Yacc/Bison tool overview in Compiler Design - Mini Project: Build & Apply

Yacc/Bison Tool Overview

📖 Scenario: You are learning about tools that help build compilers. Yacc and Bison are popular tools used to create parsers, which understand the structure of programming languages.Imagine you want to build a simple calculator that can read and understand math expressions. Yacc/Bison can help you define the rules for these expressions and generate code to process them.

🎯 Goal: Build a simple Yacc/Bison project outline that shows how to define tokens, grammar rules, and actions to parse arithmetic expressions.

📋 What You'll Learn

Create a list of tokens used in arithmetic expressions

Define a precedence configuration for operators

Write grammar rules for expressions using Yacc/Bison syntax

Add actions to compute the result of expressions

💡 Why This Matters

🌍 Real World

Yacc and Bison are used to build parsers for programming languages, calculators, and data formats by defining grammar and actions.

💼 Career

Understanding Yacc/Bison is important for compiler developers, language designers, and software engineers working on interpreters or custom data processors.

1

Define tokens for arithmetic expressions

Create a list called tokens that includes these exact tokens: NUMBER, PLUS, MINUS, MULTIPLY, and DIVIDE.

Compiler Design

# Define tokens list
# Your code here

Hint

Tokens represent the basic symbols like numbers and operators that the parser will recognize.

2

Set operator precedence and associativity

Create a variable called precedence that defines operator precedence and associativity as a list of tuples. Include these exact entries in order: ('left', 'PLUS', 'MINUS') and ('left', 'MULTIPLY', 'DIVIDE').

Compiler Design

tokens = ['NUMBER', 'PLUS', 'MINUS', 'MULTIPLY', 'DIVIDE']
# Define operator precedence and associativity
# Your code here

Hint

Precedence tells the parser which operators to evaluate first. Associativity tells how to group operators of the same precedence.

3

Write grammar rules for expressions

Define a dictionary called grammar_rules with these exact keys and values: 'expression : expression PLUS expression', 'expression : expression MINUS expression', 'expression : expression MULTIPLY expression', 'expression : expression DIVIDE expression', and 'expression : NUMBER'.

Compiler Design

tokens = ['NUMBER', 'PLUS', 'MINUS', 'MULTIPLY', 'DIVIDE']
precedence = [
    ('left', 'PLUS', 'MINUS'),
    ('left', 'MULTIPLY', 'DIVIDE')
]
# Define grammar rules for expressions
# Your code here

Hint

Grammar rules define how expressions are built from tokens and other expressions.

4

Add actions to compute expression results

Update the grammar_rules dictionary so each key maps to a string describing the action to compute the result. Use these exact values: 'p[0] = p[1] + p[3]' for expression PLUS expression, 'p[0] = p[1] - p[3]' for expression MINUS expression, 'p[0] = p[1] * p[3]' for expression MULTIPLY expression, 'p[0] = p[1] / p[3]' for expression DIVIDE expression, and 'p[0] = p[1]' for expression NUMBER.

Compiler Design

tokens = ['NUMBER', 'PLUS', 'MINUS', 'MULTIPLY', 'DIVIDE']
precedence = [
    ('left', 'PLUS', 'MINUS'),
    ('left', 'MULTIPLY', 'DIVIDE')
]
grammar_rules = {
    'expression : expression PLUS expression': None,
    'expression : expression MINUS expression': None,
    'expression : expression MULTIPLY expression': None,
    'expression : expression DIVIDE expression': None,
    'expression : NUMBER': None
}
# Add actions to grammar rules
# Your code here

Hint

Actions tell the parser what to do when it recognizes a rule. Here, they compute the value of the expression.

Practice

1. What is the primary purpose of the Yacc/Bison tool in compiler design?

easy

A. To optimize machine code for faster execution

B. To generate graphical user interfaces

C. To convert grammar rules into code that parses language syntax

D. To manage memory allocation during program execution

Solution

1. Step 1: Understand Yacc/Bison role

   Yacc/Bison are tools that take grammar rules and produce code that can parse input based on those rules.

2. Step 2: Compare options with purpose

   Only To convert grammar rules into code that parses language syntax describes converting grammar rules into parsing code, which matches Yacc/Bison's function.

3. Final Answer:

   To convert grammar rules into code that parses language syntax -> Option C

4. Quick Check:

   Yacc/Bison = grammar to parser code [OK]

Hint: Yacc/Bison turn grammar into code that reads language [OK]

Common Mistakes:

• Confusing Yacc/Bison with code optimizers
• Thinking Yacc/Bison create user interfaces
• Assuming Yacc/Bison handle memory management

2. Which of the following is the correct way to define a token in a Yacc/Bison grammar file?

easy

A. %token NUMBER

B. token NUMBER;

C. #define NUMBER

D. var NUMBER;

Solution

1. Step 1: Recall Yacc/Bison token syntax

   Tokens in Yacc/Bison are declared using the %token directive followed by the token name.

2. Step 2: Check each option

   Only %token NUMBER uses the correct %token syntax. Others are invalid in Yacc/Bison files.

3. Final Answer:

   %token NUMBER -> Option A

4. Quick Check:

   Token declaration uses %token [OK]

Hint: Tokens start with %token keyword in Yacc/Bison [OK]

Common Mistakes:

• Using C or other language syntax for tokens
• Omitting the % sign before token
• Confusing token declaration with variable declaration

3. Given the following Yacc/Bison grammar snippet:

%token NUMBER
%%
expr: expr '+' NUMBER { printf("Sum\n"); };

What will happen when parsing the input "3 + 4"?

medium

A. The parser will produce a syntax error

B. The parser will print "Sum" twice

C. The parser will print nothing

D. The parser will print "Sum" once

Solution

1. Step 1: Analyze grammar rule

   The rule expr: expr '+' NUMBER triggers action printing "Sum" when matched.

2. Step 2: Parse input "3 + 4"

   Input matches expr '+' NUMBER once, so action runs once printing "Sum".

3. Final Answer:

   The parser will print "Sum" once -> Option D

4. Quick Check:

   One matching rule triggers one print [OK]

Hint: Each matched rule action runs once per match [OK]

Common Mistakes:

• Assuming multiple prints for single input
• Thinking input causes syntax error
• Ignoring action code in grammar

4. Identify the error in this Yacc/Bison grammar snippet:

%token NUMBER
%%
expr: expr '+' NUMBER { printf("Sum\n"); }
    | NUMBER;

medium

A. Incorrect token declaration syntax

B. Missing semicolon after first rule action

C. Missing %start directive

D. Using printf inside action is invalid

Solution

1. Step 1: Check rule syntax

   Each grammar rule must end with a semicolon. The first rule lacks a semicolon after the action block.

2. Step 2: Verify other parts

   Token declaration is correct, %start is optional if default is used, and printf is allowed in actions.

3. Final Answer:

   Missing semicolon after first rule action -> Option B

4. Quick Check:

   Rules end with semicolon [OK]

Hint: Grammar rules must end with semicolon [OK]

Common Mistakes:

• Forgetting semicolon after action block
• Confusing token syntax errors
• Assuming printf is disallowed in actions

5. You want to extend a Yacc/Bison grammar to parse simple arithmetic expressions with addition and multiplication. Which approach correctly handles operator precedence?

hard

A. Define separate rules for addition and multiplication and declare precedence using %left and %right directives

B. Use only one rule combining '+' and '*' without precedence declarations

C. Declare all tokens as %token without precedence directives

D. Handle precedence in the lexer instead of grammar

Solution

1. Step 1: Understand operator precedence in Yacc/Bison

   Yacc/Bison uses %left, %right, and %nonassoc directives to declare operator precedence and associativity.

2. Step 2: Evaluate options

   Define separate rules for addition and multiplication and declare precedence using %left and %right directives correctly uses separate rules and precedence declarations. Options B and C ignore precedence, causing incorrect parsing. Handle precedence in the lexer instead of grammar is incorrect because lexer does not handle precedence.

3. Final Answer:

   Define separate rules for addition and multiplication and declare precedence using %left and %right directives -> Option A

4. Quick Check:

   Precedence via %left/%right = correct parsing [OK]

Hint: Use %left/%right to set operator precedence in grammar [OK]

Common Mistakes:

• Ignoring precedence causing wrong parse trees
• Trying to handle precedence in lexer
• Declaring tokens without precedence directives