Conditionals — if/elif/else

Why read this? Programs rarely follow a straight line. They need to make choices: "If the user is logged in, show their profile. Otherwise, show the login page." Conditionals are how programs make decisions based on data.

🎯 Learning Objectives

1. Write if statements to execute code conditionally
2. Use if-else for two-way decisions
3. Chain multiple conditions with elif
4. Nest conditionals inside each other
5. Use boolean expressions effectively
6. Understand truthiness — which values are "falsy"

📋 Prerequisites

• Operators & Expressions — Understanding comparison and logical operators.
• Variables & Data Types — Basic variable usage.

📖 Core Content

6.1 What Problem Do Conditionals Solve?

6.2 The if Statement — Simplest Decision

python
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# runnable
age = 18
if age >= 18:
    print("You are eligible to vote.")
    print("Please register.")
print("This always runs.")

pseudo
You are eligible to vote.
Please register.
This always runs.

1. Python evaluates age >= 18 → True.
2. Since the condition is True, the indented block under if executes.
3. After the block, execution continues with the next unindented line. Syntax rules:

• The condition must end with a colon :.
• The body must be indented (usually 4 spaces).
• All lines in the block must have the same indentation. (Diagram)

6.3 if-else — Two Paths

python
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# runnable
temperature = 35
if temperature > 30:
    print("It's hot outside!")
    print("Stay hydrated.")
else:
    print("It's cool outside.")
    print("Enjoy the weather!")
print("Stay safe!")

pseudo
It's hot outside!
Stay hydrated.
Stay safe!

6.4 elif — Multiple Conditions

python
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# runnable
score = 85
if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
elif score >= 60:
    grade = "D"
else:
    grade = "F"
print(f"Score: {score}, Grade: {grade}")

pseudo
Score: 85, Grade: B

• Python checks conditions top to bottom.
• The first True condition's block executes.
• The rest are skipped — even if later conditions are also True.
• else at the end catches everything not caught above.

6.5 Truthiness and Falsy Values

python
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# runnable
# These are all FALSY (treated as False):
if 0:
    print("0 is truthy")
if "":
    print("Empty string is truthy")
if None:
    print("None is truthy")
if []:
    print("Empty list is truthy")
if {}:
    print("Empty dict is truthy")
print("None of the above printed because all are falsy.")
# Everything else is TRUTHY:
if 42:
    print("42 is truthy")
if "Hello":
    print("Non-empty string is truthy")
if [1, 2]:
    print("Non-empty list is truthy")

pseudo
None of the above printed because all are falsy.
42 is truthy
Non-empty string is truthy
Non-empty list is truthy

6.6 Worked Example 1: Even or Odd

python
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# runnable
number = int(input("Enter a number: "))
if number % 2 == 0:
    print(f"{number} is even.")
else:
    print(f"{number} is odd.")
# Also check divisibility by 5 and 10
if number % 10 == 0:
    print(f"{number} is divisible by 10.")
elif number % 5 == 0:
    print(f"{number} is divisible by 5 but not 10.")

pseudo
Enter a number: 15
15 is odd.
15 is divisible by 5 but not 10.

6.7 Worked Example 2: Number Classifier

python
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# runnable
num = float(input("Enter a number: "))
if num > 0:
    print(f"{num} is positive.")
    if num.is_integer() and num > 100:
        print("It's a large positive integer.")
elif num < 0:
    print(f"{num} is negative.")
    if num > -10:
        print("It's a small negative number (between -10 and 0).")
else:
    print("The number is zero.")

pseudo
Enter a number: -5
-5.0 is negative.
It's a small negative number (between -10 and 0).

6.8 Worked Example 3: Login System

python
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# runnable
username = input("Username: ")
password = input("Password: ")
# Simulated database
correct_user = "admin"
correct_pass = "secret123"
if username == correct_user and password == correct_pass:
    print("Login successful!")
    print("Welcome, admin.")
elif username == correct_user:
    print("Incorrect password.")
elif password == correct_pass:
    print("Username not found.")
else:
    print("Both username and password are incorrect.")

pseudo
Username: admin
Password: wrong
Incorrect password.

6.9 Worked Example 4: Nested Conditionals — BMI Calculator

python
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# runnable
weight = float(input("Weight (kg): "))
height = float(input("Height (m): "))
bmi = weight / (height ** 2)
print(f"Your BMI: {bmi:.1f}")
if bmi < 18.5:
    print("Category: Underweight")
    print("Tip: Consider a balanced diet to gain healthy weight.")
elif bmi < 25:
    print("Category: Normal weight")
    print("Tip: Maintain your healthy lifestyle!")
elif bmi < 30:
    print("Category: Overweight")
    print("Tip: Regular exercise can help.")
else:
    print("Category: Obese")
    if bmi >= 40:
        print("(Class III - Severe obesity)")
    elif bmi >= 35:
        print("(Class II - Moderate obesity)")
    else:
        print("(Class I - Mild obesity)")
    print("Tip: Consult a healthcare professional.")

pseudo
Weight (kg): 85
Height (m): 1.75
Your BMI: 27.8
Category: Overweight
Tip: Regular exercise can help.

6.10 Worked Example 5: Short-Circuit Evaluation in Conditionals

python
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# runnable
x = 0
y = 10
# Safe division using short-circuit
if x != 0 and y / x > 2:
    print("Condition met")
else:
    print("Either x is 0 or y/x <= 2")
    # Thanks to short-circuit, y/x never executed when x==0!

pseudo
Either x is 0 or y/x <= 2

📐 Key Concepts Reference

⚠️ Common Pitfalls

Pitfall 1: Using = Instead of ==

Pitfall 2: Indentation Errors

python
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if x > 0:
print("Positive")   # no indent!

Pitfall 3: Using elif After else

python
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if x > 0:
    print("Positive")
else:
    print("Not positive")
elif x == 0:
    print("Zero")   # This is unreachable!

Pitfall 4: Overlapping Conditions

python
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score = 75
if score >= 70:
    print("B")
elif score >= 80:
    print("A")   # This NEVER runs!

Pitfall 5: Forgot else Catches Everything

python
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if x > 0:
    print("Positive")
elif x < 0:
    print("Negative")
# What if x == 0? No output!

📝 Practice Questions

Q1: What does this code output?

python
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x = 10
if x > 5:
    print("Big")
elif x > 8:
    print("Huge")
else:
    print("Small")

Answer:

pseudo
Big

x > 5 is True (10 > 5), so the first block runs. The elif is never checked. Q2: Fix the error:

python
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if score = 100:
    print("Perfect!")

Answer: Error: SyntaxError: invalid syntax Fix: Use == for comparison: if score == 100: Q3: What's the output of this code?

python
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value = 0
if value:
    print("Truthy")
else:
    print("Falsy")

Answer:

pseudo
Falsy

0 is a falsy value in Python. So the else branch executes. Q4: Write a program that takes a number and prints "Positive", "Negative", or "Zero".

Answer:

python
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# runnable
num = float(input("Enter a number: "))
if num > 0:
    print("Positive")
elif num < 0:
    print("Negative")
else:
    print("Zero")

Q5: What does this output?

python
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n = 15
if n % 3 == 0 and n % 5 == 0:
    print("FizzBuzz")
elif n % 3 == 0:
    print("Fizz")
elif n % 5 == 0:
    print("Buzz")
else:
    print(n)

Answer:

pseudo
FizzBuzz

15 is divisible by both 3 and 5, so the first condition (and) is True. Q6: Why won't "A" ever print?

python
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score = 95
if score > 60:
    print("C")
elif score > 75:
    print("B")
elif score > 90:
    print("A")

Answer: Because score > 60 is True (95 > 60), the first block runs and prints "C". The elif conditions are never evaluated. Fix by ordering from HIGHEST to LOWEST:

python
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if score > 90: print("A")
elif score > 75: print("B")
elif score > 60: print("C")

Q7: Write a program that checks if a year is a leap year.

Answer:

python
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# runnable
year = int(input("Enter year: "))
if year % 400 == 0:
    print(f"{year} is a leap year.")
elif year % 100 == 0:
    print(f"{year} is NOT a leap year.")
elif year % 4 == 0:
    print(f"{year} is a leap year.")
else:
    print(f"{year} is NOT a leap year.")

Q8: What does bool("False") return — True or False?

Answer: True

The string "False" is non-empty, so it's truthy. bool("False") → True. Only the empty string "" is falsy. Q9: Write a ternary expression that returns "adult" if age >= 18, else "minor".

Answer:

python
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# runnable
age = 20
status = "adult" if age >= 18 else "minor"
print(status)  # adult

Q10: What does this output?

python
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a, b, c = 5, 10, 3
if a > b and a > c:
    print(a)
elif b > a and b > c:
    print(b)
else:
    print(c)

Answer:

pseudo
10

• a > b → 5 > 10 → False, so first condition fails
• b > a → 10 > 5 → True AND b > c → 10 > 3 → True, so prints b (10)

🔗 Cross-References

• Next Topic: Nested Conditionals & Logical Operators
• Previous Topic: Strings — Basics & Operations
• BSCS1001 Computational Thinking: Conditional logic maps directly to decision diamonds in flowcharts.
• BSCS2002 PDSA: Conditionals are used in all search and sort algorithms.
• Reference: Python for Everybody, Chapter 3 — "Conditional execution"
• Video: L21: Introduction to the if statement, L22: Tutorial on if, else and elif Join Discord Previous5. Strings BasicsNext7. Nested Conditionals