Questions and Answers
What are the 3 reasons lexical analysis is separated from syntax analysis?
Portability, Simplicity, Efficiency
What are top-down parsers?
Top-down parsers trace or build a parse tree in preorder such that each node is visited before its branches are followed. Branches of a particular node utilize leftmost derivation.
What does the LL Algorithm mean?
The first L means a left to right scan of the input; the second L means a leftmost derivation is generated.
What are bottom-up parsers?
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What is the time complexity of ambiguous grammars?
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What is the time complexity of syntax analyzers and commercial compilers?
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Which parser is concerned with getting the right A rule in order to formulate the next sentential form in a leftmost derivation?
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You would never want left derivation with right recursion.
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What are the 3 advantages of LR parsers?
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Remove direct left recursion from the following grammar rules: A → Aa | Abc | bc | d. The new grammar rules are A → _____ | dA', A' → _____ | ε.
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Remove direct left recursion from the following grammar rules: X → Xyy | Xz | Yx | w, Y → Yy | Yw | zy | x. The new grammar rules are X → _____ | wX', X' → _____ | ε, Y → _____, Y' → _____.
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Perform the pairwise disjointness test for the grammar rules: a. S → aSb | bAA, b. A → b{aB} | a, c. B → aB | a. Which rule passes or fails?
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Study Notes
Lexical Analysis and Syntax Analysis
- Lexical analysis is separated from syntax analysis due to portability, simplicity, and efficiency considerations.
Top-Down Parsers
- Top-down parsers build a parse tree in a preorder fashion, visiting each node before its branches.
- They utilize leftmost derivation to construct the branches of the tree.
LL Algorithm
- The first 'L' stands for a left-to-right scan of the input.
- The second 'L' refers to generating a leftmost derivation.
Bottom-Up Parsers
- Bottom-up parsers construct a parse tree from the leaves upward to the root.
- This process involves reversing the right derivation to establish the tree structure.
Time Complexity
- Ambiguous grammars have a time complexity of O(n^3), indicating higher computational demand.
- Syntax analyzers and commercial compilers typically operate with a time complexity of O(n), ensuring efficiency.
Purpose of Top-Down Parser
- Top-down parsers focus on determining the correct A rule to formulate the next sentential form during leftmost derivation.
Left Derivation with Right Recursion
- It is false to assert that left derivation with right recursion is desirable; in practice, this approach is avoided.
Advantages of LR Parsers
- LR parsers can be constructed for all programming languages, offering versatility.
- They detect syntax errors earlier in a left-to-right scan.
- The LR grammar class is a proper superset of LL grammars, capable of handling left recursive grammars excluded from LL.
Direct Left Recursion Removal (Example: A → Aa | Abc | bc | d)
- Removed direct left recursion by introducing a secondary nonterminal A':
- A → bcA' | dA'
- A' → aA' | bcA' | ε
Direct Left Recursion Removal (Example: X → Xyy | Xz | Yx | w)
- Removed direct left recursion using new nonterminals X' and Y':
- X → YxX' | wX'
- X' → zX' | yyX' | ε
- Y → zyY' | xY'
- Y' → yY' | wY' | ε
Pairwise Disjointness Test
- For each grammar rule, the FIRST() function is calculated to determine if the left-hand side rule passes or fails pairwise disjointness:
- a. S → aSb | bAA
- Passes: FIRST(aSb) = {a}, FIRST(bAA) = {b}
- b. A → b{aB} | a
- Passes: FIRST(b{aB}) = {b}, FIRST(a) = {a}
- c. B → aB | a
- Fails: FIRST(aB) = {a}, FIRST(a) = {a}
- a. S → aSb | bAA
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Description
Explore the key concepts of lexical analysis and parsing strategies in this quiz covering Chapter 4 of Compiler Design. Test your understanding of fundamental terms like top-down parsers and the LL Algorithm, along with their definitions and importance in the compilation process.
