Class 17: Shift-Reduce Parsing, Continued

Held Monday, October 7, 2002

Summary

Today we consider how to build shift-reduce automata and the tables that represent them..

Notes

• Does anyone read the class news (the summary of the notes from the outlines) or should I stop making them?
• No, I still haven't finished grading.
• Very few of you sent me notes on group work. Please do so by the end of the day today.
• I'll create groups by Wednesday. To make it easier for me to create groups, each of you must send me the following information by 5pm today (anyone who doesn't gets put in the "doesn't respond to requests group"):
  • Your expertise/comfort with Java on a 6 point scale (1=barely able to use it; 6=Java wizard)
  • Your expertise/comfort with assembly code on a 6 point scale (1="I think I've heard of it"; 6=program as well in assembly as any other language)
  • Your comfort with the material in the class (1="ummmm..."; 6="I could teach this class")

Assignments

• Project, Phase 2: Syntactic Analysis

Overview

• Building LR(0) automata
  • Example

Constructing LR(0) Automata, Formalized

• Let us now formalize what we did.
• We need a method that fills in the rest whenever we advance the mark. We'll call this closure

  closure(State S)
    repeat
      for each item, N ::= alpha . M beta in S
        for each production M ::= gamma
          add M ::= . gamma to S
        end for each production
      end for each item
    until no changes are made to S
    return S
  end closure
  

• We need a method that describes the edges in the automata. We'll call this goto

  goto(State S, Symbol s)
    newS = {}
    for each item N ::= alpha . s beta in S
      newS = newS union { N ::= alpha s . beta }
    end for
    return closure(newS)
  end goto
  

• Finally, we can build the automaton as follows

  S0 = { S' ::= . S $ }
  S0 = closure(S0);
  while there are unmarked states
    pick an unmarked state, S
    mark S
    for each symbol, s, add state goto(S,s) with edge labelled s
  end while