Control Flow & Loops

Control flow lets you make decisions in your code. Loops let you repeat actions.

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Conditional Statements

Basic if/elif/else

status_code = 404

if status_code == 200:
    print("OK")
elif status_code == 404:
    print("Not Found")
elif status_code >= 500:
    print("Server Error")
else:
    print("Unknown status")

Comparison Operators

Operator	Meaning
==	Equal to
!=	Not equal to
<	Less than
>	Greater than
<=	Less than or equal
>=	Greater than or equal

Logical Operators

age = 25
has_license = True

if age >= 18 and has_license:
    print("Can drive")

if age < 18 or not has_license:
    print("Cannot drive")

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Ternary Expression

A short way to write a simple if/else:

status = "adult" if age >= 18 else "minor"

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For Loops

Use for to iterate over a sequence:

fruits = ["apple", "banana", "cherry"]

for fruit in fruits:
    print(fruit)

Loop with Index

for index, fruit in enumerate(fruits):
    print(f"{index}: {fruit}")

Loop over a Range

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

for i in range(1, 6):    # 1, 2, 3, 4, 5
    print(i)

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

Loop over a Dictionary

user = {"name": "Alice", "age": 30}

for key, value in user.items():
    print(f"{key}: {value}")

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While Loops

Use while when you do not know how many times to repeat:

count = 0
while count < 5:
    print(count)
    count += 1

Avoid infinite loops

Always make sure the condition will become False at some point.

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Loop Control

break — Stop the Loop

for number in range(10):
    if number == 5:
        break
    print(number)  # prints 0, 1, 2, 3, 4

continue — Skip to Next Iteration

for number in range(5):
    if number == 2:
        continue
    print(number)  # prints 0, 1, 3, 4

pass — Do Nothing (Placeholder)

for item in range(5):
    pass  # will add logic later

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List Comprehensions

A short way to create lists from loops:

squares = [x ** 2 for x in range(5)]
# [0, 1, 4, 9, 16]

even_numbers = [x for x in range(10) if x % 2 == 0]
# [0, 2, 4, 6, 8]

Keep comprehensions simple

If a comprehension is hard to read, use a regular for loop instead.

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Nested Loops

matrix = [[1, 2], [3, 4], [5, 6]]

for row in matrix:
    for item in row:
        print(item, end=" ")
    print()

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Best Practices

• Use for loops when you know the number of iterations
• Use while loops when the end condition is dynamic
• Prefer enumerate() over manual index tracking
• Keep loops simple — extract complex logic into functions
• Use list comprehensions for simple transformations
• Avoid deep nesting (more than 2-3 levels)