Python Elif Statement

In programming, we often need to make decisions based on multiple conditions. While the if-else structure allows us to handle two possible scenarios, real-world situations frequently require more complex decision-making. This is where Python's elif statement comes into play.

What is the Elif Statement?

The elif statement (short for "else if") is used when you need to check multiple conditions in sequence. It allows you to create decision trees where Python evaluates each condition in order until it finds one that's True.

The basic structure looks like this:

python

if condition1:
    # Code to execute if condition1 is True
elif condition2:
    # Code to execute if condition1 is False and condition2 is True
elif condition3:
    # Code to execute if condition1 and condition2 are False and condition3 is True
else:
    # Code to execute if all conditions are False

How Elif Works

When Python encounters an if-elif-else structure:

1. It evaluates the first if condition.
2. If that condition is True, it executes the code block under the if statement and skips all other conditions.
3. If the first condition is False, it moves to the first elif condition.
4. It continues this process through all elif conditions until it finds one that's True.
5. If none of the conditions are True, it executes the else block (if one exists).

Basic Elif Example

Let's look at a simple example that grades a student's score:

python

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}")

Output:

Your grade is: B

In this example:

• Python checks if the score is 90 or above. It's not, so it moves to the first elif.
• It checks if the score is 80 or above. It is (85), so it assigns 'B' to grade and skips all remaining conditions.
• Finally, it prints the grade.

Multiple Elif Statements

You can have as many elif blocks as necessary. Here's an example that recommends activities based on weather:

python

weather = "rainy"

if weather == "sunny":
    activity = "Go for a hike"
elif weather == "cloudy":
    activity = "Visit a museum"
elif weather == "rainy":
    activity = "Read a book indoors"
elif weather == "snowy":
    activity = "Build a snowman"
else:
    activity = "Check the weather forecast again"

print(f"Recommended activity: {activity}")

Output:

Recommended activity: Read a book indoors

Combining Conditions in Elif Statements

You can use logical operators (and, or, not) to create more complex conditions:

python

temperature = 28
weather_condition = "sunny"

if temperature > 30 and weather_condition == "sunny":
    recommendation = "Stay hydrated and find shade"
elif temperature > 25 and weather_condition == "sunny":
    recommendation = "Perfect for the beach"
elif temperature > 20 or weather_condition != "rainy":
    recommendation = "Good for outdoor activities"
else:
    recommendation = "Maybe stay indoors today"

print(recommendation)

Output:

Perfect for the beach

Nested Elif Statements

You can also nest if-elif-else structures inside each other for more complex decision trees:

python

day = "weekend"
weather = "sunny"
temperature = 22

if day == "weekend":
    if weather == "sunny" and temperature > 25:
        plan = "Go to the beach"
    elif weather == "sunny":
        plan = "Go for a picnic"
    else:
        plan = "Watch a movie at home"
elif day == "weekday":
    if temperature < 0:
        plan = "Work from home if possible"
    else:
        plan = "Go to work"
else:
    plan = "Check your calendar"

print(f"Today's plan: {plan}")

Output:

Today's plan: Go for a picnic

Real-World Applications

1. Simple Calculator

Let's create a basic calculator that performs different operations based on user input:

python

num1 = 10
num2 = 5
operation = "multiply"

if operation == "add":
    result = num1 + num2
elif operation == "subtract":
    result = num1 - num2
elif operation == "multiply":
    result = num1 * num2
elif operation == "divide":
    if num2 != 0:  # Avoid division by zero
        result = num1 / num2
    else:
        result = "Error: Division by zero"
else:
    result = "Invalid operation"

print(f"Result of {operation}: {result}")

Output:

Result of multiply: 50

2. Time-Based Greeting

A program that greets users differently based on the time of day:

python

import datetime

current_hour = datetime.datetime.now().hour

if current_hour < 12:
    greeting = "Good morning!"
elif current_hour < 17:
    greeting = "Good afternoon!"
elif current_hour < 21:
    greeting = "Good evening!"
else:
    greeting = "Good night!"

print(greeting)

Output: (will vary based on the time you run it)

Good afternoon!

3. Discount Calculator

A program that calculates discounts based on purchase amount:

python

purchase_amount = 120

if purchase_amount >= 300:
    discount = 0.25  # 25% discount
elif purchase_amount >= 200:
    discount = 0.15  # 15% discount
elif purchase_amount >= 100:
    discount = 0.10  # 10% discount
else:
    discount = 0  # No discount

discount_amount = purchase_amount * discount
final_price = purchase_amount - discount_amount

print(f"Purchase amount: ${purchase_amount}")
print(f"Discount: ${discount_amount:.2f} ({discount*100:.0f}%)")
print(f"Final price: ${final_price:.2f}")

Output:

Purchase amount: $120
Discount: $12.00 (10%)
Final price: $108.00

Best Practices for Using Elif

1. Order your conditions carefully: Place more specific or common conditions first for efficiency.
2. Keep readability in mind: If you have many elif statements, consider using a dictionary or match-case statement (Python 3.10+) instead.
3. Avoid deep nesting: Too many nested if-elif structures can make your code hard to read.
4. Use meaningful variable names: This helps clarify the purpose of your conditionals.

Summary

The elif statement is a powerful tool in Python that allows you to create multi-way decision structures. By chaining multiple conditions together, you can handle complex decision-making scenarios in your programs.

To use elif effectively:

• Start with an if statement for your first condition
• Add as many elif statements as needed for additional conditions
• Optionally end with an else statement as a catch-all
• Remember that Python stops checking conditions once it finds a True one

Practice Exercises

1. Create a BMI calculator that categorizes results as underweight, normal, overweight, or obese.
2. Write a program that converts a numeric month (1-12) to its name (January-December).
3. Create a movie ticket pricer that gives different prices based on age (child, adult, senior) and day of the week.
4. Make a simple adventure game where users choose paths with different outcomes using if-elif structures.

Additional Resources

• Python Documentation on Control Flow
• Real Python: Conditional Statements in Python
• W3Schools Python Conditions

Now that you understand the elif statement, you have a powerful tool for handling multiple conditions in your Python programs. This is a fundamental concept you'll use in almost every program you write!