Control Flow

Control flow determines which lines of your program run, in what order, and under what conditions. Without control flow, every program would run top to bottom with no decisions and no repetition.

Why This Chapter Matters

Almost every real-world program needs to make decisions. Login systems check credentials. Games check scores. Forms validate inputs. Control flow is how code reacts to data.

• you will write programs that respond differently based on conditions
• you will understand if, elif, else, and how Python evaluates conditions
• you will use for and while loops to repeat actions
• you will know how to exit loops and skip iterations

Conditional Statements

The if Statement

The if statement runs a block of code only when its condition is True.

temperature = 35

if temperature > 30:
    print("It is a hot day.")

The colon : ends the condition line, and the indented block is the body.

if / else

else provides a fallback when the if condition is False.

score = 55

if score >= 60:
    print("You passed!")
else:
    print("You need to study more.")

if / elif / else

elif (else if) checks additional conditions when the first if fails.

score = 82

if score >= 90:
    print("Grade: A")
elif score >= 80:
    print("Grade: B")
elif score >= 70:
    print("Grade: C")
elif score >= 60:
    print("Grade: D")
else:
    print("Grade: F")

Only the first matching condition runs. Once matched, Python skips the rest.

Order Matters

# Bad — score 95 prints "Passed" and never reaches "Excellent"
if score >= 60:
    print("Passed")
elif score >= 90:
    print("Excellent")   # unreachable!

Always check the most specific condition first.

Comparison Operators

print(5 == 5)    # True
print(5 != 3)    # True
print(10 > 10)   # False
print(10 >= 10)  # True

Logical Operators

Combine conditions using and, or, not.

and — Both Must Be True

has_ticket = True
has_id = True

if has_ticket and has_id:
    print("Welcome to the concert!")

or — At Least One Must Be True

is_admin = False
is_moderator = True

if is_admin or is_moderator:
    print("Access granted")

not — Reverse the Truth

is_banned = False

if not is_banned:
    print("User can log in")

Truthiness in Conditions

Python does not require conditions to be strictly True or False. Any value is evaluated for truthiness.

username = ""

if username:
    print("Username provided")
else:
    print("Username is empty")   # runs because empty string is falsy

The Ternary (Inline) Expression

Python supports a one-line conditional expression:

status = "adult" if age >= 18 else "minor"
print(status)

This is clean for simple cases. Avoid nesting ternaries — they become hard to read.

for Loops

A for loop iterates over a sequence of values.

Iterating a List

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

for fruit in fruits:
    print(fruit)

Iterating a String

for char in "Python":
    print(char)

The range() Function

range() generates a sequence of numbers.

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

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

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

Iterating with Index (enumerate())

names = ["Asha", "Leo", "Mina"]

for index, name in enumerate(names):
    print(f"{index}: {name}")

while Loops

A while loop keeps running as long as its condition is True.

count = 0

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

Always make sure the loop variable changes, or you will create an infinite loop.

break, continue, and else on Loops

break — Exit the Loop Immediately

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

continue — Skip to the Next Iteration

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

else on a Loop

The else block runs only when the loop completes without hitting a break:

for number in range(5):
    if number == 10:
        break
else:
    print("Loop finished without break")   # this runs

Nested Loops

Loops can be placed inside other loops.

for row in range(1, 4):
    for col in range(1, 4):
        print(f"({row}, {col})", end=" ")
    print()

Output:

(1, 1) (1, 2) (1, 3)
(2, 1) (2, 2) (2, 3)
(3, 1) (3, 2) (3, 3)

Real-World Examples

Grade Checker

def grade(score):
    if score >= 90:
        return "A"
    elif score >= 80:
        return "B"
    elif score >= 70:
        return "C"
    else:
        return "F"

print(grade(85))   # B

Password Validator

password = input("Enter password: ")

if len(password) < 8:
    print("Too short")
elif not any(c.isdigit() for c in password):
    print("Must contain a number")
else:
    print("Password is valid")

Number Guessing Loop

secret = 42
guess = 0

while guess != secret:
    guess = int(input("Guess: "))
    if guess < secret:
        print("Higher!")
    elif guess > secret:
        print("Lower!")

print("Correct!")

Common Mistakes

• forgetting the colon : after if, elif, else, for, while
• incorrect indentation inside blocks
• using = (assignment) instead of == (comparison) in conditions
• creating infinite while loops by forgetting to update the condition variable
• off-by-one errors with range() (remember: range(n) goes from 0 to n-1)

Mini Exercises

1. Write an if/elif/else chain that categorizes a temperature as cold, warm, or hot.
2. Use a for loop to print the multiplication table for 7.
3. Write a while loop that keeps asking for input until the user types "quit".
4. Use break to find the first number in a list that is divisible by 7.
5. Use continue to print all numbers from 1~20 that are NOT divisible by 3.

Review Questions

1. What is the difference between if/elif/else and multiple separate if statements?
2. Why does the order of elif conditions matter?
3. What is the difference between break and continue in a loop?
4. When does the else block on a for loop run?
5. What is the output of range(2, 10, 3)?

Reference Checklist

• I can write if, elif, and else chains correctly
• I understand all comparison and logical operators
• I can use for loops with lists, strings, and range()
• I can use enumerate() to loop with an index
• I can use while loops and know how to avoid infinite loops
• I know when to use break, continue, and else on loops