BASIC Compiler

Overview

This project is a compiler for a simple interpreted language inspired by BASIC. The compiler reads a source file written in this language and generates equivalent C++ code.

Features

• Lexical analysis using a custom lexer (Lexer class).
• Parsing with a recursive descent parser (Parser class).
• Code emission to generate valid C++ output (Emitter class).
• Supports basic arithmetic operations, variable assignments, conditional statements, loops, and input/output operations.
• Error handling for invalid syntax and undeclared labels.

Supported Language Constructs

• Variable Declaration and Assignment:

  LET a = 10
  

• Input and Output:

  PRINT "Hello, world!"
  INPUT x
  

• Conditional Statements:

  IF x > 10 THEN
      PRINT "X is greater than 10"
  ENDIF
  

• Loops:

  WHILE x < 5 REPEAT
      PRINT x
      LET x = x + 1
  ENDWHILE
  

  FOR i TO 5
    PRINT i
  ENDFOR
  

• Goto and Labels:

  LABEL start
  PRINT "Looping"
  GOTO start
  

Project Structure

.
├── basic.py        # Main compiler driver
├── lex.py          # Lexer implementation
├── parse.py        # Parser implementation
├── emit.py         # Code emitter for generating C++ output
└── README.md       # Project documentation

Installation & Usage

Prerequisites

• Python 3.x
• C++ compiler (e.g., g++)

Running the Compiler

python basic.py <sourcefile>

This generates an out.cpp file containing the compiled C++ code.

Compiling the Generated Code

g++ out.cpp -o output
./output

Example Program

BASIC Source Code

# Computer average of numbers
LET a = 0
WHILE a < 1 REPEAT
    PRINT "Enter number of scores: "
    INPUT a
ENDWHILE

LET b = 0
LET s = 0
PRINT "Enter one value at a time: "
WHILE b < a REPEAT
    INPUT c
    LET s = s + c
    LET b = b + 1
ENDWHILE

PRINT "Average: "
PRINT s / a

Generated C++ Code

#include <iostream>
using namespace std;

int main() {
    double a = 0;
    while (a < 1) {
        cout << "Enter number of scores: " << endl;
        cin >> a;
    }

    double b = 0;
    double s = 0;
    cout << "Enter one value at a time: " << endl;
    while (b < a) {
        double c;
        cin >> c;
        s = s + c;
        b = b + 1;
    }
    cout << "Average: " << endl;
    cout << s / a << endl;
    return 0;
}