.. _module1_interpreter_vs_compiler:

Interpreter vs Compiler
=================================

1. Introduction
---------------

Understanding the difference between interpreters and compilers is
fundamental to learning how programming languages are executed. Python
is an interpreted language, but it’s important to know how both
approaches work.

--------------

2. What is an Interpreter?
--------------------------

- An interpreter translates and executes code **line by line**.
- It does not produce a separate machine code file; instead, it runs the
  code directly.
- Errors are reported as soon as they are encountered, making debugging
  easier for beginners.
- Example languages: Python, JavaScript, Ruby, PHP.

How Python Uses an Interpreter
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

- Python source code (``.py`` files) is executed by the Python
  interpreter.
- The interpreter converts code to an intermediate form called
  **bytecode**, which is then executed by the Python Virtual Machine
  (PVM).
- No separate compilation step is required.

--------------

3. What is a Compiler?
----------------------

- A compiler translates the entire source code into **machine code** (or
  intermediate code) before execution.
- Produces an executable file (e.g., ``.exe`` on Windows).
- Errors are reported after the entire code is analyzed.
- Example languages: C, C++, Java (compiles to bytecode, then
  interpreted by JVM).

Compilation Process
~~~~~~~~~~~~~~~~~~~

- Source code → Compiler → Machine code (executable)
- The executable can be run multiple times without recompiling.

--------------

4. Key Differences
------------------

+-----------------+--------------------------+-------------------------+
| Feature         | Interpreter              | Compiler                |
+=================+==========================+=========================+
| Translation     | Line by line             | Whole program at once   |
+-----------------+--------------------------+-------------------------+
| Output          | No separate file         | Generates executable    |
|                 |                          | file                    |
+-----------------+--------------------------+-------------------------+
| Error Detection | Stops at first error     | Reports all errors      |
|                 |                          | after analysis          |
+-----------------+--------------------------+-------------------------+
| Execution Speed | Slower (interpreted      | Faster (runs compiled   |
|                 | every time)              | code)                   |
+-----------------+--------------------------+-------------------------+
| Memory Usage    | Less (no executable      | More (stores            |
|                 | stored)                  | executable)             |
+-----------------+--------------------------+-------------------------+
| Example         | Python, JS, Ruby         | C, C++, Java            |
| Languages       |                          |                         |
+-----------------+--------------------------+-------------------------+

--------------

5. Python: Interpreted Language
-------------------------------

- Python code is executed by the interpreter, making it easy to test and
  debug.
- No need to compile before running scripts.
- Interactive mode (REPL) allows quick experimentation.

6. How Python Code is Executed
------------------------------

1. **Source Code**: You write Python code in a text file with a ``.py``
   extension.
2. **Interpreter**: When you run the code, the Python interpreter reads
   the file.
3. **Tokenization**: The interpreter breaks the code into tokens
   (keywords, operators, etc.).
4. **Parsing**: The tokens are analyzed to create a parse tree,
   representing the code structure.
5. **Bytecode Compilation**: The parse tree is compiled into bytecode, a
   low-level representation.
6. **Execution**: The bytecode is executed by the Python Virtual Machine
   (PVM).
7. **Error Handling**: If an error occurs during execution, the
   interpreter stops and displays an error message, allowing the
   programmer to fix the issue.
8. **Output**: The result of the execution is returned to the user,
   whether it’s a printed output or a returned value from a function.
9. **Garbage Collection**: Unused memory is reclaimed by the garbage
   collector, which automatically manages memory allocation and
   deallocation.

--------------

Summary
-------

- **Interpreter:** Executes code line by line, easier for beginners,
  slower execution.
- **Compiler:** Converts code to machine code before execution, faster
  but less flexible for rapid testing.
- Python uses an interpreter, which is why you can run scripts directly
  without compiling.

--------------

*Prepared by Zaid Kamil.*