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Lectures 7 and 8. Parsing (syntax analysis)
Wei Le 2015.9
SLIDE 2 Bottom Up Parsing
◮ Recognize many programming languages, used in practice ◮ More general, more powerful parser: less restrictions on grammars,
e.g., can handle left-recursive grammar
◮ no backtracking ◮ LR parsing: L – tokens are read from the left to right, R – rightmost
derivation
◮ LR parsing reduces a string to the start symbol by inverting
productions
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LR Parsing
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LR Parsing: An Example
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LR Parsing: An Example
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LR Parsing: An Example
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LR Parsing: An Example
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LR Parsing: An Example
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LR Parsing: An Example
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LR Parsing: Important Factor #1
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LR Parsing: Where Reduction Happens
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LR Parsing: Split Notation
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LR Parsing: Shift and Reduce
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Shift and Reduce Example
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Shift and Reduce Example
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Shift and Reduce Example
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Shift and Reduce Example
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Shift and Reduce Example
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Shift and Reduce Example
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Shift and Reduce Example
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Shift and Reduce Example
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Shift and Reduce Example
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Shift and Reduce Example
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Shift and Reduce Example
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The stack
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Key issues: when to shift or reduce
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Key issues: when to shift or reduce
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LR Parsing Algorithm
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Parsing Table
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Parsing Table
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Table-Driven Bottom Up Parsing
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LR Parsing Algorithm
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Review: Key Ideas of LR Parsing
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Review: Key Ideas of LR Parsing
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How to Decide When to Shift or Reduce
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How to Construct the Parsing Table
Run stack on DFA: search which context matches the stack?
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LR(1) Item Defines the State (or Context)
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Notation Clarification
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LR(1) Items Are Used to Define DFA
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LR(1) Items Are Used to Define DFA: An Example
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LR(1) Items Are Used to Define DFA: An Example
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How To Determine a set of LR(1) Items for the State?
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Closure Operation: An Example
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Closure Operation: An Example
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Closure Operation: An Example
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LR(1) Items Are Used to Define DFA: An Example
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LR(1) Items Are Used to Define DFA: Transitions
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Automatically Generating Parsing Tables
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Shift and Reduce Conflict
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Shift and Reduce Conflict
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Shift and Reduce Conflict
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Shift and Reduce Conflict
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Reduce Reduce Conflict
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Reduce Reduce Conflict
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Reduce Reduce Conflict
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Motivation of LALR
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Motivation of LALR
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Motivation of LALR
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LALR
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LALR
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LALR: Example
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LALR
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LALR
SLIDE 64 Parsing: A Summary
◮ Parse tree: how to derive a string from the grammar ◮ Context free grammars ◮ Algorithms: Recursive Decedent Parsers, LL (1), LR (1), LALR (1) ◮ General approach: table-driven parsing (the stack, streams of
tokens, general parsing algorithm), using table to guide each step of building parsing trees
◮ Challenge: Building parsing table
