How to Solve Assignments on Building a Parser for a Mini C-Like Language

Before diving into the coding process, it is crucial to grasp the assignment's expectations. Understanding the grammar, the role of different components, and how the provided files interact is essential for successfully implementing the parser.

Reviewing the Provided Grammar

Most assignments of this nature provide an Extended Backus-Naur Form (EBNF) grammar. This grammar dictates the structure of the Mini C-Like Language (MCL). A thorough review of the provided grammar will help in structuring the parser correctly. Key grammar components include:

• Program Structure: The parser typically starts parsing from a root production rule, such as Prog ::= PROGRAM IDENT CompStmt. This defines the main structure of the program.
• Statements: These include declaration statements, assignment statements, control flow statements (if, while, etc.), and compound statements enclosed within {}.
• Expressions and Operators: Handling arithmetic expressions, relational expressions, logical expressions, and ensuring proper precedence and associativity of operators.

Understanding Parser Expectations

The primary objective of an MCL parser is to:

• Read and process tokens generated by the lexical analyzer.
• Match tokens with grammar rules and build a parse tree.
• Detect syntax errors and provide meaningful error messages.
• Ensure correct precedence and associativity when parsing expressions.
• Handle compound statements and nested control structures correctly.

Identifying the Provided Resources

Many assignments come with partially implemented files or starter templates. These typically include:

• parser.h – Header file containing function declarations for parsing routines.
• GivenParserPart.cpp – A partially implemented parser file requiring completion.
• lex.h and lex.cpp – Lexical analyzer responsible for tokenization.
• prog2.cpp – A test driver to execute the parser.

Familiarizing yourself with these files will help you understand what has already been implemented and what needs to be completed.

Implementing the Recursive-Descent Parser

Once the grammar is clear, the next step is to implement the recursive-descent parser. This involves writing functions that correspond to the non-terminals in the grammar.

Structuring Parser Functions

Each non-terminal in the grammar should have a dedicated function in the parser. Commonly required functions include:

• Prog(): Parses the overall program structure.
• StmtList(): Parses a sequence of statements.
• Stmt(): Parses individual statements such as declarations, assignments, and control structures.
• Expr(): Parses expressions while ensuring correct precedence.
• CompStmt(): Handles compound statements enclosed within {}.

Handling Statement Parsing

Parsing Declaration Statements

Declaration statements define variables and assign them types. The parser must ensure proper syntax and type specification.

void DeclStmt() { Token t = GetNextToken(); if (t != INT && t != FLOAT && t != BOOL && t != CHAR && t != STRING) { ParserError("Invalid Declaration Statement."); return; } VarList(); Expect(';'); }

Parsing If Statements

If-statements introduce conditional branching. The parser must ensure that conditions are enclosed in parentheses and statements follow correct syntax.

void IfStmt() { Expect(IF); Expect('('); Expr(); Expect(')'); Stmt(); if (PeekToken() == ELSE) { GetNextToken(); Stmt(); } }

Parsing Compound Statements

Compound statements group multiple statements within curly braces.

void CompStmt() { Expect('{'); StmtList(); Expect('}'); }

Handling Expressions and Operator Precedence

Expressions in MCL involve different precedence levels. Implementing a correct precedence parser ensures that expressions evaluate as expected.

Parsing Logical and Relational Expressions

Logical AND and OR

void LogANDExpr() { EqualExpr(); while (PeekToken() == AND) { GetNextToken(); EqualExpr(); } }

Relational Operators

Relational operators define comparisons.

void RelExpr() { AddExpr(); if (PeekToken() == '<' || PeekToken() == '>') { GetNextToken(); AddExpr(); } }

Handling Arithmetic Expressions

Addition and Subtraction

void AddExpr() { MultExpr(); while (PeekToken() == '+' || PeekToken() == '-') { GetNextToken(); MultExpr(); } }

Multiplication, Division, and Modulus

void MultExpr() { UnaryExpr(); while (PeekToken() == '*' || PeekToken() == '/' || PeekToken() == '%') { GetNextToken(); UnaryExpr(); } }

Implementing Error Handling and Debugging

A parser must detect syntax errors and report them effectively.

Error Reporting Mechanisms

Basic Error Reporting

void ParserError(string msg) { cout << "Error at line " << lineNumber << ": " << msg << endl; }

Detecting Missing Symbols

if (token != ';') { ParserError("Missing semicolon at end of statement."); }

Debugging Strategies

• Printing tokens while parsing helps track progress.
• Using test cases ensures correctness.
• Comparing expected vs. actual output identifies discrepancies.

Testing and Validation

Thorough testing ensures the parser functions correctly.

Running Provided Test Cases

A well-implemented parser should:

• Parse correct programs successfully.
• Detect syntax errors and provide informative messages.

Edge Case Handling

Consider scenarios such as:

• Nested if-statements.
• Incorrect operator usage.
• Missing delimiters.

Conclusion

Building a parser for a Mini C-Like Language is a complex but rewarding task. By analyzing grammar rules, implementing recursive-descent parsing, managing operator precedence, and handling errors effectively, you can create a robust parser. Testing thoroughly and debugging systematically will enhance its reliability. Mastering these skills prepares you for advanced compiler construction tasks in the future.