Control Flow 🔀

What You'll Learn

• ✅ How Python makes decisions with if, elif, else
• ✅ Ternary (conditional) expressions
• ✅ match statement (Python 3.10+)
• ✅ for and while loops with real examples
• ✅ break, continue, pass, and else on loops
• ✅ Nested conditions and common mistakes to avoid

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📖 Introduction

Control flow is how a program decides what to do next. By default, Python executes code line by line from top to bottom. Control flow statements let you branch, repeat, and skip code based on conditions.

New to Python?

Start with Basics → Complexity Analysis → Data Structures → Algorithms before jumping into practice problems.

Already know Python?

Jump straight to Data Structures or Algorithms depending on what you're revising.

Keep in mind

Every topic includes real code examples, memory diagrams, and practical use cases — read them carefully, don't just skim the code.

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🔀 Conditionals

1️⃣ if / elif / else

The most fundamental decision-making structure in Python.

Real-Life Example

• If it's raining → take an umbrella
• Elif it's cloudy → maybe take one
• Else → enjoy the sun ☀️

Basic Syntax

age = 20

if age < 13:
    print("Child")
elif age < 18:
    print("Teenager")
elif age < 60:
    print("Adult")
else:
    print("Senior")

Output:

Adult

How Python Evaluates It

age = 20

if age < 13  → False  ✗ skip
elif age < 18 → False  ✗ skip
elif age < 60 → True   ✓ execute → print "Adult"
else          → skipped (a branch already ran)

Practical Example: Grade System

marks = int(input("Enter your marks (0-100): "))

if marks >= 90:
    grade = "A"
    remark = "🌟 Excellent!"
elif marks >= 80:
    grade = "B"
    remark = "👍 Good!"
elif marks >= 70:
    grade = "C"
    remark = "😊 Average"
elif marks >= 60:
    grade = "D"
    remark = "😐 Below Average"
elif marks >= 0:
    grade = "F"
    remark = "😞 Fail"
else:
    grade = "?"
    remark = "❌ Invalid marks"

print(f"Grade: {grade} — {remark}")

Output:

Enter your marks (0-100): 85
Grade: B — 👍 Good!

Nested if

num = 15

if num > 0:
    print("Positive")
    if num % 2 == 0:
        print("and Even")
    else:
        print("and Odd")
else:
    print("Non-positive")

Output:

Positive
and Odd

Indentation is Everything

# ✅ Correct
if True:
    print("inside if")

# ❌ IndentationError
if True:
print("no indent")  # IndentationError!

Python uses indentation (4 spaces) instead of {} braces. Getting this wrong crashes your program.

Comparison Operators

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

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2️⃣ Logical Operators (and, or, not)

Combine multiple conditions into one.

age = 25
has_id = True

# and — both must be True
if age >= 18 and has_id:
    print("Entry allowed ✅")

# or — at least one must be True
is_weekend = False
is_holiday = True
if is_weekend or is_holiday:
    print("Day off! 🎉")

# not — flips True/False
is_raining = False
if not is_raining:
    print("Go for a walk 🚶")

Output:

Entry allowed ✅
Day off! 🎉
Go for a walk 🚶

Truth Table

AND:                    OR:                     NOT:
┌───────┬───────┬───┐  ┌───────┬───────┬───┐  ┌───────┬───────┐
│   A   │   B   │A∧B│  │   A   │   B   │A∨B│  │   A   │  ¬A   │
├───────┼───────┼───┤  ├───────┼───────┼───┤  ├───────┼───────┤
│ True  │ True  │ T │  │ True  │ True  │ T │  │ True  │ False │
│ True  │ False │ F │  │ True  │ False │ T │  │ False │ True  │
│ False │ True  │ F │  │ False │ True  │ T │  └───────┴───────┘
│ False │ False │ F │  │ False │ False │ F │
└───────┴───────┴───┘  └───────┴───────┴───┘

Short-Circuit Evaluation

# 'and' stops at first False
x = 0
if x != 0 and 10 / x > 1:   # safe! — stops at x != 0
    print("big")

# 'or' stops at first True
name = "Alice"
if name or input("Enter name: "):  # stops at name — no input needed
    print(f"Hello, {name}")

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3️⃣ Ternary Expression

A one-line if-else for simple conditions.

# Syntax: value_if_true if condition else value_if_false

age = 20
status = "Adult" if age >= 18 else "Minor"
print(status)

# Nested ternary (use sparingly!)
score = 75
grade = "A" if score >= 90 else "B" if score >= 80 else "C"
print(grade)

Output:

Adult
C

Traditional if-elseTernary Expression ✅

age = 20
if age >= 18:
    status = "Adult"
else:
    status = "Minor"
print(status)

age = 20
status = "Adult" if age >= 18 else "Minor"
print(status)

Don't Overuse Ternary

Keep ternary expressions simple. If you need more than one else, use a regular if-elif-else for readability.

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4️⃣ match Statement (Python 3.10+)

Python's version of switch-case — cleaner than long elif chains.

command = "quit"

match command:
    case "start":
        print("Starting the engine 🚀")
    case "stop":
        print("Stopping... 🛑")
    case "pause":
        print("Paused ⏸️")
    case "quit" | "exit":
        print("Goodbye! 👋")
    case _:
        print(f"Unknown command: {command}")

Output:

Goodbye! 👋

Match with Values

point = (0, 1)

match point:
    case (0, 0):
        print("Origin")
    case (x, 0):
        print(f"On X-axis at {x}")
    case (0, y):
        print(f"On Y-axis at {y}")
    case (x, y):
        print(f"Point at ({x}, {y})")

Output:

On Y-axis at 1

match vs if-elif

	match	if-elif
Python version	3.10+ only	All versions
Pattern matching	✅ Powerful	❌ Basic
Readability	✅ Cleaner	Verbose for many cases
Use case	Multiple exact values	Complex conditions

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🔁 Loops

5️⃣ for Loop

Iterates over any iterable — list, string, range, dict, etc.

Real-Life Example

• For each student in the class → print their name
• For each item in cart → calculate total price

Basic Syntax

fruits = ["apple", "banana", "cherry"]

for fruit in fruits:
    print(fruit)

Output:

apple
banana
cherry

range() Function

range(stop)range(start, stop)range(start, stop, step)Reverse range

for i in range(5):
    print(i, end=" ")

Output:

0 1 2 3 4

for i in range(1, 6):
    print(i, end=" ")

Output:

1 2 3 4 5

for i in range(0, 10, 2):
    print(i, end=" ")

Output:

0 2 4 6 8

for i in range(5, 0, -1):
    print(i, end=" ")

Output:

5 4 3 2 1

enumerate() — Index + Value

fruits = ["apple", "banana", "cherry"]

for index, fruit in enumerate(fruits):
    print(f"{index}: {fruit}")

# Start index from 1
for index, fruit in enumerate(fruits, start=1):
    print(f"{index}. {fruit}")

Output:

0: apple
1: banana
2: cherry
1. apple
2. banana
3. cherry

Iterating Over Strings

for char in "Python":
    print(char, end="-")

Output:

P-y-t-h-o-n-

Iterating Over Dictionaries

student = {"name": "Alice", "age": 20, "gpa": 3.8}

for key in student:
    print(key)

for key, value in student.items():
    print(f"{key}: {value}")

Output:

name
age
gpa
name: Alice
age: 20
gpa: 3.8

Nested for Loops

# Multiplication table
for i in range(1, 4):
    for j in range(1, 4):
        print(f"{i}×{j}={i*j}", end="  ")
    print()

Output:

1×1=1  1×2=2  1×3=3
2×1=2  2×2=4  2×3=6
3×1=3  3×2=6  3×3=9

Memory Visualization

for fruit in ["apple", "banana", "cherry"]:

Iteration 1:  fruit → "apple"   → body executes
Iteration 2:  fruit → "banana"  → body executes
Iteration 3:  fruit → "cherry"  → body executes
              (no more items)   → loop ends

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6️⃣ while Loop

Repeats as long as a condition is True. Use when you don't know how many times to loop.

Real-Life Example

• While the ATM has balance → allow withdrawals
• While user hasn't guessed correctly → keep asking

Basic Syntax

count = 1

while count <= 5:
    print(count, end=" ")
    count += 1

print("Done!")

Output:

1 2 3 4 5 Done!

Practical Example: Number Guessing Game

import random

secret = random.randint(1, 10)
attempts = 0

while True:
    guess = int(input("Guess (1-10): "))
    attempts += 1

    if guess < secret:
        print("Too low! 📉")
    elif guess > secret:
        print("Too high! 📈")
    else:
        print(f"🎉 Correct! Got it in {attempts} attempts!")
        break

Output:

Guess (1-10): 5
Too low! 📉
Guess (1-10): 8
Too high! 📈
Guess (1-10): 6
🎉 Correct! Got it in 3 attempts!

Infinite Loop

# ❌ This runs forever — forgot to update count!
count = 1
while count <= 5:
    print(count)
    # count += 1  ← missing!

# ✅ Always make sure the condition eventually becomes False
count = 1
while count <= 5:
    print(count)
    count += 1

for loopwhile loop

# Use when you know how many times
for i in range(10):
    print(i)

# Use when condition-based
user_input = ""
while user_input != "quit":
    user_input = input("Enter command: ")

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🎛️ Loop Control Statements

7️⃣ break

Exits the loop immediately, regardless of the condition.

numbers = [1, 3, 7, 2, 9, 4, 6]

for num in numbers:
    if num == 2:
        print(f"Found 2! Stopping.")
        break
    print(num)

Output:

1
3
7
Found 2! Stopping.

Memory Visualization

for num in [1, 3, 7, 2, 9, 4, 6]:

  num=1 → print(1)
  num=3 → print(3)
  num=7 → print(7)
  num=2 → condition True → BREAK ──────────────────┐
  num=9 → never reached                             │
  num=4 → never reached                             │
  num=6 → never reached                             │
                                                    ▼
                                          continues after loop

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8️⃣ continue

Skips the current iteration and jumps to the next one.

for i in range(1, 11):
    if i % 2 == 0:
        continue        # skip even numbers
    print(i, end=" ")

Output:

1 3 5 7 9

Practical: Skip Invalid Input

data = [10, -5, 20, -3, 15, 0, 8]

total = 0
for num in data:
    if num <= 0:
        continue        # skip non-positive
    total += num

print(f"Sum of positives: {total}")

Output:

Sum of positives: 53

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9️⃣ pass

Does absolutely nothing — a placeholder for empty blocks.

# Placeholder for future code
for i in range(5):
    pass    # TODO: add logic later

# Empty function body
def my_function():
    pass

# Empty class
class MyClass:
    pass

print("No errors! ✅")

Output:

No errors! ✅

Why pass?

Python doesn't allow empty blocks — an if, for, def, or class with no body causes a SyntaxError. pass is the fix.

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🔟 else on Loops

Python has a unique feature — else on loops. The else block runs only if the loop completed without break.

# Search for a number
numbers = [1, 3, 5, 7, 9]
target = 6

for num in numbers:
    if num == target:
        print(f"Found {target}!")
        break
else:
    print(f"{target} not found in list.")

Output:

6 not found in list.

With break (else skipped)

numbers = [1, 3, 5, 6, 9]
target = 6

for num in numbers:
    if num == target:
        print(f"Found {target}!")
        break
else:
    print(f"{target} not found.")   # skipped because break ran

Output:

Found 6!

while-else

attempts = 0
max_attempts = 3

while attempts < max_attempts:
    password = input("Enter password: ")
    if password == "secret123":
        print("✅ Access granted!")
        break
    attempts += 1
else:
    print("❌ Too many failed attempts. Locked out!")

Loop else — When to Use

Perfect for search loops where you want to know if something was found or not, without using a flag variable.

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✅ Quick Reference Summary

Statement	Purpose	Runs when
if	Check condition	condition is True
elif	Alternative condition	previous was False, this is True
else	Fallback	all above were False
match	Pattern matching (3.10+)	value matches a case
for	Iterate over iterable	each item in sequence
while	Repeat while condition	condition is True
break	Exit loop early	immediately on execution
continue	Skip current iteration	immediately on execution
pass	Do nothing (placeholder)	immediately, no effect
for-else	Run after loop	loop finished without break