CS453 Colorado State University
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Context-Free Grammars and Top-Down Parsing
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class announcements
- recitation this week will be about svn and will need to show a lexer
  that can handle about half the tokens in MiniSVG, you can provide your own
  input file

- reading that was due today
    Ch. 2.1, 2.2, 4 thru 4.2

- reading for next week
    Ch 2.3 thru 2.5, 4.3, 4.4


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Goals for today and Tuesday

    - Learn context free grammars and associated terminology
    
    - Relationship between parse trees and context free grammars
    
    - Syntax-directed translation
    
    - Top-down predictive parsing, an initial overview
 
 
 
        
 
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Context free grammars
        
-useful because we can express programming languages with context free
 grammars
    - show grammar for MiniSVG 
    - show grammar for MeggyJava (webpage, MeggyJava grammar)
    - CF grammars for most languages: SSL, python bytecode, Java bytecode,
      etc.
    
-why not just use regular expressions to describe programming languages?



    
    balanced parentheses example



    
    



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vocabulary 









    terminal
        "characters" of the grammar alphabet, in our case tokens
    
    nonterminal
        represents a set of strings made up of terminals
        
    symbol
        a terminal, nonterminal, or epsilon
        
    production 
        has a nonterminal on the left side and a string of 
        terminals and nonterminals or epsilon on the right side,
        also called a rewrite rule

    start symbol
        a nonterminal from which derivations begin
    
    epsilon
        a special symbol
    
    derivations
        step by step replacement of nonterminals with the right hand side
        of production rules
    
    sentential form
        a string of symbols that derive from the start symbol of a grammar
    
    

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parse trees (slides 14 through 21)
    
    create them directly from derivations
    -root is the start symbol
    
    -each nonterminal in the derivation is an internal node
    
    -each leaf is a terminal or epsilon
    
    -each production links the left hand side nonterminal with each of the
    symbols on the right hand side
            
    Ambiguity
    
            (slide 22) No ambiguity


    
    
            (slide 23) show derivation for 42 + 7 * 6
            expr1 -> expr2 + expr3              used production (1)
            -> expr2 + expr4 * expr5            used production (2)
            -> NUM(42) + NUM(7) * NUM(6)        used production (3), 3 times

    
            --> have students do all both derivations and parse trees
           
           
           SUGGESTED EXERCISE: Is the MiniSVG grammar ambiguous?
           

        -We use parse trees to do syntax directed translation or 
        interpretation...
            1) translate from one language to another, 
            
            2) create a data structure that represents the program, or
            
            3) evaluate the program like an interpreter.
           

            
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syntax-directed translation or interpretation (slides 25-27)
    -associate an action or rule with each production, can think
    of the action as being one more entity on the right-hand side of 
    the production
    

    --------------------------------------
    How this is related to parsing MiniSVG
    --------------------------------------
    -> show example MiniSVG parse tree (slide ?)
        - syntax directed translation corresponds to rendering objects to 
        the screen and printing to the log
        
    - The trick is that parse tree will be implicit.  We won't actually
    be constructing an explicit data structure.
            

    Example: Using parse trees to evaluate expressions
    
        (slide 27)
        -semantics of Exp --> (Stm, Exp) is that Stm is executed and then
        the lhs Exp gets the value of the rhs expression
            -like the C comma operator
            
 
 
 
 
 
 
 
 
 
        
        -Do a depth-first, post-order traversal of nodes in the parse 
        tree and map each expression node to a number and each
        statement node to a side effect.
        
 
 
 
 

        (slide 28)
        Two possible parse trees for 42 + 7 * 6
            -> do interpretation of both
            
            -> which is correct?

            
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parsing (slides 29-43)

    - naive algorithm and its complexity
          
    - need for O(N) solutions
    
    - top-down, predictive parsing example
    
    - Next time we will be covering top-down, predictive parsing in detail


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mstrout@cs.colostate.edu, 1/27/10