (int i = 0; i < nwords; i++) Vector s = (Vector) statetab.get(prefix); if (s == null) System.err.println("Markov: internal error: no state"); System.exit(1);

3 int r = Math.abs(rand.nextInt()) % s.size(); String suf = (String) s.elementat(r); if (suf.equals(nonword)) break; System.out.println(suf); prefix.pref.removeelementat(0); prefix.pref.addelement(suf); Markov Class class Markov static final int MAXGEN = 10000; // maximum words generated public static void main(string[] args) throws IOException Chain chain = new Chain(); int nwords = MAXGEN; chain.build(system.in); chain.generate(nwords); 2. Reading/Writing in Java Reading and Writing Examples (i) The following code illustrates the use of classes Input Stream Reader, Buffered Reader and the method readline() public static void main(string[] args)throws IOException //We construct an input stream is //It allows us to read input text line by line InputStreamReader converter = new InputStreamReader(System.in); BufferedReader is = new BufferedReader(converter); //We prompt the user for N strings //We store input strings inside a String array B String[] B=new String[N]; System.out.println("Enter input strings, one string per line:"); for(int i=0;i<n;i++) B[i]=is.readLine();

4 System.out.println(); (ii) This is a modification of Markov chain algorithm. The program takes input from the file alice30.txt and it sends the output to the file output.txt. class MarkovModifiedInOut static final int MAXGEN = 10000; // maximum words generated public static void main(string[] args) throws IOException BufferedReader in = new BufferedReader(new FileReader("alice30.txt")); PrintWriter out = new PrintWriter(new FileWriter("output.txt")); Chain chain = new Chain(); int nwords = MAXGEN; chain.build(in); in.close(); chain.generate(nwords, out); out.close(); Modifications inside Chain class void build(bufferedreader in) throws IOException void generate(int nwords, PrintWriter out) prefix = new Prefix(NPREF, NONWORD); for (int i = 0; i < nwords; i++) out.println(suf); String Tokenizer Class (iii) This is yet another modification of Markov chain algorithm. We keep the changes done in example (ii) and now we substitute the Stream Tokenizer object of the method build by an object of the String Tokenizer class.

5 Modifications inside build method of Chain class void build(bufferedreader in) throws IOException String s; StringTokenizer st; while((s = in.readline())!=null) st = new StringTokenizer(s, " "); while (st.hasmoretokens()) add(st.nexttoken()); add(nonword); 3. Programming Practice Debugging programs Debugging an overview To reduce debugging time stick to the following principles: (i) Good design (ii) Good style (iii) Boundary conditions tests (iv) Assertions (v) Sanity checks (vi) Defensive programming (vii) Well-designed interfaces (viii) Limited global data (ix) Checking tools When a hard bug occurs: (i) Make the bug reproducible (ii) Divide and conquer (iii) Study the numerology of failures (iv) Display output to localize your search (v) Write self checking code (vi) Write a log file (vii) Draw a picture (viii) Use tools (ix) Keep records

6 4. Programming Practice Examples of tests based on proofs Loop Invariants and Loop Testing Loop Syntax: while condition B do P end while Loop Invariant Q is a condition, which we design in order to prove loop correctness. Once the invariant Q is ready we need to verify the following statements: Invariant Q holds after initializing loop variables. If Q holds before execution of P, then it holds after execution of P. Execution of the loop terminates. It is possible to deduce post-condition from the fact that invariant Q holds and that the loop terminated its execution. With the help of the following two examples we will discuss how one may use loop invariants to design tests of loops. Example 1: Product(int x, int y) //pre-condition: x,y 0 //post-condition: j=x*y int i,j i:=0 j:=0 while i x do j:=j+y i:=i+1 end while return j end function Product Loop invariant: Q: j=i*y Example 2: Euclid s Algorithm GCD(int a, int b) //precondition: a,b> 0, a b //postcondition: i=gcd(a,b) int i,j i:=a j:=b

7 while j 0 do compute i=q j+r, 0 r< j i:=j j:=r end while return i end function GCD Loop invariant: Q: gcd(i,j)=gcd(a,b) Problem 1: Prepare C++ code for the following test of Example 1: Design an integer-valued function with two integer arguments, which implements the loop of Example 1. Design a Boolean valued function Q, which checks whether the invariant holds. In the implementation of Q perform direct calculation of the product. Prepare C++ code for testing your implementation of the loop. Your code should verify whether the invariant holds after the initialization of variables and after every execution of the update. It should prompt the user for two input integers and report the result of the test. Problem 2: Prepare C++ code for the following test of Euclid s algorithm: Design an integer-valued function with two integer arguments implementing the loop of Example 2. Design a Boolean valued function Q, which checks whether the invariant holds. In the implementation of Q perform direct calculation of the greatest common divisor. Find the largest positive integer (no larger than the smaller of the arguments), which divides both input arguments. Prepare C++ code for testing your implementation of the loop. Your code should verify whether the invariant holds after the initialization of variables and after every execution of the update. It should prompt the user for two input integers and report the result of the test.