Lex and Yacc

1. Compiler Design
Presentation
Prepared by: Aniket Mali (12202040501007)
Daiwik Korat (12202040501012)
Dhyan Shah (12202040501023)
Academic Year: 2024-25
Batch: 1A04
Mini programming
Language parser using
Lex & Yacc

2. Introduction
 What is a Parser?
• A parser analyzes source code and checks if it follows language
grammar.
 Why use Lex & Yacc?
• Lex: Generates scanner (tokenizer).
• Yacc: Generates parser (syntax analyzer).
 Mini Programming Language Concept:
• A simplified C-like language designed for learning compiler
construction.
• Easier to implement but models real-world compiler behavior.

3. Motivation
3
 Why Compiler Construction?
• Every programming language needs a compiler/interpreter.
• Understanding compilers helps in language design and optimization.
 Importance of Scanners & Parsers:
• Scanner (Lex): Breaks raw input into tokens (words with meaning).
• Parser (Yacc): Ensures tokens follow the grammar rules.
 Why Mini-Languages?
• Easier to design and understand.
• A stepping stone towards building real compilers.

4. Role of Lex and Yacc
4
 Lex:
• Converts input source code into a sequence of tokens.
• Uses regular expressions to define patterns.
 Yacc:
• Uses context-free grammar to validate syntax.
• Builds parse tree for semantic meaning.
 Workflow:
• Source Code → Lex (Scanner) → Tokens → Yacc (Parser) → Parse Tree

5. Problem Objectives
5
• Design and implement a parser for a C-like mini language.
 Features supported:
• Declarations: int x;, float y;
• Assignments: x = 5;, y = 2.5 * 3.0;
• If-Else Statements: if (x > 0) x = x - 1;
• Switch-Case Constructs:
• Return Statements: return x;

6. Language Features
6
 Data Types:
• int → integer variables.
• float → floating-point variables.
 Control Flow:
• if, else for decisions.
• switch, case, default for multiple choices.
• return for function exit.
 Identifiers:
• User-defined variable names like x, y, sum.
 Operators:
• Arithmetic: +, -, *
• Assignment: =
• Relational: >, <, ==
• Delimiters: ;, {, }, (, )
 Constants:
• Integers (NUM) → e.g., 10, 25
• Floating-point (FLOAT_NUM) → e.g., 3.14, 2.0

7. 7
 Purpose: Define lexical tokens using regex.
 Token Categories:
• Keywords → int, float, if, else, switch, return, case, default.
• Identifiers → [a-zA-Z][a-zA-Z0-9]*
• Numbers → Integers [0-9]+, Floats [0-9]+.[0-9]+
 Example Mapping:
• "int" → INT
• "float" → FLOAT
• 123 → NUM
• 3.14 → FLOAT_NUM
Lex(Scanner) Overview

8. 8
 Purpose:
• Check if sequence of tokens follows grammar.
• Define syntax in CFG style.
 Rules Example:
• decl → type ID ;
• assign → ID = expr ;
• if_stmt → IF ( cond ) stmt [ELSE stmt]
• switch_stmt → SWITCH (ID) { case_list }
• return_stmt → RETURN expr ;
 Output:
• Produces Parse Tree if syntax is valid.
Yacc(Parser) overview

9. 9
Lex Code Snippet

10. 10
Yacc Code Snippet

11. Yacc Code Snippet
11

12. Error Handling
12
 Examples of Errors:
• int ; → Error: missing identifier.
• x = ; → Error: missing expression.
• switch { case: } → Error: missing condition.
 Error Recovery in Yacc:
• Use special error token to capture invalid input.
• Allows parser to continue instead of terminating.

13. Example run
• Input:
• Output:

14. Applications
14
 Education:
• Teaching compiler construction concepts.
 Programming Tools:
• Syntax checking in IDEs.
• Custom interpreters for small languages.
 Industry:
• Designing Domain-Specific Languages (DSLs).
• Validation of config files or query languages.
 Research:
• Foundation for building optimized compilers.

15. 15
 Conclusion:
• Lex + Yacc simplify compiler front-end design.
• Mini programming language parser successfully implemented.
 Supports: Declarations, Assignments, If-Else, Switch-Case, Return, Integers & Floats.
 Future Enhancements:
• Loops (while, for).
• Functions & parameter passing.
• Arrays and pointers.
• Error diagnostics and semantic checks.
Conclusion & Future Scope

16. 16
Thank
You!