Loops

Master Python loops with for and while statements. Learn iteration patterns, loop control, and practical examples for working with collections.

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.

For Loops

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

Basic For Loop

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

# Output:
# manzana
# banana
# naranja

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() Variations

# 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

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.

While Loops

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

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!

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

Iterating Over Different Data Structures

Iterating Over Lists

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

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

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

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}")

Iterating Over Strings

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

Loop Control Statements

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

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)

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.

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

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"]}
    """)

Loop Patterns and Best Practices

Pattern: Counter Loop

# 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.

Pattern: User-Controlled Loop

continuar = True
while continuar:
    # Do something
    respuesta = input("Continue? (si/no): ")
    if respuesta != "si":
        continuar = False

Use when the user controls loop termination.

Pattern: Accumulator

total = 0
for numero in [1, 2, 3, 4, 5]:
    total += numero
print(total)  # 15

Use when building up a result across iterations.

Pattern: Filter and Process

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.

For vs While: When to Use

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)

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")

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.

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)

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.