Predictive parsing in Compiler Design - Mini Project: Build & Apply

Building a Simple Predictive Parser

📖 Scenario: You are learning how a predictive parser works in compiler design. Predictive parsing uses a parsing table and a stack to decide which grammar rule to apply next without backtracking.Imagine you want to parse simple arithmetic expressions with addition and multiplication using a predictive parser.

🎯 Goal: Build a step-by-step predictive parser setup that includes the grammar rules, the parsing table, the input string, and the parsing stack operations.

📋 What You'll Learn

Define the grammar rules as a dictionary with exact keys and values

Create a parsing table as a nested dictionary with exact entries

Set the input string as a list of tokens with exact values

Simulate one step of the predictive parsing process using the stack and parsing table

💡 Why This Matters

🌍 Real World

Predictive parsing is used in compilers to analyze source code and check if it follows the grammar rules of a programming language.

💼 Career

Understanding predictive parsing helps in compiler development, language design, and building tools that process structured text.

1

Define the Grammar Rules

Create a dictionary called grammar with these exact entries: 'E': ['T E\''], 'E\'': ['+ T E\'', 'ε'], 'T': ['F T\''], 'T\'': ['* F T\'', 'ε'], 'F': ['( E )', 'id'].

Compiler Design

# Define the grammar rules dictionary
# Your code here

Hint

Use a Python dictionary with non-terminals as keys and lists of productions as values.

2

Create the Parsing Table

Create a nested dictionary called parsing_table with these exact entries: parsing_table['E']['id'] = 'T E\'', parsing_table['E']['('] = 'T E\'', parsing_table['E\'']['+'] = '+ T E\'', parsing_table['E\''][')'] = 'ε', parsing_table['T']['id'] = 'F T\'', parsing_table['T']['('] = 'F T\'', parsing_table['T\'']['+'] = 'ε', parsing_table['T\'']['*'] = '* F T\'', parsing_table['T\''][')'] = 'ε', parsing_table['F']['id'] = 'id', parsing_table['F']['('] = '( E )'.

Compiler Design

grammar = {
    'E': ['T E\''],
    'E\'': ['+ T E\'', 'ε'],
    'T': ['F T\''],
    'T\'': ['* F T\'', 'ε'],
    'F': ['( E )', 'id']
}

# Create the parsing table dictionary
# Your code here

Hint

Use a nested dictionary where the first key is the non-terminal and the second key is the terminal symbol.

3

Set the Input String

Create a list called input_tokens with these exact values in order: 'id', '+', 'id', '*', 'id'.

Compiler Design

grammar = {
    'E': ['T E\''],
    'E\'': ['+ T E\'', 'ε'],
    'T': ['F T\''],
    'T\'': ['* F T\'', 'ε'],
    'F': ['( E )', 'id']
}

parsing_table = {
    'E': {'id': 'T E\'', '(': 'T E\''},
    'E\'': {'+': '+ T E\'', ')': 'ε'},
    'T': {'id': 'F T\'', '(': 'F T\''},
    'T\'': {'+': 'ε', '*': '* F T\'', ')': 'ε'},
    'F': {'id': 'id', '(': '( E )'}
}

# Define the input tokens list
# Your code here

Hint

Use a list with the exact tokens in the given order.

4

Simulate One Parsing Step

Create a list called stack initialized with ['$', 'E']. Then create a variable called top that stores the last element popped from stack. Finally, create a variable called current_input that stores the first element of input_tokens.

Compiler Design

grammar = {
    'E': ['T E\''],
    'E\'': ['+ T E\'', 'ε'],
    'T': ['F T\''],
    'T\'': ['* F T\'', 'ε'],
    'F': ['( E )', 'id']
}

parsing_table = {
    'E': {'id': 'T E\'', '(': 'T E\''},
    'E\'': {'+': '+ T E\'', ')': 'ε'},
    'T': {'id': 'F T\'', '(': 'F T\''},
    'T\'': {'+': 'ε', '*': '* F T\'', ')': 'ε'},
    'F': {'id': 'id', '(': '( E )'}
}

input_tokens = ['id', '+', 'id', '*', 'id']

# Initialize the stack and simulate one parsing step
# Your code here

Hint

Use a list for the stack starting with '$' and 'E'. Pop the top element and store it in top. Access the first token from input_tokens as current_input.

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