Lecture 06: Control Flow

Control flow allows your program to make decisions and execute different paths of code based on conditions.

1. If/Elif/Else Statements

Python uses the keywords if, elif (short for else-if), and else to control decision making:

age = 18

if age >= 21:
    print("You can enter the club and buy drinks.")
elif age >= 18:
    print("You can enter the club but cannot buy drinks.")
else:
    print("You are too young to enter the club.")

In Python, code blocks are defined by indentation (typically 4 spaces).
The colon (:) at the end of conditional lines is required.

2. Logical Operators

You can combine multiple conditions using logical operators:

and: True if both conditions are True.
or: True if at least one condition is True.
not: Reverses the boolean value of a condition.

has_id = True
is_vip = False

if has_id and (age >= 18 or is_vip):
    print("Welcome!")

3. Truthy and Falsy Values

In Python, values can evaluate to True or False when used in a conditional statement.

Falsy values:

None
False
0 (integer) or 0.0 (float)
Empty sequences or collections: "", [], (), {}, set()

Any other value in Python is considered Truthy.

name = input("Enter your name: ")
if name:  # Checks if name is not an empty string
    print(f"Hello, {name}!")
else:
    print("You didn't enter a name.")

4. Ternary Operators

Ternary operators provide a shorthand way to write simple if-else statements.

Syntax: value_if_true if condition else value_if_false

x = 10
status = "even" if x % 2 == 0 else "odd"
print(status)  # Output: even

5. Match Statements (Python 3.10+)

Similar to switch-case statements in other languages, match statements offer structural pattern matching.

command = "start"

match command:
    case "start":
        print("Starting the system...")
    case "stop" | "halt":
        print("Stopping the system...")
    case _:
        print("Unknown command.")  # Default wildcard pattern

🧠 Try It Yourself

Write a program that takes a score (0–100) and prints grades: A (>=90), B (>=80), C (>=70), and F (otherwise).
Use a ternary operator to assign "allowed" if age is >= 18, otherwise "denied".

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