Parsing
Techniques
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Top-Down and Bottom-Up Parsing Techniques, Slides of Compiler Construction
The differences between top-down and bottom-up parsing techniques, including the use of production rules, input matching, and internal state encoding. It also covers the concept of left recursion and its elimination in top-down parsing. Predictive parsing and the use of first sets are also discussed.
Typology: Slides
2011/2012
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Parsing
Techniques
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Parsing Techniques
Top-down parsers Start at the root of the parse tree and grow towards leaves. Pick a production and try to match the input
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Parsing Techniques
Top-down parsers Start at the root of the parse tree and grow towards leaves. Pick a production and try to match the input
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Parsing Techniques
Top-down parsers Bad “pick” ⇒ may need to backtrack Some grammars are backtrack-free.
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Parsing Techniques
Bottom-up parsers Start at the leaves and grow toward root As input is consumed, encode possibilities in an internal state.
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Parsing Techniques
Bottom-up parsers Start at the leaves and grow toward root As input is consumed, encode possibilities in an internal state.
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Parsing Techniques
Bottom-up parsers Start in a state valid for legal first tokens Bottom-up parsers handle a large class of grammars
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Parsing Techniques
Bottom-up parsers Start in a state valid for legal first tokens Bottom-up parsers handle a large class of grammars
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Top-Down Parser
A top-down parser starts with the root of the parse tree. The root node is labeled with the goal symbol of the grammar
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Top-Down Parsing Algorithm
Construct the root node of the parse tree Repeat until the fringe of the parse tree matches input string
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Top-Down Parsing
At a node labeled A , select a production with A on its lhs for each symbol on its rhs, construct the appropriate child
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Top-Down Parsing
At a node labeled A , select a production with A on its lhs for each symbol on its rhs, construct the appropriate child
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Top-Down Parsing
When a terminal symbol is added to the fringe and it does not match the fringe, backtrack Find the next node to be expanded
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Top-Down Parsing
The key is picking right production in step 1.
That choice should be guided by the input string