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lecture07

# lecture07 - Predictive Parsers Like recursive-descent but...

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1 Prof. Aiken CS 143 Lecture 7 1 Top-Down Parsing and Intro to Bottom-Up Parsing Lecture 7 Prof. Aiken CS 143 Lecture 7 2 Predictive Parsers Like recursive-descent but parser can “predict” which production to use By looking at the next few tokens No backtracking Predictive parsers accept LL(k) grammars L means “left-to-right” scan of input L means “leftmost derivation” k means “predict based on k tokens of lookahead” In practice, LL(1) is used Prof. Aiken CS 143 Lecture 7 3 LL(1) vs. Recursive Descent In recursive-descent, At each step, many choices of production to use Backtracking used to undo bad choices In LL(1), At each step, only one choice of production That is When a non-terminal A is leftmost in a derivation The next input symbol is t There is a unique production A   to use Or no production to use (an error state) LL(1) is a recursive descent variant without backtracking Prof. Aiken CS 143 Lecture 7 4 Predictive Parsing and Left Factoring Recall the grammar E T + E | T T int | int * T | ( E ) Hard to predict because For T two productions start with int For E it is not clear how to predict We need to left-factor the grammar Prof. Aiken CS 143 Lecture 7 5 Left-Factoring Example Recall the grammar E T + E | T T int | int * T | ( E ) Factor out common prefixes of productions E T X X + E | T ( E ) | int Y Y * T | Prof. Aiken CS 143 Lecture 7 6 LL(1) Parsing Table Example Left-factored grammar E T X X + E | T ( E ) | int Y Y * T | The LL(1) parsing table: int * + ( ) \$ E T X T X X + E T int Y ( E ) Y * T leftmost non-terminal next input token rhs of production to use

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2 Prof. Aiken CS 143 Lecture 7 7 LL(1) Parsing Table Example (Cont.) Consider the [E, int] entry “When current non-terminal is E and next input is int , use production E T X This can generate an int in the first position Consider the [Y,+] entry “When current non-terminal is Y and current token is + , get rid of Y Y can be followed by + only if Y   Prof. Aiken CS 143 Lecture 7 8 LL(1) Parsing Tables. Errors Blank entries indicate error situations Consider the [E,*] entry “There is no way to derive a string starting with * from non-terminal E Prof. Aiken CS 143 Lecture 7 9 Using Parsing Tables Method similar to recursive descent, except For the leftmost non-terminal S We look at the next input token a And choose the production shown at [S,a] A stack records frontier of parse tree Non-terminals that have yet to be expanded Terminals that have yet to matched against the input Top of stack = leftmost pending terminal or non-terminal Reject on reaching error state Accept on end of input & empty stack Prof. Aiken CS 143 Lecture 7 10 LL(1) Parsing Algorithm initialize stack = <S \$> and next repeat case stack of <X, rest> : if T[X,*next] = Y 1 …Y n then stack <Y 1 … Y n rest>; else error (); <t, rest> : if t == *next ++ then stack <rest>;
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