Compiled vs. Interpreted

When you do programming, your computer does not directly understand your program. For the computer to understand it, your program needs to be translated into machine language first. This translation is handled by either a compiler or and interpreter.

This difference determines how fast programs run, how easy they are to debug, and which languages are better for certain tasks than others. After this article you will know:

• What compilation and interpretation actually mean
• Basic knowledge about how compilers and interpreters operate
• Important pros and cons of them
• Which popular languages use which method

What does compilation and interpretation actually mean?

• Compiler: A compiler takes the entire source code and converts it into machine code before the program runs. This process checks for errors and optimizes the code for better performance. Once compiled, the program can be executed directly by the processor. making it very fast. The program only needs to be compiled once: The binary that is output by the compiler can be run over and over again.

Pro	Contra
Program is executed faster.	Debugging can be more complicated.
Programs can be executed without installing additional components / dependencies.	Specifically needs to be compiled for every platform.
Ability to distribute compiled executables without exposing the source code.	Big programs take long time to compile.
Less overhead when executed because the translation is already done.	Changes made to the program require recompilation.

• Interpreter: An interpreter reads and executes code line by line, translating each instruction into machine code just before running it. If the interpreter notices an error, it stops immediately, which makes debugging easier. But because the program is interpreted every time it is executed, it takes longer to start.

How do compilers and interpreters operate?

Compiler:

A compiler reads your source code, checks it for correctness (both syntactically and semantically), optimizes it, and foutputs a standalone executable.

• Lexical Analysis: This is the first step. The compiler breaks the source code up into tokens / chunks like keywords (int, return), identifiers (variable names), operators (+, *), etc. Basically scanning the code to recognize each component.
• Syntax Analysis / Parsing: For the second step, the compiler checks the structure of the code and builds a syntax tree (or parse tree) based on grammar rules of the language. For example, it verifies that loops, functions, and conditionals are formed properly. E.g. if there is a semicolon missing or an if/else statement is not done right, the compiler will throw an error.
• Semantic Analysis: The compiler now checks whether the code actually makes sense. It verifies things like:
  • Is the program using variables before declaring them?
  • Are function calls using the correct arguments?
  • etc.
• Intermediate Code Generation and Optimization: The compiler then converts the syntax tree into an intermediate form, which is kind of a halfway language between the source code and machine code. During this step, it also optimizes your program, improving efficiency without changing what it does by simplifying expressions, inline functions, or removing unused variables.
• Code Generation: Now the compiler produces actual machine code, suited for the specific CPU and operating system. This is where your ".c" file turns into a ".o" or ".obj" file (an object file), which contains low-level instructions, almost ready to be executed.
• Linking: This is the final step. Most programs are not isolated. They rely on libraries (like the C standard library) or are split into multiple source files. The linker is the tool that takes all these pieces, the object files, any libraries the program is using, and some startup code the OS needs. It puts them together into one final, executable file (like a.out on Linux or program.exe on Windows).

Interpreter:

An interpreter reads your source code and executes it line by line. It performs many of the same checks as a compiler but instead of giving the user a binary file, it runs the source directly.

• Lexical Analysis: This is the first step. The interpreter breaks the source code into tokens / keywords, variable names, literals, operators, etc. Just like a compiler, scans the code to identify its components.
• Syntax Analysis / Parsing: Next, the interpreter parses the tokens into a syntax tree or an abstract syntax tree (AST), ensuring the code follows the syntactical rules of the language. IE.g. if you forget a semicolon or have a wrong loop, the interpreter will throw a syntax error.
• Semantic Analysis: The interpreter then checks if the code makes logical sense. It verifies:
  • Are all variables declared before use?
  • Are functions called with the right number/type of arguments?
  • Does the code follow type rules (e.g., adding numbers to strings)?
• Interpretation / Execution: This is where the main difference to a compiler is. Instead of converting the whole program into machine code, the interpreter directly executes the syntax tree or another intermediate form, instruction by instruction. This means it translates and runs the code on the fly, without creating a separate executable.
• Runtime Handling: Since the code is executed dynamically, runtime errors (e.g., division by zero, accessing undefined variables) are detected and thrown while the code runs. This is why interpreted languages often feel more flexible but may have slower performance.

JIT: Just-in-Time compilation:

Just-In-Time (JIT) compilation is a hybrid approach that combines features of interpreters and compilers. Instead of compiling the whole program before or interpreting it line by line during execution, a JIT compiler begins by interpreting the code or executing it in an intermediate form, such as bytecode. While the program runs, the system identifies frequently used sections—called "hot spots". Then it compiles only those hot spots into optimized machine code while the program is still running. A popular example for this is Java's JVM (Java Virtual Machine).

Which popular programming languages use which method?