Predictive parsing in Compiler Design - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is predictive parsing?

Predictive parsing is a top-down method of syntax analysis in compilers that uses lookahead symbols to decide which grammar rule to apply without backtracking.

beginner

What is the role of the lookahead symbol in predictive parsing?

The lookahead symbol helps the parser decide which production rule to use next by examining the next input token before making a parsing decision.

intermediate

What is an LL(1) grammar in the context of predictive parsing?

An LL(1) grammar is a type of grammar that can be parsed by a predictive parser using one lookahead symbol, reading input from Left to right, and producing a Leftmost derivation.

intermediate

Why is backtracking not needed in predictive parsing?

Because predictive parsing uses lookahead symbols and a parsing table to choose the correct production rule, it can decide the next step without trying multiple options and backtracking.

beginner

What is a parsing table in predictive parsing?

A parsing table is a two-dimensional table used by the predictive parser to decide which production rule to apply based on the current non-terminal and the lookahead symbol.

What does the '1' in LL(1) grammar stand for?

A One production rule

B One lookahead symbol

C One non-terminal

D One terminal symbol

The '1' in LL(1) means the parser uses one lookahead symbol to make parsing decisions.

Which parsing method uses a parsing table and no backtracking?

A Bottom-up parsing

B Recursive descent with backtracking

C Predictive parsing

D Shift-reduce parsing

Predictive parsing uses a parsing table and lookahead symbols to avoid backtracking.

In predictive parsing, what is the main purpose of the parsing table?

A To decide which production to use

B To store tokens

C To hold the input string

D To generate machine code

The parsing table helps the parser decide which production rule to apply based on the current input.

Predictive parsing is a type of which parsing approach?

A Top-down parsing

B Bottom-up parsing

C LR parsing

D Operator precedence parsing

Predictive parsing is a top-down parsing technique.

Which of the following is a requirement for a grammar to be parsed by predictive parsing?

A Grammar must be context-sensitive

B Grammar must be ambiguous

C Grammar must have left recursion

D Grammar must be LL(1)

Predictive parsing requires the grammar to be LL(1), meaning no ambiguity and no left recursion.

Explain how predictive parsing uses lookahead symbols to parse input without backtracking.

Think about how the parser chooses the next rule based on the next input token.

Describe the characteristics of an LL(1) grammar and why it is important for predictive parsing.

Focus on the grammar rules that make predictive parsing possible.

Practice

1. What is the main purpose of predictive parsing in compiler design?

easy

A. To optimize the runtime performance of a program

B. To analyze sentences using a stack and parsing table without backtracking

C. To generate machine code from source code

D. To translate high-level code into assembly language

Solution

1. Step 1: Understand predictive parsing function

   Predictive parsing analyzes sentences by using a stack and a parsing table to decide the next step without guessing or backtracking.

2. Step 2: Differentiate from other compiler tasks

   Generating machine code, optimizing runtime, and translating to assembly are later compiler stages, not parsing.

3. Final Answer:

   To analyze sentences using a stack and parsing table without backtracking -> Option B

4. Quick Check:

   Predictive parsing = stack + table analysis [OK]

Hint: Parsing means analyzing structure, not generating code [OK]

Common Mistakes:

• Confusing parsing with code generation
• Thinking predictive parsing guesses or backtracks
• Mixing parsing with optimization tasks

2. Which of the following is a correct component used in predictive parsing?

easy

A. Parsing table

B. Backtracking stack

C. Hash map for variables

D. Symbolic debugger

Solution

1. Step 1: Identify key components of predictive parsing

   Predictive parsing uses a parsing table and a stack to decide which grammar rule to apply next.

2. Step 2: Eliminate unrelated options

   Backtracking stack is not used because predictive parsing avoids backtracking. Hash maps and debuggers are unrelated to parsing tables.

3. Final Answer:

   Parsing table -> Option A

4. Quick Check:

   Parsing table is essential in predictive parsing [OK]

Hint: Parsing table guides decisions without guessing [OK]

Common Mistakes:

• Assuming backtracking is part of predictive parsing
• Confusing parsing table with data structures for variables
• Mixing debugging tools with parsing components

3. Given the grammar rules:
S -> a A b
A -> c | ε
and the input string acb, what will the predictive parser do at the point it reads c?

medium

A. Throw a syntax error

B. Apply rule A -> ε and skip c

C. Backtrack to try another rule

D. Apply rule A -> c and consume c

Solution

1. Step 1: Check the input and grammar at A

   The input symbol is c. The grammar allows A -> c or A -> ε.

2. Step 2: Use predictive parsing table logic

   Since the next input is c, the parser chooses A -> c to match the input symbol and consume it.

3. Final Answer:

   Apply rule A -> c and consume c -> Option D

4. Quick Check:

   Input matches rule A -> c [OK]

Hint: Match input symbol with rule start, no backtracking [OK]

Common Mistakes:

• Choosing epsilon rule when input symbol matches
• Thinking parser backtracks on mismatch
• Assuming syntax error without checking rules

4. Identify the error in this predictive parsing step:
Grammar: S -> a S b | ε
Input: a a b b
Parser applies S -> ε immediately at the start.

medium

A. Parser should backtrack after epsilon

B. Input string is invalid for the grammar

C. Incorrect application of epsilon rule too early

D. No error, parsing is correct

Solution

1. Step 1: Analyze grammar and input

   The grammar allows S -> a S b or S -> ε. The input starts with a, so epsilon should not be chosen immediately.

2. Step 2: Understand predictive parsing rules

   Predictive parsing uses lookahead to decide rules. Since input is a, it should apply S -> a S b first, not epsilon.

3. Final Answer:

   Incorrect application of epsilon rule too early -> Option C

4. Quick Check:

   Epsilon only if input doesn't match other rules [OK]

Hint: Check input symbol before choosing epsilon rule [OK]

Common Mistakes:

• Applying epsilon rule without checking input
• Assuming input is invalid without parsing
• Expecting backtracking in predictive parsing

5. You have a grammar with rules:
E -> T E'
E' -> + T E' | ε
T -> int
How would you construct the predictive parsing table entry for non-terminal E' and input symbol +?

hard

A. Use rule E' -> + T E' because input is '+'

B. Use rule E' -> ε because '+' is not in T

C. No entry needed because '+' is terminal

D. Use rule E' -> T E' to start parsing

Solution

1. Step 1: Identify first sets for E'

   The first set of E' includes '+' because of the rule E' -> + T E'.

2. Step 2: Match input symbol '+' with rule

   Since input is '+', the parser uses the rule E' -> + T E' in the parsing table for E' and '+'.

3. Final Answer:

   Use rule E' -> + T E' because input is '+' -> Option A

4. Quick Check:

   Input '+' matches E' -> + T E' [OK]

Hint: Match input with first set of non-terminal rules [OK]

Common Mistakes:

• Choosing epsilon rule when input matches '+'
• Confusing terminals with non-terminals in table
• Ignoring first sets in table construction