Checking Multiple Conditions (elif)

Sometimes you have more than two choices. For example, if you want to assign a grade to a student based on their marks:

• Marks 90 or above → Grade A
• Marks 75 or above → Grade B
• Marks 50 or above → Grade C
• Anything below 50 → Grade F (Fail)

To check multiple conditions one after the other, Python uses elif (which is short for "else if").

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1. The if-elif-else Chain

Python checks each condition from top to bottom. The moment it finds a condition that is True, it runs that block of code and ignores all other options below it.

marks = 82

if marks >= 90:
    print("Grade: A")
elif marks >= 75:
    print("Grade: B")
elif marks >= 50:
    print("Grade: C")
else:
    print("Grade: F (Fail)")

# Output: Grade: B

⚠️ Golden Rule: The Order Matters!

Always place your checks starting with the most restrictive or highest value at the top.

If we wrote if marks >= 50: at the very top, a student with 95 marks would trigger that first condition and get a "Grade C" immediately, and Python would skip the rest of the checks!

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2. Nested Conditions (Choices Inside Choices)

Sometimes, you need to check a condition only after another condition has passed. This is called nesting (putting one if-else statement inside another one).

Real-Time Example: Blood Donation Eligibility

To donate blood, you must meet two rules:

1. You must be 18 years or older.
2. If you are old enough, you must weigh 50 kg or more.

Let's see how this works in Python:

age = 20
weight = 55

# Outer Check: Age
if age >= 18:
    print("Age check passed.")
    
    # Inner (Nested) Check: Weight
    if weight >= 50:
        print("You are eligible to donate blood!")
    else:
        print("Sorry, you must weigh 50 kg or more to donate.")
        
else:
    print("Sorry, you must be 18 or older to donate blood.")

Note on Nesting:

Notice the indentation! The inner if-else block is pushed even further to the right (8 spaces) because it belongs inside the outer if statement block. While nesting is very useful, try to keep it simple - deep nesting can make your code hard to read!

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What's Next?

If you have many exact options to match, writing multiple elif blocks can become tedious. Let's learn about Python's modern match-case statement and try some exercises in the next lesson!