Week 1 • Tuesday

Block 3: if/else and Conditionals

Control program flow with if/elif/else.

Concepts

if/elif/else syntax and indentation
Comparison operators: ==, !=, <, >, <=, >=
Logical operators: and, or, not
Nested conditionals

Code Examples

Basic if/elif/else

score = 85
if score >= 90:
    print('A')
elif score >= 80:
    print('B')
else:
    print('C or below')

Logical operators

age = 25
has_ticket = True
if age >= 18 and has_ticket:
    print('Entry allowed')
if age < 13 or age > 65:
    print('Eligible for discount')

Exercise

Write a BMI category classifier (underweight, normal, overweight, obese). Given a score (0–100), print the letter grade A/B/C/D/F.

Solution

def bmi_category(bmi):
    if bmi < 18.5: return 'underweight'
    elif bmi < 25: return 'normal'
    elif bmi < 30: return 'overweight'
    else: return 'obese'

def letter_grade(score):
    if score >= 90: return 'A'
    elif score >= 80: return 'B'
    elif score >= 70: return 'C'
    elif score >= 60: return 'D'
    else: return 'F'

print(bmi_category(22))   # normal
print(letter_grade(85))  # B

Practice Problems

Problem 1: Write a function that returns 'even' or 'odd' for any integer.

Hint: Use % 2

Problem 2: Given age, print 'minor' if <18, 'adult' if 18-64, 'senior' otherwise.

Hint: Use elif chain

Application

Conditionals are used everywhere: filtering data, validating inputs, branching in ML pipelines, and business logic.

Case Study

An e-commerce site uses conditionals: if cart_total > 100 and is_member: apply 10% discount. Nested conditions handle complex rules.

Homework

Explain why indentation matters in Python. Write an example that would break without correct indentation.