Lecture 4 – Conditional Statements (if / elif / else) in Python

Learn conditional statements in Python with simple examples. This Lecture 4 guide explains if, elif, else, comparison operators, real-world use cases, MCQs, and practice questions for Data Science beginners.

What You Will Learn

By the end of this lecture, you will be able to:

• Understand why and where conditional statements are used
• Write if, elif, and else correctly
• Use comparison operators
• Apply logical operators (AND, OR, NOT)
• Build real Data Science-style conditions
• Solve MCQs + practical exercises

1) What Are Conditional Statements?

A conditional statement allows Python to make a decision based on a condition.

Real-world examples:

• If rain starts → you open umbrella
• If battery < 15% → phone alerts
• If score ≥ 80 → Grade A
• If customer buys > 3 items → give discount

Python works exactly the same way.

2) IF Statement

Runs code only when condition is TRUE.

Example 1

age = 18

if age >= 18:
    print("You are an adult")

Example 2

marks = 95

if marks > 90:
    print("Excellent!")

3) IF–ELSE Statement

Used for two outcomes.

Example 1 – Pass / Fail

score = 40

if score >= 50:
    print("Pass")
else:
    print("Fail")

Example 2 – Even / Odd

number = 11

if number % 2 == 0:
    print("Even Number")
else:
    print("Odd Number")

4) IF–ELIF–ELSE Statement

Used when you have multiple conditions.

Example 1 – Grading

score = 85

if score >= 90:
    print("A+")
elif score >= 80:
    print("A")
elif score >= 70:
    print("B")
else:
    print("C")

Example 2 – Temperature Classification

temp = 32

if temp > 40:
    print("Very Hot")
elif temp > 30:
    print("Warm")
elif temp > 20:
    print("Normal")
else:
    print("Cold")

5) Comparison Operators

Used to compare values inside conditions.

Lecture 3 – Python Data Structures (Lists, Tuples, Sets, Dictionaries)

6) Logical Operators (VERY IMPORTANT!)

Used when combining multiple conditions.

and → All conditions must be TRUE

or → At least one condition must be TRUE

not → Reverses the condition

Example – Admission Eligibility

marks = 85
age = 17

if marks >= 80 and age <= 18:
    print("Eligible for admission")

Example – Discount Eligibility

customer_type = "member"
amount = 5000

if customer_type == "member" or amount > 4000:
    print("Discount applied!")

Example – NOT

logged_in = False

if not logged_in:
    print("Please log in first.")

7) Real Data Science Examples

Example 1 – Categorizing BMI

bmi = 27

if bmi < 18.5:
    print("Underweight")
elif bmi < 25:
    print("Normal")
elif bmi < 30:
    print("Overweight")
else:
    print("Obese")

Example 2 – Classifying Customer Spending

spent = 1200

if spent >= 2000:
    print("Premium Customer")
elif spent >= 1000:
    print("Regular Customer")
else:
    print("Basic Customer")

Example 3 – Weather Condition from Dataset

humidity = 82

if humidity > 90:
    print("Rain Expected")
elif humidity > 70:
    print("Humid")
else:
    print("Normal Weather")

https://docs.python.org/3/tutorial/controlflow.html

Example 4 – Fraud Detection Logic

transaction_amount = 50000
location_changed = True

if transaction_amount > 30000 and location_changed:
    print("High Risk Transaction")
else:
    print("Normal Transaction")

Which operator checks equality?

a) =
b) ==
c) !=
d) eq

Answer: b) ==

Which keyword is used for multiple conditions?

a) and
b) elif
c) else
d) check

Answer: b) elif

What will this print?

age = 16
if age >= 18:
    print("Adult")
else:
    print("Minor")

a) Error
b) Adult
c) Minor
d) Nothing

Answer: c) Minor

What is the output?

x = 10
if x > 5 and x < 20:
    print("Valid")
else:
    print("Invalid")

a) Valid
b) Invalid
c) Error
d) Both

Answer: a) Valid

age = 17

if age >= 18:
    print("Eligible")
else:
    print("Not Eligible")

units = 450

if units >= 500:
    print("High Usage")
elif units >= 300:
    print("Medium Usage")
else:
    print("Low Usage")

num = -4

if num > 0:
    print("Positive")
elif num < 0:
    print("Negative")
else:
    print("Zero")

marks = 92
attendance = 75

if marks >= 90 and attendance >= 80:
    print("Top Student")
else:
    print("Needs Improvement")