Topic 03: Control Flow — Conditions

🎯 Learning Objectives

Write if, if-else, and if-elif-else statements
Use comparison operators: ==, !=, <, >, <=, >=
Use logical operators: and, or, not
Write nested if statements
Understand truthy and falsy values in Python

📚 Key Concepts

Basic if Statement

Executes a block only if the condition is True. Indentation (4 spaces) defines the block.

if condition:
    # runs when condition is True
    statement

score = 75
if score >= 50:
    print("Pass")

if-else Statement

One branch runs when condition is True, the other when it's False.

score = 40
if score >= 50:
    print("Pass")
else:
    print("Fail")

if-elif-else Chain

Checks multiple conditions in order — only the first matching branch runs.

score = 72
if score >= 80:
    grade = "HD"
elif score >= 70:
    grade = "D"
elif score >= 60:
    grade = "C"
elif score >= 50:
    grade = "P"
else:
    grade = "N"
print("Grade:", grade)   # Grade: D

Comparison Operators

Operator	Meaning	Example
`==`	Equal to	`x == 5`
`!=`	Not equal to	`x != 5`
`<`	Less than	`x < 10`
`>`	Greater than	`x > 10`
`<=`	Less than or equal	`x <= 10`
`>=`	Greater than or equal	`x >= 10`

Logical Operators

Operator	Meaning	Example
`and`	Both must be True	`x > 0 and x < 10`
`or`	At least one True	`x < 0 or x > 100`
`not`	Inverts True/False	`not (x == 5)`

age = 20
if age >= 18 and age <= 65:
    print("Working age")

Truthy and Falsy Values

In Python, the following values are considered False (falsy):

0, 0.0 — zero numbers
"" — empty string
[], (), {} — empty collections
None
False

Everything else is True (truthy).

Nested if Statements

An if inside another if. Be careful with indentation.

age = 20
has_license = True

if age >= 18:
    if has_license:
        print("Can drive")
    else:
        print("Need a license")
else:
    print("Too young")

Chaining Comparisons

Python allows chained comparisons — very readable:

# Checks if 0 < x and x < 10
0 < x < 10        # Pythonic way

# Equivalent but longer:
x > 0 and x < 10  # Same result

💻 Code Examples

Example 1 — BMI calculator with conditions

weight = float(input("Weight (kg): "))
height = float(input("Height (m): "))
bmi = weight / (height ** 2)

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

Example 2 — Using logical operators

username = input("Username: ")
password = input("Password: ")

if username == "admin" and password == "1234":
    print("Login successful")
else:
    print("Invalid credentials")

⚠️ Exam Focus

Know that = is assignment, == is comparison — mixing them is a very common error.
In an if-elif-else chain, only the first matching branch runs — order matters!
Trace through nested if statements and identify exactly what gets printed for given inputs.
Know logical operators: and requires both True; or needs at least one True.
Indentation errors are syntax errors — be precise in exam code.

❌ Common Mistakes

Using = instead of == in conditions — assignment vs comparison.
Wrong indentation — Python uses indentation to define blocks; wrong indent = wrong logic.
Putting conditions in wrong order in elif chain — most specific first.
Forgetting the colon : after if, elif, else.

⚡ Quick Recap

if cond: runs when True; else: runs when False.
elif checks additional conditions — only first match runs.
Comparison: ==, !=, <, >, <=, >=.
Logical: and, or, not — combine boolean expressions.
Indentation defines which statements belong to which block.

← Topic 02: Expressions Topic 04: Loops →

Control Flow: Conditions