MASTER YOUR LOGIC BUILDING:Phase 1 - Level 2: Nested If & Multiple Conditions

Phase 1 - Level 2: Nested If & Multiple Conditions

Why This Matters

At a theme park, the ride operator doesn't just ask your age — they also check your height. You need to be both old enough and tall enough to ride. Sometimes one condition isn't enough; you need to combine several checks.

That's what this level is about: asking multiple questions at once, or asking a follow-up question only after the first answer is yes.

What You'll Learn

• How to combine conditions with and, or, and not
• How to nest one if inside another
• When to use elif instead of nesting
• How to solve problems like grade calculation and shape validation

Combining Conditions

AND — both must be true

if age >= 18 and height >= 140:
    print("You can ride!")

Think: "I need this AND that."

OR — at least one must be true

if day == "Saturday" or day == "Sunday":
    print("It's the weekend!")

Think: "Either this OR that is fine."

NOT — flip the answer

if not is_raining:
    print("Let's go outside!")

Think: "Do the opposite of what this says."

Nested If — A Question Inside a Question

Sometimes you check one thing first, then ask a follow-up:

if sides_form_valid_triangle:
    if all_sides_equal:
        print("Equilateral")
    elif two_sides_equal:
        print("Isosceles")
    else:
        print("Scalene")
else:
    print("Not a valid triangle")

Step-by-step for triangle type:

1. First ask: "Can these three sides even make a triangle?"
2. Only if yes, ask: "Are all sides equal? Two equal? Or all different?"
3. Handle the invalid case in else

elif — Cleaner Than Deep Nesting

When you're picking one outcome from several (like grades), use elif:

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

Python checks from top to bottom and stops at the first match. That's why we check 90 before 80 — order matters!

Common Mistakes

• Deep nesting — more than 2–3 levels gets hard to read; try elif or and instead
• Wrong operator — "18 or older AND student ID" needs and, not or
• Checking grades in wrong order — always check highest marks first when using elif
• Forgetting parentheses in complex conditions — use them to make intent clear: (a > 0) and (b > 0)

Tips

• Write each condition as a plain-English sentence first, then translate to code
• For "pick one category" problems → use if / elif / else
• For "must pass all rules" problems → use and
• For "any one rule is enough" problems → use or

Practice Focus

Try the examples below on your own. Pay special attention to:

• Checking validity first, then type (triangle problem)
• Using an elif chain where order matters (grade calculator)
• Combining multiple conditions with and and or