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Documentation Index

Fetch the complete documentation index at: https://mintlify.com/jcomte23/Python_vanilla/llms.txt

Use this file to discover all available pages before exploring further.

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Loops in Python

Loops allow you to execute code repeatedly. Python provides two main loop types: for loops for iterating over sequences and while loops for conditional repetition.

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For Loops

The for loop iterates over sequences (lists, tuples, strings, ranges, etc.).

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Basic For Loop

# Iterate over a list
frutas = ["manzana", "banana", "naranja"]
for fruta in frutas:
    print(fruta)

# Output:
# manzana
# banana
# naranja

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Using range()

The range() function generates a sequence of numbers: 
# range(n) generates numbers from 0 to n-1
for contador in range(5):
    print(contador)
# Output: 0, 1, 2, 3, 4

# range(start, stop)
for i in range(2, 6):
    print(i)  # 2, 3, 4, 5

# range(start, stop, step)
for i in range(0, 10, 2):
    print(i)  # 0, 2, 4, 6, 8

# Backwards
for i in range(5, 0, -1):
    print(i)  # 5, 4, 3, 2, 1

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Practical Example: Building a List

Here’s a real example collecting user input: 
listaNumeros = []

# Collect 5 numbers from user
for numeroVuelta in range(5):
    numero = int(input(f"ingrese un numero # {numeroVuelta}=>"))
    listaNumeros.append(numero)
    
print(listaNumeros)
Use f-strings (f”text ”) to include variables in strings - it’s more readable than concatenation.

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While Loops

The while loop continues as long as a condition is true:

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Basic While Loop

listaNumeros = []
numeroVuelta = 0

while numeroVuelta < 5:
    numero = int(input(f"ingrese un numero # {numeroVuelta}=>"))
    listaNumeros.append(numero)
    numeroVuelta += 1  # Increment counter

print(listaNumeros)
Always ensure your while loop has a way to exit - otherwise it will run forever!

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While Loop with User Control

listaEstudiantesRiwi = []
ingresarOtroEstudiante = "si"

while ingresarOtroEstudiante == "si":
    print("Nuevo estudiante")
    nombre = input("ingrese el nombre =>")
    apellido = input("ingrese el apellido =>")
    edad = input("ingrese la edad =>")
    direccion = input("ingrese la direccion =>")
    correo = input("ingrese su correo =>")

    estudiante = {
        "nombre": nombre,
        "apellido": apellido,
        "edad": edad,
        "direccion": direccion,
        "correo": correo
    }
    
    listaEstudiantesRiwi.append(estudiante)
    respuesta = input("vas a ingresar otro estudiante? =>")
    
    if respuesta != "si":
        ingresarOtroEstudiante = False

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Iterating Over Different Data Structures

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Iterating Over Lists

miembrosDeLaFamilia = ["papa", "mama", "yo"]

for miembro in miembrosDeLaFamilia:
    print("#->", miembro.upper())

# Output:
# #-> PAPA
# #-> MAMA
# #-> YO

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Iterating Over Dictionaries

estudiante = {
    "nombre": "Carlos",
    "edad": 20,
    "carrera": "Ingeniería"
}

# Iterate over keys
for clave in estudiante:
    print(clave)

# Iterate over values
for valor in estudiante.values():
    print(valor)

# Iterate over key-value pairs
for clave, valor in estudiante.items():
    print(f"{clave}: {valor}")

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Iterating Over Strings

palabra = "Python"
for letra in palabra:
    print(letra)
# Output: P, y, t, h, o, n

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Loop Control Statements

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break - Exit Loop Early

for numero in range(10):
    if numero == 5:
        break  # Exit loop when numero is 5
    print(numero)
# Output: 0, 1, 2, 3, 4

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continue - Skip Current Iteration

for numero in range(10):
    if numero % 2 == 0:  # If even
        continue  # Skip to next iteration
    print(numero)
# Output: 1, 3, 5, 7, 9 (only odd numbers)

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else with Loops

The else clause executes when the loop completes normally (not via break): 
for numero in range(5):
    print(numero)
else:
    print("Loop completed!")

# With break - else won't execute
for numero in range(10):
    if numero == 5:
        break
    print(numero)
else:
    print("This won't print")  # Skipped because of break
The else clause with loops is uncommon but useful for search operations where you need to know if the loop completed without finding something.

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Nested Loops

Loops can be nested inside other loops: 
# Multiplication table
for i in range(1, 4):
    for j in range(1, 4):
        print(f"{i} x {j} = {i*j}")
    print("---")  # Separator between tables

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Real-World Example: Student Management

Here’s a complete example combining loops and data structures: 
listaEstudiantesRiwi = []
ingresarOtroEstudiante = "si"

# Collect student data
while ingresarOtroEstudiante == "si":
    print("Nuevo estudiante")
    nombre = input("ingrese el nombre =>")
    apellido = input("ingrese el apellido =>")
    edad = input("ingrese la edad =>")
    direccion = input("ingrese la direccion =>")
    correo = input("ingrese su correo =>")

    estudiante = {
        "nombre": nombre,
        "apellido": apellido,
        "edad": edad,
        "direccion": direccion,
        "correo": correo
    }
    
    listaEstudiantesRiwi.append(estudiante)
    respuesta = input("vas a ingresar otro estudiante? =>")
    
    if respuesta != "si":
        ingresarOtroEstudiante = False

# Display all students
for estudiante in listaEstudiantesRiwi:
    print(f"""
        Nombre=> {estudiante["nombre"]}
        Apellido=> {estudiante["apellido"]}
        Edad=> {estudiante["edad"]}
        Correo=> {estudiante["correo"]}
        Direccion=> {estudiante["direccion"]}
    """)

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Loop Patterns and Best Practices

# When you need to track iterations
for i in range(10):
    print(f"Iteration {i}")
Use when you need to know which iteration you’re on.
continuar = True
while continuar:
    # Do something
    respuesta = input("Continue? (si/no): ")
    if respuesta != "si":
        continuar = False
Use when the user controls loop termination.
total = 0
for numero in [1, 2, 3, 4, 5]:
    total += numero
print(total)  # 15
Use when building up a result across iterations.
numeros = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
pares = []
for num in numeros:
    if num % 2 == 0:
        pares.append(num)
print(pares)  # [2, 4, 6, 8, 10]
Use when filtering and collecting items.

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For vs While: When to Use

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Use FOR loops when:

  • You know how many iterations you need
  • You’re iterating over a collection
  • You’re using range() for a counted loop
# Perfect for FOR loop
for i in range(10):
    print(i)

for item in lista:
    process(item)

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Use WHILE loops when:

  • The number of iterations is unknown
  • You’re waiting for a condition to change
  • User input controls the loop
# Perfect for WHILE loop
while user_wants_to_continue:
    # do something
    response = input("Continue? ")
    user_wants_to_continue = (response == "yes")

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List Comprehensions (Advanced Loop Pattern)

A more Pythonic way to create lists: 
# Traditional loop
cuadrados = []
for x in range(10):
    cuadrados.append(x**2)

# List comprehension (more Pythonic)
cuadrados = [x**2 for x in range(10)]
print(cuadrados)  # [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]

# With condition
pares = [x for x in range(20) if x % 2 == 0]
print(pares)  # [0, 2, 4, 6, 8, 10, 12, 14, 16, 18]
List comprehensions are more concise and often faster than traditional loops for creating lists.

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Common Loop Mistakes to Avoid

Modifying list while iterating:
# ❌ Bad - modifying list during iteration
lista = [1, 2, 3, 4, 5]
for item in lista:
    if item % 2 == 0:
        lista.remove(item)  # Can skip elements!

# ✅ Good - iterate over a copy
lista = [1, 2, 3, 4, 5]
for item in lista[:]:
    if item % 2 == 0:
        lista.remove(item)

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Key Takeaways

  • For loops are for iterating over sequences
  • While loops continue until a condition becomes false
  • Use break to exit loops early
  • Use continue to skip to next iteration
  • range() generates number sequences for counting
  • Always ensure while loops have an exit condition
  • Consider list comprehensions for creating lists from loops
  • Use enumerate() when you need both index and value (see next section)
For more advanced iteration with indices, see the Enumerate page.

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