Lecture 06 syntax analysis 3
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The document discusses predictive parsing and LL(1) grammars. It begins by providing an example grammar and walking through calculating FIRST and FOLLOW sets. It then explains predictive parsing, including that it works for LL(1) grammars where the next input symbol uniquely determines the production rule. The document outlines table-driven predictive parsing, including constructing the parsing table. It provides algorithms and examples for predictive parsing and reconstructing the parse tree. Finally, it discusses properties of LL(1) grammars and different error recovery techniques for predictive parsing.
![ERROR RECOVERY
a + b $
Y
X
$
Z
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](https://image.slidesharecdn.com/lecture06syntaxanalysis3-151007044556-lva1-app6892/85/Lecture-06-syntax-analysis-3-75-320.jpg)
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Lecture 06 syntax analysis 3
- 2. FOLLOW EXAMPLE S a S e | B B b B C f | C C c C g | d | ε FIRST(C) = FIRST(B) = FIRST(S) = FOLLOW(C) = FOLLOW(B) = FOLLOW(S) = {$} Assume the first non-terminal is the start symbol 1. If A is start symbol, put $ in FOLLOW(A) 2. Productions of the form B α A β, Add FIRST(β) – {ε} to FOLLOW(A) 3. Productions of the form B α A or B α A β where β ⇒* ε Add FOLLOW(B) to FOLLOW(A) 2
- 3. FOLLOW EXAMPLE S a S e | B B b B C f | C C c C g | d | ε FIRST(C) = {c,d,ε} FIRST(B) = {b,c,d,ε} FIRST(S) = {a,b,c,d,ε} FOLLOW(C) = FOLLOW(B) = FOLLOW(S) = { }$, e {c,d} ∪ FOLLOW(S) = {c,d,e,$} {f,g} ∪ FOLLOW(B) = {c,d,e,f,g,$} 3
- 5. 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
- 61. EXAMPLE: THE “DANGLING ELSE” GRAMMAR 61
- 62. EXAMPLE: THE “DANGLING ELSE” GRAMMAR 62
- 63. EXAMPLE: THE “DANGLING ELSE” GRAMMAR 63
- 64. EXAMPLE: THE “DANGLING ELSE” GRAMMAR 64
- 65. EXAMPLE: THE “DANGLING ELSE” GRAMMAR 65
- 66. EXAMPLE: THE “DANGLING ELSE” GRAMMAR 66
- 67. EXAMPLE: THE “DANGLING ELSE” GRAMMAR 67
- 68. EXAMPLE: THE “DANGLING ELSE” GRAMMAR 68
- 69. EXAMPLE: THE “DANGLING ELSE” GRAMMAR 69
- 70. EXAMPLE: THE “DANGLING ELSE” GRAMMAR 70
- 71. EXAMPLE: THE “DANGLING ELSE” GRAMMAR 71
- 72. 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 72
- 73. LL(1) GRAMMAR 73
- 74. PROPERTIES OF LL(1) GRAMMAR A grammar G is LL(1) if an only if whenever A →α | β are two distinct productions of G the following conditions hold: 1. For no terminal a do both α and β derive strings beginning with a. 2. At most one of α and β can derive the empty string. 3. If then β ⇒* ε , then α does not derive any string beginning with a terminal in FOLLOW(A). 74
- 75. ERROR RECOVERY a + b $ Y X $ Z 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
- 77. ERROR RECOVERY: SKIP INPUT SYMBOLS 77
- 78. ERROR RECOVERY: POP THE STACK 78
- 79. ERROR RECOVERY: PANIC MODE 79
- 80. ERROR RECOVERY - TABLE ENTRIES 80
- 81. QUESTIONS ? 81