Control Flow: If Statements

Programs often need to make decisions. If statements let your code take different paths based on conditions.

Basic If Statement

age = 18

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

The code inside the if block only runs when the condition is True.

If-Else

Handle both cases:

temperature = 72

if temperature > 80:
    print("It's hot outside!")
else:
    print("The weather is nice.")

If-Elif-Else

Check multiple conditions:

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}")

Important: Python checks conditions from top to bottom and stops at the first True condition.

Indentation Matters!

Python uses indentation (spaces) to define code blocks:

if True:
    print("This is inside the if block")
    print("This too!")
print("This is outside the if block")

Use 4 spaces for indentation (this is the Python standard).

Nested If Statements

You can put if statements inside other if statements:

has_ticket = True
age = 15

if has_ticket:
    if age >= 13:
        print("Enjoy the movie!")
    else:
        print("Sorry, this movie is PG-13")
else:
    print("Please buy a ticket first")

Combining Conditions

Use and, or, and not to combine conditions:

age = 25
has_license = True
has_car = False

# Need both license AND car to drive
if has_license and has_car:
    print("You can drive!")
else:
    print("You can't drive right now")

# Can vote if 18 or older
if age >= 18:
    print("You can vote")

# Check if NOT a minor
if not age < 18:
    print("You're an adult")

Common Patterns

Checking for empty values

name = ""

if name:
    print(f"Hello, {name}!")
else:
    print("Name is empty")

Checking membership

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

if "banana" in fruits:
    print("We have bananas!")

Practice Problem

Write a program that checks if a number is positive, negative, or zero:

number = -5

if number > 0:
    print("Positive")
elif number < 0:
    print("Negative")
else:
    print("Zero")