Introduction to if-elif-else Statement

The if-elif-else statement is a powerful tool in Python that allows you to check multiple conditions and execute different blocks of code based on which condition is true. It is particularly useful when you have multiple conditions to evaluate, but only want one block of code to run.

When you need to choose between several conditions, elif is preferred over multiple if statements. This is because elif ensures that once a condition is met, the rest of the conditions are skipped, improving efficiency. In contrast, using multiple if statements results in all conditions being evaluated independently, which can lead to unnecessary checks and redundant code.

Let's track a user's sleep duration and categorize it into different ranges. We will check whether the user has met their sleep goal, is close to meeting it, or needs more rest.


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hours_slept = 6
sleep_goal = 8

if hours_slept < 0:
    print("Sleep hours cannot be negative.")
if hours_slept >= sleep_goal:
    print("Great job! You've met your sleep goal!")
if hours_slept >= sleep_goal - 2:
    print("You're almost there! Keep going, you'll reach your goal soon!")
if hours_slept < sleep_goal - 2:
    print("You need more rest. Try to sleep a bit longer tonight.")

In this version, all conditions are evaluated independently, even if one condition is already true. This leads to redundant checks. For example, if the user has already met their goal, the code still checks if they're close to the goal or need more rest.


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hours_slept = 6
sleep_goal = 8

if hours_slept < 0:
    print("Sleep hours cannot be negative.")
elif hours_slept >= sleep_goal:
    print("Great job! You've met your sleep goal!")
elif hours_slept >= sleep_goal - 2:
    print("You're almost there! Keep going, you'll reach your goal soon!")
else:
    print("You need more rest. Try to sleep a bit longer tonight.")

Using elif helps make the code more readable and efficient, as once a condition is met, no further conditions need to be checked.

This diagram visualizes the flow of an if-elif-else decision structure in Python. The process starts by evaluating the initial if condition:

If the first condition is true, the code inside the if block executes;
If the first condition is false, it moves to the next condition, which is the elif. If the elif condition is true, the corresponding elif block is executed;
If the first elif is false, another elif can be checked, and if it's true, its respective code block runs;
If none of the if or elif conditions are met (i.e., all are false), the final else block is executed, ensuring one code block is always run.

This flow ensures that only the first true condition is executed, providing a clean and efficient way to handle multiple conditions. The if-elif-else structure avoids redundant checks, and only one code block is executed from the entire structure.

1. What is the advantage of using `if-elif-else` instead of multiple `if` statements?

2. What will be printed from the following code?

Everything was clear?

Thanks for your feedback!

Section 3. Chapter 1