Python Control Structures - If Elif And Else in Python

Python, one of the most popular programming languages, is praised for its readability and simplicity. A major reason behind its user-friendly nature is its control structures, which allow programmers to control the flow of a program. In this blog, we will dive deep into two essential control structures: if and else statements in Python.

Understanding these foundational elements is key to writing efficient and logical programs. So, let’s explore how Python handles decision-making, starting with the basics of if, moving to else, and extending to more advanced concepts like elif.

What are Control Structures?

Control structures allow you to manage the flow of your code based on certain conditions. In Python, the if, else, and elif statements are used to make decisions and execute specific blocks of code based on whether a condition is true or false.

• if statement: Runs a block of code if a specified condition evaluates to True.
• else statement: Executes a block of code if the condition specified in the if statement is False.
• elif statement: Short for "else if," this allows you to check multiple expressions for True and execute code for the first one that evaluates to True.

Now, let’s look at each of these in detail.

The if Statement

The if statement is the simplest form of decision-making control. It allows you to run a block of code only if a specific condition is met. If the condition evaluates to True, Python executes the block; if it’s False, Python skips the block entirely.

Syntax:

if condition:
    # code to execute if condition is True

Example:

age = 18
if age >= 18:
    print("You are eligible to vote!")

Breakdown:

• Condition: The condition is an expression that returns either True or False. In this case, age >= 18 checks if the value of age is greater than or equal to 18.
• Indentation: The block of code inside the if statement must be indented (usually 4 spaces). Python relies on this indentation to determine the start and end of a block of code.

In this example, if the value of age is 18 or more, the message "You are eligible to vote!" will be printed.

The else Statement

The else statement works in conjunction with the if statement. If the condition in the if statement is False, the code inside the else block is executed.

Syntax:

if condition:
    # code to execute if condition is True
else:
    # code to execute if condition is False

Example:

age = 16
if age >= 18:
    print("You are eligible to vote!")
else:
    print("You are not old enough to vote.")

Breakdown:

• When age is 16, the condition age >= 18 evaluates to False, so the else block is executed, printing "You are not old enough to vote."

In this case, the else statement acts as a fallback when the if condition fails.

The elif Statement (Else If)

What if you need to check multiple conditions? This is where elif comes in handy. The elif statement allows you to evaluate multiple conditions, and Python will execute the first True condition it encounters.

Syntax:

if condition1:
    # code to execute if condition1 is True
elif condition2:
    # code to execute if condition2 is True
else:
    # code to execute if all conditions are False

Example:

marks = 85

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

Breakdown:

• Python evaluates each condition in order:
  • If marks >= 90, the first block is executed, and "Grade: A" is printed.
  • If the first condition is False, it moves to marks >= 80 (checking if the marks are 80 or more).
  • This continues until a True condition is found, or the else block is reached, which acts as a final fallback.

In this case, since marks is 85, the second condition marks >= 80 is True, so "Grade: B" is printed.

Nesting if, else, and elif Statements

Sometimes, you may need to include one if statement inside another, creating a hierarchy of conditions. This is called nesting.

Example:

age = 20
country = "USA"

if age >= 18:
    if country == "USA":
        print("You can vote in the USA.")
    else:
        print("You cannot vote in the USA.")
else:
    print("You are not old enough to vote.")

Breakdown:

• First, the outer if statement checks if the person is at least 18 years old.
• If True, the inner if statement checks if the person is from the USA. If both conditions are satisfied, "You can vote in the USA" is printed.

Nested statements allow for more complex logic but can become difficult to manage if overused. Always try to write clear and readable code!

Conditional Expressions (Ternary Operator)

For simpler conditions, Python offers a shorthand form called the ternary operator or conditional expression. This is a one-liner for if-else statements.

Syntax:

result = value_if_true if condition else value_if_false

Example:

age = 17
eligibility = "Eligible" if age >= 18 else "Not eligible"
print(eligibility)

In this case, the value of eligibility is set to "Eligible" if age is greater than or equal to 18, otherwise, it’s set to "Not eligible." This can be useful for shorter, more concise decisions.

Key Takeaways

• The if statement is used to execute a block of code based on a condition.
• The else statement provides a fallback if the if condition is False.
• The elif statement allows you to check multiple conditions in sequence.
• You can nest if statements for more complex decision-making.
• Python provides a ternary operator for concise conditional expressions.

Conclusion

Mastering if, else, and elif is crucial for any Python programmer. These control structures form the backbone of decision-making in Python programs, allowing you to control the flow of execution based on varying conditions.

As you continue to develop your Python skills, you’ll find yourself using these statements in nearly every project, from simple scripts to complex applications. The more you practice, the more intuitive they will become, enabling you to write smarter, more efficient code.