Differences

This shows you the differences between two versions of the page.

--- lfa:lab [2018/10/02 11:02]
pdmatei
+++ lfa:lab [2018/10/02 11:20] (current)
pdmatei
@@ Line 108: / Line 108: @@
 <code java>
+import java.io.*;
+import java.util.*;
+public class Hello {
+  public static void main (String[] args) throws IOException {
+    HelloLexer l = new HelloLexer(new FileReader(args[0]));
+    l.yylex();
+    System.out.println(l.words);
+  }
+}
 </code>
+  * Note that the lexer constructor method receives a java Reader as input (other options are possible, see the docs), and we take the name of the file to-be-scanned from standard input.
+  * Each lexer implements the method ''yylex'' which starts the scanning process.
+After compiling:
+<code>
+javac HelloLexer.java Hello.java
+</code>
+and running:
+<code>
+java Hello
+</code>
+we obtain:
+<code>
+</code>
+at standard output.
+Recall that the option ''standalone'' tells the lexer to print unmatched words. In our example, those unmatched words are whitespaces.
 ==== Application - parsing lists ====
+Consider the following BNF grammar which describes lists:
+<code>
+<integer> ::= [0-9]+
+<op> ::= "++" | ":"
+<element> ::= <integer> | <op> | <list>
+<sequence> ::= <element> | <element> " " <sequence>
+<list> ::= | "()" | "(" <sequence> ")"
+</code>
+The following are examples of lists:
+<code>
+(1 2 3)
+(1 (2 3) 4 ())
+(1 (++ (: 2 (3)) (4 5)) 6)
+</code>
+Your task is to:
+  * correctly parse such lists:
+    * write a JFlex file to implement the lexer:
+      * Since the language describing lists is Context Free, in order to parse a list, you need to keep track of the opened/closed parenthesis.
+      * Start by write a PDA (on paper) which accepts correctly-formed lists. Treat each regular expression you defined (for numbers and operators) as a single symbol;
+      * Implement the PDA (strategy) in the lexer file;
+  * given a correctly-defined list, write a procedure which evaluates lists operations (in the standard way); For instance, ''(1 (++ (: 2 (3)) (4 5)) 6)'' evaluates to ''(1 (2 3 4 5) 6)''
+  * write a procedure which checks if a list is **semantically valid**. What type of checks do you need to implement?