Conditionals: Teaching Your Program to Make Choices

What You Might Be Wondering

"Code runs top to bottom, right? Why would it branch?"

Because the real world is full of "if X then Y, otherwise Z" scenarios. Conditionals turn business rules into executable logic.

One-Line Definition

Conditionals use if / elif / else to pick different execution paths based on boolean results.

Real-Life Analogy

A subway turnstile:

• Enough balance -> pass through
• Insufficient balance -> denied

That's your basic if/else right there.

Minimal Working Example

balance = 120
price = 99

if balance >= price:
    print("Payment successful")
else:
    print("Insufficient balance")

Multi-Branch Example

score = 82

if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 60:
    grade = "C"
else:
    grade = "D"

print(grade)

Combining Conditions

is_member = True
order_amount = 180

if is_member and order_amount >= 100:
    print("Eligible for member bulk discount")

Quick Quiz (5 min)

1. Given a score, output the grade (A/B/C/D).
2. Check whether "is a member AND balance > 100" is met.
3. Check if a username is an empty string.

Quiz Rubric & Grading Criteria

• Direction: write runnable code that covers the core requirements and edge cases from the prompt.
• Criterion 1 (Correctness): main flow produces correct results, key branches execute.
• Criterion 2 (Readability): clear variable names, no excessive nesting.
• Criterion 3 (Robustness): basic protection against empty values, type errors, or unexpected input.

Take-Home Task

Write a "coupon eligibility checker" function:

• Conditions: member + minimum amount + not expired
• Output: eligible/not eligible + reason

Acceptance Criteria

You can independently:

• Write single-branch, two-branch, and multi-branch logic
• Combine and/or/not for business conditions
• Maintain correct branch structure through proper indentation

Common Errors & Debugging Steps (Beginner Edition)

• Error message looks like gibberish: read the last line for the error type (TypeError, NameError, etc.), then trace back to the offending line.
• Not sure what a variable holds: drop a temporary print(variable, type(variable)) to check.
• Changed code but nothing happened: make sure you saved the file, you're running the right file, and your terminal environment (venv) is correct.

Common Misconceptions

• Misconception: mixing up = and ==.

• Reality: = assigns. == compares.

• Misconception: nesting branches deeply is fine.

• Reality: extract complex rules into functions. Keep the main flow readable.