Chapter 4: Flow Control

Conditional Statements: if/elif/else

In everyday life, we're constantly making decisions: "If it's raining, bring an umbrella; otherwise, wear a hat." Python's if statement works the same way:

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 score is {score}, grade is {grade}")

Your score is 85, grade is B

⚠️ Note: ⚠️ Common mistake: Python uses indentation (usually 4 spaces) to denote code blocks, not curly braces! Inconsistent indentation will cause an IndentationError. Never mix tabs and spaces.

# Ternary expression (shorthand if-else)
age = 20
status = "Adult" if age >= 18 else "Minor"
print(status)

Adult

for Loops

A for loop is like a "conveyor belt" — bringing items one by one for processing:

# Iterate over a list
fruits = ["Apple", "Banana", "Orange", "Watermelon"]
for fruit in fruits:
    print(f"I like eating {fruit}!")

print("---")

# range() generates number sequences
for i in range(1, 6):
    print(f"Iteration {i}")

print("---")

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

I like eating Apple!
I like eating Banana!
I like eating Orange!
I like eating Watermelon!
---
Iteration 1
Iteration 2
Iteration 3
Iteration 4
Iteration 5
---
0: Apple
1: Banana
2: Orange
3: Watermelon

while Loops

# Number guessing game
import random
secret = random.randint(1, 10)
guess = 0
attempts = 0

while guess != secret:
    guess = int(input("Guess a number between 1-10: "))
    attempts += 1
    if guess > secret:
        print("Too high!")
    elif guess < secret:
        print("Too low!")

print(f"🎉 Congratulations! You got it! The answer is {secret}, it took {attempts} tries.")

Guess a number between 1-10: 5
Too high!
Guess a number between 1-10: 3
Too low!
Guess a number between 1-10: 4
🎉 Congratulations! You got it! The answer is 4, it took 3 tries.

break, continue, and for...else

# break: immediately exit the loop
print("break example: stop after finding the first even number")
numbers = [1, 3, 5, 8, 9, 10]
for n in numbers:
    if n % 2 == 0:
        print(f"Found even number: {n}")
        break
    print(f"  Checking {n}...")

print("\ncontinue example: skip even numbers")
for n in range(1, 8):
    if n % 2 == 0:
        continue
    print(f"Odd: {n}")

print("\nfor...else example: check if all numbers are positive")
test_list = [1, 2, 3, 4, 5]
for n in test_list:
    if n < 0:
        print("Found a negative number!")
        break
else:
    # Executes when loop completes normally (no break)
    print("All numbers are positive ✅")

break example: stop after finding the first even number
  Checking 1...
  Checking 3...
  Checking 5...
Found even number: 8

continue example: skip even numbers
Odd: 1
Odd: 3
Odd: 5
Odd: 7

for...else example: check if all numbers are positive
All numbers are positive ✅

📝 Note: 📝 Fun fact: for...else is a Python-unique syntax. The else block only executes when the loop completes normally (without hitting break). Commonly used in search scenarios: "If we searched everything and didn't find it, execute the else block."