Mastering Conditional Statements in Python: If, Elif, and Else

In Python, conditional statements allow your programs to make decisions and execute different blocks of code based on specific conditions. These decisions are made using the if, elif, and else statements. Think of them as a set of rules that guide your program's behavior.

Chapter 1: The Basics of if Statement

The if statement checks if a condition is True. If it is, the block of code under the if statement is executed.

Syntax:

if condition:
    # Code to run if the condition is True

Example:

age = 18

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

In this example, the condition age >= 18 is True, so the program prints "You are an adult.".

Chapter 2: Using else – Handling the False Condition

The else statement allows you to run code when the condition in the if statement is False.

Syntax:

if condition:
    # Code to run if the condition is True
else:
    # Code to run if the condition is False

Example:

age = 16

if age >= 18:
    print("You are an adult.")
else:
    print("You are a minor.")

Here, the condition age >= 18 is False, so the else block is executed, and the output is "You are a minor.".

Chapter 3: Introducing elif – Multiple Conditions

What if you need to check more than two conditions? That’s where elif (short for "else if") comes in. It lets you add additional conditions to test after the initial if condition.

Syntax:

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

Example:

score = 85

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

In this example, the elif condition score >= 80 is True, so "Grade: B" is printed.

Chapter 4: Working with Comparison Operators

Conditions in if, elif, and else statements often involve comparison operators. Here are the most commonly used ones:

• ==: Equal to
• !=: Not equal to
• >: Greater than
• <: Less than
• >=: Greater than or equal to
• <=: Less than or equal to

Example:

x = 10
y = 5

if x == y:
    print("x and y are equal.")
elif x > y:
    print("x is greater than y.")
else:
    print("x is less than y.")

Here, x > y is True, so "x is greater than y." is printed.

Chapter 5: Combining Conditions with and, or, and not

You can combine multiple conditions in an if statement using logical operators: and, or, and not.

• and: Both conditions must be True.
• or: At least one condition must be True.
• not: Inverts the condition (i.e., True becomes False).

Example: Using and

age = 25
has_id = True

if age >= 18 and has_id:
    print("You can enter the club.")
else:
    print("You cannot enter the club.")

Both conditions (age >= 18 and has_id) are True, so "You can enter the club." is printed.

Example: Using or

is_student = True
is_senior = False

if is_student or is_senior:
    print("You get a discount.")
else:
    print("No discount available.")

Since is_student is True, the program prints "You get a discount.".

Example: Using not

is_raining = False

if not is_raining:
    print("Go outside!")
else:
    print("Stay indoors!")

Since is_raining is False, not is_raining becomes True, so "Go outside!" is printed.

Chapter 6: Nesting if Statements – Checking Conditions Within Conditions

You can also place an if statement inside another if statement. This is known as nesting, and it lets you check multiple conditions in a sequence.

Example:

age = 22
has_id = True

if age >= 18:
    if has_id:
        print("You can enter the club.")
    else:
        print("You need an ID to enter.")
else:
    print("You are too young to enter.")

Here, the program checks if age >= 18. If this is True, it then checks whether the person has an ID before allowing entry.

Chapter 7: Practical Example – Grading System

Let’s put everything together with a simple program that assigns letter grades based on a student's score.

Example:

score = int(input("Enter your score: "))

if score >= 90:
    print("You got an A!")
elif score >= 80:
    print("You got a B!")
elif score >= 70:
    print("You got a C!")
elif score >= 60:
    print("You got a D!")
else:
    print("You got an F.")

This program takes a score from the user and assigns a letter grade based on the conditions.

Chapter 8: Key Takeaways

1. The if statement checks a condition and executes code if the condition is True.
2. The else statement runs code when the if condition is False.
3. elif allows checking multiple conditions in sequence.
4. Comparison operators like ==, !=, >, <, >=, and <= help in forming conditions.
5. Logical operators (and, or, not) allow combining multiple conditions.
6. You can nest if statements for more complex decision-making.

Conclusion:

Conditional statements are a powerful feature in Python, allowing your programs to make decisions and act accordingly. By mastering if, elif, and else, you can write code that is both flexible and responsive to user input. Keep practicing these concepts to get a solid foundation in decision-making within your Python programs!