Core Concepts

Variables, Constants & Mutability

Interactive multi-language comparison: C, JavaScript, Python, and Java.

1. Memory, Identifiers & Values

Every variable or constant is a RAM memory location storing a value under a named identifier.

Identifier: The symbolic name in code (e.g. score).

Address: Physical RAM location (e.g. 0x7ffd9).

Value: The binary data stored inside.

SIMULATED RAM VISUALIZER

0x7FFD01 score (int) 100

0x7FFD05 MAX_LIMIT (const) 999 🔒

0x7FFD09 user_name (string) "Facundo"

2. C: Static Typing & Direct Memory

In C, exact data types are declared before use. The compiler allocates fixed byte space.

int / float: Modifiable variables anytime.
const: Compiler instruction forbidding reassignment.

main.c

#include <stdio.h>

int main() {
    int contador = 0;        // Variable mutable
    const float PI = 3.14159f; // Constante inmutable

    contador = 10;          // OK: Cambia valor
    // PI = 3.0;            // Error de compilación GCC
    return 0;
}

3. JavaScript: var, let, const & References

JS uses dynamic typing. const prevents variable reassignment, but DOES NOT freeze object properties.

let: Block-scoped variable.

const (Primitivo): Completely immutable.

const (Objeto): Reference is fixed, but object properties can mutate.

app.js

let edad = 25;
edad = 26; // OK

const config = { tema: 'oscuro' };
config.tema = 'claro'; // ¡Mutación permitida!
// config = {};       // TypeError en V8

Object.freeze(config); // Inmutabilidad real

4. Python: Names as Object References

In Python everything is an object. Variables are labels referencing objects in the Heap.

Immutables: int, float, str, tuple (create new object on change).
Mutables: list, dict, set (modify in-place).
typing.Final: Hint for static checkers (Mypy).

script.py

from typing import Final

MAX_USERS: Final[int] = 500

numeros = [1, 2, 3]
numeros.append(4) # Mutación in-place en Heap

texto = "Hola"
# texto[0] = "h"  # TypeError: str es inmutable

5. Java: Primitives, Objects & final

Java strictly distinguishes primitive types (stored by value) and objects (stored by heap reference).

final: Prevents variable or pointer reassignment.

String: Immutable class by design (String Pool).

Main.java

public class Main {
    public static void main(String[] args) {
        final double IVA = 0.21;
        // IVA = 0.15; // Error de compilador javac

        final StringBuilder sb = new StringBuilder("Hi");
        sb.append(" World"); // OK: Contenido muta
    }
}

6. Live Mutation Simulator

Select a language and trigger an invalid mutation:

C / GCC Compiler

const float PI = 3.14159f; PI = 3.0f; // Intentando reasignar...

COMPILER ERROR

main.c:6:5: error: assignment of read-only variable 'PI'