Course course mechanics mechanics but what ivariable

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: the be Grossman Fall 2011 F set of symbols tall 2011 follow A in any possible hat can deriva]on –  i.e., t is in FOLLOW(A) if any deriva]on contains At –  To compute this, we need to compute FIRST(γ) for strings γ that can follow A Winter 2013 UW CSE 401 (Michael Ringenburg) 5 Calcula]ng FIRST(γ) CSE341: Programming CSE341: Programming Languages Languages •  Sounds easy… If γ = X Y Z , then FIRST(γ) is Lecture 1 Lecture 1 FIRST(X), right? Course Course Mechanics Mechanics –  But what iVariable Bindings X ::= ε? f we have tL Variable Bindings M he rule ML –  In that case, FIRST(γ) includes FIRST(Y) … and Dan Grossman FIRST(Z) if YDcan derive ε. Fall 2011 an Grossman Fall 2011 –  So, compu]ng FIRST and FOLLOW requires knowledge of other symbols FIRST and FOLLOW, as well as which symbols can derive ε. Winter 2013 UW CSE 401 (Michael Ringenburg) 6 2/4/13 So How Do We Calculate FIRST/FOLLOW? CSE341: Programming CSE341: Programming Languages FOLLOW, and •  Actually calculate three equa]ons: FIRST, Languages nullable Lecture 1 Lecture 1 •  nullable(X) is true if X can doursetMechanicstring C erive he empty s Course Mechanics •  Given a string γ of terminals and non- terminals, FIRST(γ) is ML Variable Bindings ML Variable Bindings the set of terminals that can begin strings derived from γ. –  Actually, for SLR construc]on, jDanneed to calculate FIRST(X), ust Grossman Dan Grossman where X is a single symbol (terminal or nonterminal) Fall 2011 •  FOLLOW(X) is the Fall 2011erminals that can immediately set of t follow X in some deriva]on –  We only really need this for nonterminals, but we’ll compute it for everything for illustra]on. •  All three of these are computed together Winter 2013 UW CSE 401 (Michael Ringenburg) 7 Compu]ng FIRST, FOLLOW, and nullable CSE341: Programming CSE341: Programming Languages Languages •  We use another fixed point algorithm Lecture 1 Course Mechanics •  Basically, FIRST(a) =Ma} for all terminals a { L Variable Bindings ML Variable Bindings 1 –  Start with a Limple ini]al state s ecture Course Mechanics –  Repeatedly apply four simple observa]ons to modify the Dtate s an Grossman Dan Grossman Fall 2011 Fall 2011 –  Stop when the state no longer changes Winte...
View Full Document

This document was uploaded on 04/04/2014.

Ask a homework question - tutors are online