lecture 06 syntax analysis 3

SYNTAX ANALYSISORPARSINGLecture 06

FOLLOW EXAMPLE S a S e | B B b B C f | C C c C g | d |

FIRST(C) = FIRST(B) = FIRST(S) =

FOLLOW(C) = FOLLOW(B) = FOLLOW(S) = {$}

Assume the first non-terminal is the start symbol1. If A is start symbol, put $ in FOLLOW(A)2. Productions of the form B A ,

Add FIRST() – {} to FOLLOW(A)3. Productions of the form B A or B A where *

Add FOLLOW(B) to FOLLOW(A)

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FOLLOW EXAMPLE S a S e | B B b B C f | C C c C g | d |

FIRST(C) = {c,d,} FIRST(B) = {b,c,d,} FIRST(S) = {a,b,c,d,}

FOLLOW(C) =

FOLLOW(B) =

FOLLOW(S) = { }$, e

{c,d} FOLLOW(S)

= {c,d,e,$}

{f,g} FOLLOW(B)

= {c,d,e,f,g,$}

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PREDICTIVE PARSING

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PREDICTIVE PARSING LL(1) Grammars

Can do predictive parsing Can select the right rule Looking at only the next 1 input symbol

First L : Left to Right Scanning Second L: Leftmost derivation 1 : one input symbol look-ahead for predictive decision

LL(k) Grammars Can do predictive parsing Can select the right rule Looking at only the next k input symbols

Techniques to modify the grammar: Left Factoring Removal of Left Recursion

LL(k) Language Can be described with an LL(k) grammar 5

TABLE DRIVEN PREDICTIVE PARSING

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PARSE TABLE CONSTRUCTION

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TABLE DRIVEN PREDICTIVE PARSING

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TABLE DRIVEN PREDICTIVE PARSING

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PREDICTIVE PARSING ALGORITHM

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PREDICTIVE PARSING

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RECONSTRUCTING THE PARSE TREE

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RECONSTRUCTING THE PARSE TREE

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RECONSTRUCTING THE PARSE TREE

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EXAMPLE: THE “DANGLING ELSE” GRAMMAR

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EXAMPLE: THE “DANGLING ELSE” GRAMMAR

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EXAMPLE: THE “DANGLING ELSE” GRAMMAR

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EXAMPLE: THE “DANGLING ELSE” GRAMMAR

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EXAMPLE: THE “DANGLING ELSE” GRAMMAR

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EXAMPLE: THE “DANGLING ELSE” GRAMMAR

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EXAMPLE: THE “DANGLING ELSE” GRAMMAR

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EXAMPLE: THE “DANGLING ELSE” GRAMMAR

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EXAMPLE: THE “DANGLING ELSE” GRAMMAR

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EXAMPLE: THE “DANGLING ELSE” GRAMMAR

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EXAMPLE: THE “DANGLING ELSE” GRAMMAR

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LL(1) GRAMMAR LL(1) grammars

Are never ambiguous. Will never have left recursion.

Furthermore... If we are looking for an “A” and the next symbol

is “b”, Then only one production must be possible

Although elimination of left recursion and left factoring is easy. Some grammar will never be a LL(1) grammar.

LL(1) Grammar

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LL(1) GRAMMAR

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PROPERTIES OF LL(1) GRAMMAR A grammar G is LL(1) if an only if whenever

A are two distinct productions of G the following conditions hold:1. For no terminal a do both and derive

strings beginning with a.2. At most one of and can derive the empty

string.3. If then * then does not derive any

string beginning with a terminal in FOLLOW(A).

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ERROR RECOVERY

a + b $

Y

X

$

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Input

Predictive Parsing Program

Stack Output

Parsing Table M[A,a]

When Do Errors Occur? Recall Predictive Parser Function:

1. If X is a terminal and it doesn’t match input.

2. If M[X, Input] is empty – No allowable actions 75

ERROR RECOVERY

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ERROR RECOVERY: SKIP INPUT SYMBOLS

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ERROR RECOVERY: POP THE STACK

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ERROR RECOVERY: PANIC MODE

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ERROR RECOVERY - TABLE ENTRIES

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QUESTIONS ?

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