Control Flow

Control flow determines the order in which statements are executed. This lesson covers conditionals and loops.

What You'll Learn

Make decisions with if, elif, else
Repeat code with for and while loops
Control loop execution with break and continue
Understand truthy and falsy values
Use the match statement (Python 3.10+)

Conditionals

Conditionals allow your program to make decisions based on conditions.

┌─────────────────────────────────────────────────────────────────┐
│                    Conditional Flow                              │
├─────────────────────────────────────────────────────────────────┤
│                                                                  │
│                    ┌──────────────┐                             │
│                    │  Condition   │                             │
│                    └──────┬───────┘                             │
│                           │                                      │
│              ┌────────────┴────────────┐                        │
│              │                         │                         │
│           True?                     False?                       │
│              │                         │                         │
│              ▼                         ▼                         │
│      ┌─────────────┐           ┌─────────────┐                  │
│      │ if block    │           │ else block  │                  │
│      │ executes    │           │ executes    │                  │
│      └─────────────┘           └─────────────┘                  │
│              │                         │                         │
│              └────────────┬────────────┘                        │
│                           ▼                                      │
│                    Continue...                                   │
│                                                                  │
└─────────────────────────────────────────────────────────────────┘

if Statement

python

age = 20

if age >= 18:
    print("You are an adult")
    print("You can vote")

if-else Statement

python

age = 15

if age >= 18:
    print("You are an adult")
else:
    print("You are a minor")

if-elif-else Statement

python

score = 85

if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
elif score >= 60:
    grade = "D"
else:
    grade = "F"

print(f"Your grade is: {grade}")

Multiple Conditions Flow

┌───────────────────────────────────────────────────────────────┐
│                                                                │
│   if score >= 90:      ──▶  True? → grade = "A" → DONE       │
│                             False? ↓                           │
│   elif score >= 80:    ──▶  True? → grade = "B" → DONE       │
│                             False? ↓                           │
│   elif score >= 70:    ──▶  True? → grade = "C" → DONE       │
│                             False? ↓                           │
│   else:                ──▶  grade = "F" → DONE                │
│                                                                │
│   Python checks conditions TOP to BOTTOM, stops at first True  │
│                                                                │
└───────────────────────────────────────────────────────────────┘

Nested Conditionals

python

age = 25
has_license = True

if age >= 18:
    if has_license:
        print("You can drive")
    else:
        print("You need a license first")
else:
    print("You're too young to drive")

Conditional Expressions (Ternary)

python

# Syntax: value_if_true if condition else value_if_false
age = 20
status = "adult" if age >= 18 else "minor"
print(status)  # adult

# Useful for simple assignments
max_value = a if a > b else b
abs_value = x if x >= 0 else -x

Truthy and Falsy Values

In Python, all values have a boolean interpretation.

Falsy Values (evaluate to False)

Value	Type	Note
`False`	bool	Explicit False
`None`	NoneType	No value
`0`	int	Zero
`0.0`	float	Zero float
`""`	str	Empty string
`[]`	list	Empty list
`{}`	dict	Empty dict
`()`	tuple	Empty tuple
`set()`	set	Empty set

python

# Everything else is truthy
if [1, 2, 3]:    # Non-empty list - truthy
    print("List has items")

if "":           # Empty string - falsy
    print("This won't print")

# Practical example
name = input("Enter name: ")
if name:  # Check if not empty
    print(f"Hello, {name}!")
else:
    print("No name entered")

Loops

Loops allow you to repeat code multiple times.

for Loop

The for loop iterates over a sequence (list, string, range, etc.).

┌─────────────────────────────────────────────────────────────────┐
│                    for Loop Anatomy                              │
├─────────────────────────────────────────────────────────────────┤
│                                                                  │
│   for fruit in ["apple", "banana", "cherry"]:                   │
│       │     │             │                                      │
│       │     │             └── Sequence to iterate over          │
│       │     └── Each item goes into this variable               │
│       └── Keyword                                                │
│                                                                  │
│   Iteration:                                                     │
│   ┌─────────────────────────────────────────────────────┐       │
│   │ Loop 1: fruit = "apple"  → print("apple")          │       │
│   │ Loop 2: fruit = "banana" → print("banana")         │       │
│   │ Loop 3: fruit = "cherry" → print("cherry")         │       │
│   │ Done!                                               │       │
│   └─────────────────────────────────────────────────────┘       │
│                                                                  │
└─────────────────────────────────────────────────────────────────┘

python

# Loop through a list
fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit)

# Loop through a string
for char in "Python":
    print(char)

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

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

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

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

enumerate()

Get both index and value when looping.

python

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

# Without enumerate
for i in range(len(fruits)):
    print(f"{i}: {fruits[i]}")

# With enumerate (cleaner!)
for index, fruit in enumerate(fruits):
    print(f"{index}: {fruit}")

# Start from different index
for index, fruit in enumerate(fruits, start=1):
    print(f"{index}: {fruit}")  # 1: apple, 2: banana...

zip()

Loop through multiple sequences together.

python

names = ["Alice", "Bob", "Charlie"]
ages = [25, 30, 35]

for name, age in zip(names, ages):
    print(f"{name} is {age} years old")

# Works with multiple sequences
cities = ["NYC", "LA", "Chicago"]
for name, age, city in zip(names, ages, cities):
    print(f"{name}, {age}, from {city}")

while Loop

The while loop repeats as long as a condition is True.

┌─────────────────────────────────────────────────────────────────┐
│                    while Loop Flow                               │
├─────────────────────────────────────────────────────────────────┤
│                                                                  │
│            ┌─────────────────┐                                  │
│            │ Check condition │ ◀──────────────────────┐         │
│            └────────┬────────┘                        │         │
│                     │                                  │         │
│         ┌──────────┴──────────┐                       │         │
│         │                      │                       │         │
│       True                   False                     │         │
│         │                      │                       │         │
│         ▼                      ▼                       │         │
│   ┌───────────┐         ┌───────────┐                 │         │
│   │ Execute   │         │  Exit     │                 │         │
│   │ loop body │         │  loop     │                 │         │
│   └─────┬─────┘         └───────────┘                 │         │
│         │                                              │         │
│         └──────────────────────────────────────────────┘         │
│                                                                  │
└─────────────────────────────────────────────────────────────────┘

python

# Basic while loop
count = 0
while count < 5:
    print(count)
    count += 1  # Don't forget to update!

# User input loop
while True:
    answer = input("Continue? (yes/no): ")
    if answer.lower() == "no":
        break
    print("Continuing...")

# Find a number
number = 1
while number % 7 != 0 or number < 50:
    number += 1
print(f"Found: {number}")  # 56

Infinite Loops

python

# DANGER! This runs forever
while True:
    print("Help!")

# DANGER! Condition never changes
count = 0
while count < 5:
    print(count)
    # Forgot count += 1

# Always have an exit condition!

for vs while

for	while
Known number of iterations	Unknown number of iterations
Iterating over sequences	Condition-based
Cleaner, less error-prone	More flexible
Example: process all items	Example: wait for user input

Loop Control

break

Exit the loop immediately.

python

# Find first even number
numbers = [1, 3, 5, 8, 9, 10]
for num in numbers:
    if num % 2 == 0:
        print(f"Found even number: {num}")
        break

# Search with early exit
target = 7
found = False
for i in range(100):
    if i == target:
        found = True
        break

continue

Skip the current iteration and continue with the next.

python

# Print only odd numbers
for i in range(10):
    if i % 2 == 0:
        continue
    print(i)  # 1, 3, 5, 7, 9

# Skip invalid entries
data = [10, -5, 20, -3, 15]
for value in data:
    if value < 0:
        continue
    print(f"Processing: {value}")

else Clause

The else clause runs if the loop completes without break.

python

# Search example
numbers = [1, 3, 5, 7, 9]
target = 4

for num in numbers:
    if num == target:
        print("Found!")
        break
else:
    print("Not found")  # This runs because no break

# Prime check
num = 17
for i in range(2, num):
    if num % i == 0:
        print(f"{num} is not prime")
        break
else:
    print(f"{num} is prime!")  # This runs

pass

Placeholder that does nothing (useful for empty blocks).

python

# Placeholder for future code
def my_function():
    pass  # TODO: implement this

# Empty loop (rare, but valid)
for i in range(10):
    pass

# Minimal class definition
class MyClass:
    pass

Nested Loops

Loops inside loops.

python

# Multiplication table
for i in range(1, 4):
    for j in range(1, 4):
        print(f"{i} × {j} = {i * j}")
    print("---")

# Grid pattern
for row in range(3):
    for col in range(4):
        print("*", end=" ")
    print()  # New line

# Output:
# * * * *
# * * * *
# * * * *

┌─────────────────────────────────────────────────────────────────┐
│                    Nested Loop Execution                         │
├─────────────────────────────────────────────────────────────────┤
│                                                                  │
│   for i in range(2):      # Outer loop                          │
│       for j in range(3):  # Inner loop (runs completely         │
│           print(i, j)     #  for each outer iteration)          │
│                                                                  │
│   Execution order:                                               │
│   ┌────────────────────────────────────────────┐                │
│   │ i=0: j=0 → j=1 → j=2 (inner completes)    │                │
│   │ i=1: j=0 → j=1 → j=2 (inner completes)    │                │
│   └────────────────────────────────────────────┘                │
│                                                                  │
│   Output: (0,0) (0,1) (0,2) (1,0) (1,1) (1,2)                   │
│                                                                  │
└─────────────────────────────────────────────────────────────────┘

Match Statement (Python 3.10+)

Pattern matching - similar to switch in other languages.

python

def http_status(status):
    match status:
        case 200:
            return "OK"
        case 404:
            return "Not Found"
        case 500:
            return "Internal Server Error"
        case _:  # Default case (wildcard)
            return "Unknown"

print(http_status(200))  # OK
print(http_status(999))  # Unknown

Pattern Matching with Conditions

python

def check_value(value):
    match value:
        case 0:
            return "Zero"
        case n if n < 0:
            return "Negative"
        case n if n > 100:
            return "Large positive"
        case _:
            return "Positive"

# Matching data structures
def process_point(point):
    match point:
        case (0, 0):
            return "Origin"
        case (x, 0):
            return f"On X-axis at {x}"
        case (0, y):
            return f"On Y-axis at {y}"
        case (x, y):
            return f"Point at ({x}, {y})"

List Comprehensions

A concise way to create lists using loops.

python

# Traditional way
squares = []
for x in range(5):
    squares.append(x ** 2)

# List comprehension (same result, one line)
squares = [x ** 2 for x in range(5)]

# With condition
evens = [x for x in range(10) if x % 2 == 0]

# Nested comprehension
matrix = [[i * j for j in range(3)] for i in range(3)]

Comprehension Syntax

python

[expression for item in iterable if condition]

# Examples:
[x * 2 for x in range(5)]           # [0, 2, 4, 6, 8]
[x for x in range(10) if x % 2 == 0] # [0, 2, 4, 6, 8]
[c.upper() for c in "hello"]         # ['H', 'E', 'L', 'L', 'O']

Common Mistakes

Avoid These Common Errors

1. Forgetting the Colon

python

# ❌ WRONG
if x > 5
    print("Big")

# ✓ CORRECT
if x > 5:
    print("Big")

2. Using = Instead of ==

python

# ❌ WRONG - This is assignment, not comparison!
if x = 5:    # SyntaxError

# ✓ CORRECT
if x == 5:
    print("Equal")

3. Infinite Loops

python

# ❌ WRONG - Never terminates!
i = 0
while i < 10:
    print(i)
    # Forgot i += 1

# ✓ CORRECT
i = 0
while i < 10:
    print(i)
    i += 1

4. Off-by-One Errors

python

# ❌ WRONG - Doesn't include 10
for i in range(10):  # 0 to 9

# ✓ CORRECT - If you want 1 to 10
for i in range(1, 11):  # 1 to 10

5. Modifying List While Iterating

python

# ❌ WRONG - Can cause unexpected behavior
numbers = [1, 2, 3, 4, 5]
for num in numbers:
    if num % 2 == 0:
        numbers.remove(num)

# ✓ CORRECT - Create new list
numbers = [1, 2, 3, 4, 5]
numbers = [num for num in numbers if num % 2 != 0]

Python vs JavaScript

Coming from JavaScript?

Feature	Python	JavaScript
If statement	`if x > 5:`	`if (x > 5) {`
Else if	`elif`	`else if`
For loop	`for i in range(10):`	`for (let i = 0; i < 10; i++) {`
For-each	`for item in list:`	`for (const item of array) {`
While	`while x > 0:`	`while (x > 0) {`
Ternary	`a if cond else b`	`cond ? a : b`
Switch	`match value:` (3.10+)	`switch (value) {`
AND	`and`	`&&`
OR	`or`	`\|\|`
NOT	`not`	`!`

Real-World Examples

Example 1: User Authentication

python

MAX_ATTEMPTS = 3
CORRECT_PASSWORD = "secret123"

attempts = 0
logged_in = False

while attempts < MAX_ATTEMPTS:
    password = input("Enter password: ")
    attempts += 1

    if password == CORRECT_PASSWORD:
        logged_in = True
        print("Login successful!")
        break
    else:
        remaining = MAX_ATTEMPTS - attempts
        if remaining > 0:
            print(f"Wrong password. {remaining} attempts remaining.")
        else:
            print("Account locked. Too many failed attempts.")

if logged_in:
    print("Welcome to the system!")

python

def show_menu():
    print("\n" + "=" * 30)
    print("       MAIN MENU")
    print("=" * 30)
    print("1. View Profile")
    print("2. Edit Settings")
    print("3. Help")
    print("4. Exit")
    print("=" * 30)

while True:
    show_menu()
    choice = input("Select option (1-4): ")

    match choice:
        case "1":
            print("\n📋 Displaying your profile...")
        case "2":
            print("\n⚙️ Opening settings...")
        case "3":
            print("\n❓ Help: Contact support@example.com")
        case "4":
            print("\n👋 Goodbye!")
            break
        case _:
            print("\n❌ Invalid option. Please try again.")

Example 3: Data Validation

python

def get_valid_age():
    """Get a valid age from user with validation."""
    while True:
        try:
            age = int(input("Enter your age: "))

            if age < 0:
                print("Age cannot be negative!")
            elif age > 150:
                print("Age seems unrealistic!")
            else:
                return age

        except ValueError:
            print("Please enter a valid number!")

def get_valid_email():
    """Get a valid email from user."""
    while True:
        email = input("Enter your email: ").strip()

        if not email:
            print("Email cannot be empty!")
        elif "@" not in email:
            print("Email must contain @!")
        elif "." not in email.split("@")[-1]:
            print("Invalid email domain!")
        else:
            return email

# Usage
age = get_valid_age()
email = get_valid_email()
print(f"\nAge: {age}, Email: {email}")

Example 4: Shopping Cart with Discount

python

cart = []
total = 0

print("Shopping Cart (type 'done' to checkout)")
print("-" * 40)

while True:
    item = input("Item name (or 'done'): ").strip()
    if item.lower() == 'done':
        break

    try:
        price = float(input(f"Price of {item}: $"))
        qty = int(input("Quantity: "))

        if price < 0 or qty < 1:
            print("Invalid price or quantity!")
            continue

        cart.append({"item": item, "price": price, "qty": qty})
        subtotal = price * qty
        total += subtotal
        print(f"Added: {item} x{qty} = ${subtotal:.2f}")
        print()

    except ValueError:
        print("Invalid input! Try again.\n")

# Checkout
print("\n" + "=" * 40)
print("CHECKOUT")
print("=" * 40)

for item in cart:
    line_total = item['price'] * item['qty']
    print(f"{item['item']:20} x{item['qty']:3} ${line_total:8.2f}")

print("-" * 40)
print(f"{'Subtotal:':24} ${total:8.2f}")

# Apply discount
if total >= 100:
    discount = total * 0.1
    print(f"{'Discount (10%):':24} -${discount:7.2f}")
    total -= discount

print(f"{'TOTAL:':24} ${total:8.2f}")

Exercises

Exercise 1: FizzBuzz

Print numbers 1-100. For multiples of 3, print "Fizz". For multiples of 5, print "Buzz". For multiples of both, print "FizzBuzz".

Solution

python

for i in range(1, 101):
    if i % 3 == 0 and i % 5 == 0:
        print("FizzBuzz")
    elif i % 3 == 0:
        print("Fizz")
    elif i % 5 == 0:
        print("Buzz")
    else:
        print(i)

Exercise 2: Prime Numbers

Print all prime numbers between 2 and 50.

Solution

python

for num in range(2, 51):
    is_prime = True
    for i in range(2, int(num ** 0.5) + 1):
        if num % i == 0:
            is_prime = False
            break
    if is_prime:
        print(num)

Exercise 3: Guess the Number

Create a number guessing game with limited attempts.

Solution

python

import random

secret = random.randint(1, 100)
attempts = 7

print("Guess the number between 1 and 100!")
print(f"You have {attempts} attempts.")

while attempts > 0:
    guess = int(input("Your guess: "))
    attempts -= 1

    if guess == secret:
        print("Congratulations! You won!")
        break
    elif guess < secret:
        print(f"Too low! {attempts} attempts left.")
    else:
        print(f"Too high! {attempts} attempts left.")
else:
    print(f"Game over! The number was {secret}.")

Exercise 4: Pyramid Pattern

Print a pyramid pattern with n rows.

Solution

python

n = int(input("Enter number of rows: "))

for i in range(1, n + 1):
    # Print spaces
    print(" " * (n - i), end="")
    # Print stars
    print("*" * (2 * i - 1))

# Output for n=5:
#     *
#    ***
#   *****
#  *******
# *********

Exercise 5: Factorial Calculator

Calculate factorial using a loop.

Solution

python

num = int(input("Enter a number: "))

if num < 0:
    print("Factorial not defined for negative numbers")
elif num == 0:
    print("0! = 1")
else:
    factorial = 1
    for i in range(1, num + 1):
        factorial *= i
    print(f"{num}! = {factorial}")

Exercise 6: Password Strength Checker

Check password strength based on criteria.

Solution

python

password = input("Enter password: ")

score = 0
feedback = []

# Check length
if len(password) >= 8:
    score += 1
else:
    feedback.append("At least 8 characters")

# Check for uppercase
if any(c.isupper() for c in password):
    score += 1
else:
    feedback.append("Add uppercase letter")

# Check for lowercase
if any(c.islower() for c in password):
    score += 1
else:
    feedback.append("Add lowercase letter")

# Check for digit
if any(c.isdigit() for c in password):
    score += 1
else:
    feedback.append("Add a number")

# Check for special character
special = "!@#$%^&*()_+-="
if any(c in special for c in password):
    score += 1
else:
    feedback.append("Add special character")

# Display result
print(f"\nStrength: {'█' * score}{'░' * (5 - score)} ({score}/5)")

if score < 3:
    print("Status: Weak 🔴")
elif score < 5:
    print("Status: Medium 🟡")
else:
    print("Status: Strong 🟢")

if feedback:
    print("\nSuggestions:")
    for item in feedback:
        print(f"  • {item}")

Quick Reference

Control Flow Cheat Sheet

python

# Conditionals
if condition:
    # code
elif other_condition:
    # code
else:
    # code

# Ternary
result = "yes" if condition else "no"

# for loop
for item in sequence:
    # code

for i in range(start, stop, step):
    # code

for index, value in enumerate(sequence):
    # code

# while loop
while condition:
    # code

# Loop control
break      # Exit loop
continue   # Skip iteration
pass       # Do nothing

# List comprehension
[expr for item in sequence if condition]

Summary

Concept	Description	Example
`if/elif/else`	Conditional branching	`if x > 0:`
`for`	Iterate over sequence	`for i in range(5):`
`while`	Loop while condition true	`while count < 10:`
`break`	Exit loop	`break`
`continue`	Skip iteration	`continue`
`enumerate()`	Index + value	`for i, v in enumerate(list):`
`zip()`	Combine sequences	`for a, b in zip(x, y):`
`match`	Pattern matching	`match value:`

Next Steps

Continue to Functions to learn how to create reusable code blocks.

Control Flow ​

Conditionals ​

if Statement ​

if-else Statement ​

if-elif-else Statement ​

Nested Conditionals ​

Conditional Expressions (Ternary) ​

Truthy and Falsy Values ​

Loops ​

for Loop ​

enumerate() ​

zip() ​

while Loop ​

for vs while ​

Loop Control ​

break ​

continue ​

else Clause ​

pass ​

Nested Loops ​

Match Statement (Python 3.10+) ​

Pattern Matching with Conditions ​

List Comprehensions ​

Common Mistakes ​

1. Forgetting the Colon ​

2. Using = Instead of == ​

3. Infinite Loops ​

4. Off-by-One Errors ​

5. Modifying List While Iterating ​

Python vs JavaScript ​

Real-World Examples ​

Example 1: User Authentication ​

Example 2: Menu System ​

Example 3: Data Validation ​

Example 4: Shopping Cart with Discount ​

Exercises ​

Exercise 1: FizzBuzz ​

Exercise 2: Prime Numbers ​

Exercise 3: Guess the Number ​

Exercise 4: Pyramid Pattern ​

Exercise 5: Factorial Calculator ​

Exercise 6: Password Strength Checker ​

Quick Reference ​

Summary ​

Next Steps ​

Control Flow

Conditionals

if Statement

if-else Statement

if-elif-else Statement

Nested Conditionals

Conditional Expressions (Ternary)

Truthy and Falsy Values

Loops

for Loop

enumerate()

zip()

while Loop

for vs while

Loop Control

break

continue

else Clause

pass

Nested Loops

Match Statement (Python 3.10+)

Pattern Matching with Conditions

List Comprehensions

Common Mistakes

1. Forgetting the Colon

2. Using = Instead of ==

3. Infinite Loops

4. Off-by-One Errors

5. Modifying List While Iterating

Python vs JavaScript

Real-World Examples

Example 1: User Authentication

Example 2: Menu System

Example 3: Data Validation

Example 4: Shopping Cart with Discount

Exercises

Exercise 1: FizzBuzz

Exercise 2: Prime Numbers

Exercise 3: Guess the Number

Exercise 4: Pyramid Pattern

Exercise 5: Factorial Calculator

Exercise 6: Password Strength Checker

Quick Reference

Summary

Next Steps