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Grammar Top-Down Parsing
Syntax Analysis
Sukree Sinthupinyo1
1Department of Computer Engineering
Grammar Top-Down Parsing
Syntax Analysis Sukree Sinthupinyo 1 1 Department of Computer - - PowerPoint PPT Presentation
Syntax Analysis Sukree Sinthupinyo 1 1 Department of Computer - - PowerPoint PPT Presentation
Grammar Top-Down Parsing Syntax Analysis Sukree Sinthupinyo 1 1 Department of Computer Engineering Chulalongkorn University 24 August 2012 Grammar Top-Down Parsing Outline Grammar 1 Syntax Error Handling Notational Conventions Ambiguity
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Grammar Top-Down Parsing Syntax Error Handling Notational Conventions Ambiguity Elimination of Left Recursion
Introduction
Type of parsers
Universal Top-Down Bottom-Up
Type of grammars
Left Recursive Non Left Recursive
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Grammar Top-Down Parsing Syntax Error Handling Notational Conventions Ambiguity Elimination of Left Recursion
Left recursive grammar Non-left-recursive grammar
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Grammar Top-Down Parsing Syntax Error Handling Notational Conventions Ambiguity Elimination of Left Recursion
Outline
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Grammar Syntax Error Handling Notational Conventions Ambiguity Elimination of Left Recursion
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Top-Down Parsing
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Grammar Top-Down Parsing Syntax Error Handling Notational Conventions Ambiguity Elimination of Left Recursion
Error Handling
There are two strategies of handling error
Panic-mode Phrase-level recovery
The detail of each strategy depends on specific parsing methods. Common errors
Lexical errors include misspellings of identifiers, keywords,
- r operators
Syntactic errors include misplaced semicolons or extra or missing braces. Semantic errors include type mismatches between
- perators and operands.
Logical errors can be anything from incorrect reasoning on the part of the programmer.
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Grammar Top-Down Parsing Syntax Error Handling Notational Conventions Ambiguity Elimination of Left Recursion
Grammar for Simple Arithmetic Expression
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Grammar Top-Down Parsing Syntax Error Handling Notational Conventions Ambiguity Elimination of Left Recursion
Outline
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Grammar Syntax Error Handling Notational Conventions Ambiguity Elimination of Left Recursion
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Top-Down Parsing
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Grammar Top-Down Parsing Syntax Error Handling Notational Conventions Ambiguity Elimination of Left Recursion
Terminals
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Grammar Top-Down Parsing Syntax Error Handling Notational Conventions Ambiguity Elimination of Left Recursion
Nonterminals
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Grammar Top-Down Parsing Syntax Error Handling Notational Conventions Ambiguity Elimination of Left Recursion
Others
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Grammar Top-Down Parsing Syntax Error Handling Notational Conventions Ambiguity Elimination of Left Recursion
Example
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Grammar Top-Down Parsing Syntax Error Handling Notational Conventions Ambiguity Elimination of Left Recursion
Outline
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Grammar Syntax Error Handling Notational Conventions Ambiguity Elimination of Left Recursion
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Top-Down Parsing
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Grammar Top-Down Parsing Syntax Error Handling Notational Conventions Ambiguity Elimination of Left Recursion
Example
From the grammar above we can parse id + id * id in two ways as shown below.
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Grammar Top-Down Parsing Syntax Error Handling Notational Conventions Ambiguity Elimination of Left Recursion
Eliminating Ambiguity
From the grammar above, a parse tree for a conditional statement is as below.
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Grammar Top-Down Parsing Syntax Error Handling Notational Conventions Ambiguity Elimination of Left Recursion
An Ambiguous Sentence
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Grammar Top-Down Parsing Syntax Error Handling Notational Conventions Ambiguity Elimination of Left Recursion
An Unambiguous Sentence
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Grammar Top-Down Parsing Syntax Error Handling Notational Conventions Ambiguity Elimination of Left Recursion
Outline
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Grammar Syntax Error Handling Notational Conventions Ambiguity Elimination of Left Recursion
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Top-Down Parsing
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Grammar Top-Down Parsing Syntax Error Handling Notational Conventions Ambiguity Elimination of Left Recursion
We can change the left-recursive pair of productions A → Aα | β to From the productions as We can change it to
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Grammar Top-Down Parsing Syntax Error Handling Notational Conventions Ambiguity Elimination of Left Recursion
Left Factoring
can be written in general as A → αβ1 | αβ2. The left-factored will be
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Grammar Top-Down Parsing
Recursive-Descent Parsing
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Grammar Top-Down Parsing
FIRST and FOLLOW
FIRST(α), where α is any string of grammar symbols, to be the set of terminals that begin strings derived from α. FOLLOW(A), for nonterminal A, is the set of terminals a that can appear immediately to the right of A in some sentential form.
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Grammar Top-Down Parsing
FIRST
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Grammar Top-Down Parsing
FOLLOW
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Grammar Top-Down Parsing
Constructing a Predictive Parsing Table
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Grammar Top-Down Parsing
Constructing a Predictive Parsing Table
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Grammar Top-Down Parsing