Control Flow

If/Else Statements

In this lesson, we’ll learn how to make decisions and repeat actions in Python using If/Else Statements

What are if/else statements?

In Python (or programming in general), if/else statements are decision-making tools that allow a program to take different actions based on certain conditions. Think of them as a way to make the program think and decide what to do in various situations.

• The if part checks a condition: "Is this true?"
• The else part defines what happens if the condition is not true.

Why do we need if/else statements? Programs need to handle different scenarios and make decisions just like humans do in everyday life. Without if/else statements, the program would not be able to adapt to different situations or conditions dynamically.

Key reasons:

• Decision Making: To allow the program to choose between multiple courses of action.
• Dynamic Responses: To react to different inputs or changing conditions in real-time.
• Error Handling: To guide the program when something unexpected happens.

Real-world examples (without code):

• 1. Traffic Light Decision If the traffic light is green, then you go. Else (if it's red), you stop.
• 2. Weather-based Dressing If it's raining, then carry an umbrella. Else (if it's sunny), wear sunglasses.
• 3. Restaurant Ordering If you are vegetarian, then order a veggie dish. Else (if you eat meat), order a non-veg dish.
• 4. Shopping Discounts If your total bill is above $100, then you get a 10% discount. Else, you pay the full amount.
• 5. Airline Boarding If your boarding pass is valid, then you can board the plane. Else, you are denied boarding.

How would the world look without if/else decisions?

Imagine trying to follow the same routine every day without adapting to different circumstances:

• You'd carry an umbrella every day, even when it's sunny.
• You'd try to drive through a red light because there's no decision-making involved.
• Restaurants would serve you random food because they can't decide based on your preference.

In essence, if/else statements bring logic, adaptability, and intelligence to programs, just like critical thinking brings adaptability to our lives!

Syntax:

if condition:
    # code to run if condition is true
elif another_condition:
    # code to run if another_condition is true
else:
    # code to run if all conditions are false

Example:

age = 18
if age >= 18:
    print("You are an adult.")
elif age >= 13:
    print("You are a teenager.")
else:
    print("You are a child.")

1. Basic if Statement

The if statement is the simplest form of conditional in Python. It checks if a condition is true, and if it is, the block of code inside the if statement is executed.

Example 1: Basic if Statement

# Example: Checking if a number is positive
number = 5

if number > 0:
    print("The number is positive.")

Explanation:

• The condition number > 0 is checked.
• If it is True, the message "The number is positive." is printed.

2. if with else Statement

The if-else statement allows you to execute one block of code if the condition is true, and a different block if the condition is false.

Example 2: if with else

# Example: Checking if a number is positive or not
number = -3

if number > 0:
    print("The number is positive.")
else:
    print("The number is not positive.")

Explanation:

• If number > 0, it prints "The number is positive.".
• Otherwise, it prints "The number is not positive.".

3. if-elif-else Statement

The if-elif-else statement allows checking multiple conditions. Only the first True condition is executed, and the rest are skipped.

Example 3: if-elif-else Statement

# Example: Categorizing a number
number = 0

if number > 0:
    print("The number is positive.")
elif number < 0:
    print("The number is negative.")
else:
    print("The number is zero.")

Explanation:

• If number > 0, it prints "The number is positive.".
• If number < 0, it prints "The number is negative.".
• If neither condition is true, it prints "The number is zero.".

##if-elif-else: Grade categorization
# Change the value of variable marks and see what output is printed.
marks = 85

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

Now that we have learnt the basics of if-elif-else conditional statement, its time to practice.

# Problem 1: Check if a number is positive.

# Write a program that asks the user to input a number.
# Print "The number is positive." if the number is greater than zero.

# Problem 2: Check if a person is eligible to vote.

# Ask the user to input their age.
# Print "You are eligible to vote." if the age is 18 or more.

MIN_VOTING_AGE = 18

# Problem 3: Even or Odd Number

# Write a program that asks the user to input a number.
# Print "The number is even." if it is divisible by 2, otherwise print "The number is odd.".

# Problem 4: Check String Length
# Ask the user to input two strings and print the one that is longer

# Problem 5: Check Username Validity
# Ask the user to input a username.
# Check if it meets these requirements:
# At least 5 characters long.
# Contains no spaces.
# Starts with an uppercase letter.
#If the username is invalid inform the user why the username is invalid:
  #print("Invalid username: Must be at least 5 characters long.")
  #print("Invalid username: Must start with an uppercase letter.")
  #print("Invalid username: Must not contain spaces.")
#Else inform the user that username is valid'
  #print("Valid username.")

Nested If Statements

You can also nest if statements inside another if block.

Example:

age = 20
if age >= 18:
    print("You can vote.")
    if age >= 21:
        print("You can drink alcohol in the US.")
else:
    print("You are not old enough to vote.")

Example: Determining Delivery Cost

A store offers delivery services based on the total order value and the delivery distance. The delivery cost is calculated as follows:

• If the order value is greater than 100:

  • delivery is free

• If the order value is less than 100:

  • If the delivery distance is less than 5 km, delivery is free.
  • Otherwise, delivery costs $10.

Here’s the code:

# Inputs
order_value = 80  # Example: Order value in dollars
delivery_distance = 3  # Example: Delivery distance in kilometers

# Nested if-else to determine delivery cost
if order_value > 100:
    delivery_cost = 0  # Free delivery for orders over $100
else:  # order_value <= 100
    if delivery_distance <= 5:
        delivery_cost = 0  # Free delivery within 5 km for orders under $100
    else:
        delivery_cost = 10  # Delivery cost if distance is greater than 5 km

# Output
print(f"Order value: ${order_value}")
print(f"Delivery distance: {delivery_distance} km")
print(f"Delivery cost: ${delivery_cost}")