Control Flow

Making Decisions and Repeating Actions

Control Flow

Programming Logic

Conditional Statements - Making decisions
Loops - Repeating actions
Control Statements - Managing flow
Logical Operators - Combining conditions

if Statements - Making Decisions

age = 18

if age >= 18:
    print("You are an adult")
    print("You can vote")
else:
    print("You are a minor")
    print("You cannot vote yet")

print("This always runs")

Key Point: Indentation defines code blocks in Python!

if-elif-else Chains

score = 85

if score >= 90:
    grade = "A"
    message = "Excellent!"
elif score >= 80:
    grade = "B" 
    message = "Good job!"
elif score >= 70:
    grade = "C"
    message = "Keep working!"
elif score >= 60:
    grade = "D"
    message = "Need improvement"
else:
    grade = "F"
    message = "Must retake"

print(f"Score: {score}, Grade: {grade}")

Comparison Operators

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

a, b = 10, 5
print(f"{a} > {b}: {a > b}")     # True
print(f"{a} == {b}: {a == b}")   # False

Logical Operators

age = 25
has_license = True
has_car = False

# AND operator - both conditions must be True
can_drive = age >= 18 and has_license
print(f"Can drive: {can_drive}")  # True

# OR operator - at least one condition must be True  
can_travel = has_car or age >= 18
print(f"Can travel: {can_travel}")  # True

# NOT operator - reverses the boolean value
needs_license = not has_license
print(f"Needs license: {needs_license}")  # False

# Complex conditions
can_rent_car = age >= 21 and has_license and not has_car

Membership & Identity Operators

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

# Membership operators
if my_fruit in fruits:
    print(f"{my_fruit} is available!")

if "grape" not in fruits:
    print("We don't have grapes")

# Identity operators (be careful!)
list1 = [1, 2, 3]
list2 = [1, 2, 3]  
list3 = list1

print(list1 == list2)  # True (same content)
print(list1 is list2)  # False (different objects)  
print(list1 is list3)  # True (same object)

for Loops - Iterating Collections

# Loop over a list
fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(f"I like {fruit}")

# Loop over a string
for letter in "Python":
    print(letter)

# Loop with enumerate (get index + value)
for index, fruit in enumerate(fruits):
    print(f"{index + 1}. {fruit}")

# Loop over dictionary
student = {"name": "Alice", "age": 20, "major": "CS"}
for key, value in student.items():
    print(f"{key}: {value}")

range() - Generating Number Sequences

# Basic range - numbers 0 to 4
for i in range(5):
    print(i)  # 0, 1, 2, 3, 4

# Range with start and stop
for i in range(1, 6):
    print(i)  # 1, 2, 3, 4, 5

# Range with step
for i in range(0, 11, 2):
    print(i)  # 0, 2, 4, 6, 8, 10

# Countdown
for i in range(5, 0, -1):
    print(i)  # 5, 4, 3, 2, 1

while Loops - Condition-Based Repetition

# Basic while loop
count = 0
while count < 5:
    print(f"Count: {count}")
    count += 1

# Input validation loop
while True:
    user_input = input("Enter 'quit' to exit: ")
    if user_input.lower() == 'quit':
        break
    print(f"You entered: {user_input}")

# Number guessing game
import random
secret = random.randint(1, 10)
guess = 0
attempts = 0

while guess != secret:
    guess = int(input("Guess a number (1-10): "))
    attempts += 1
    if guess < secret:
        print("Too low!")
    elif guess > secret:
        print("Too high!")

print(f"Correct! It took {attempts} attempts.")

Loop Control - break, continue, pass

# break - exit loop immediately
for i in range(10):
    if i == 5:
        break  # Stop loop when i is 5
    print(i)  # Prints: 0, 1, 2, 3, 4

# continue - skip to next iteration
for i in range(5):
    if i == 2:
        continue  # Skip when i is 2
    print(i)  # Prints: 0, 1, 3, 4

# pass - do nothing (placeholder)
for i in range(3):
    if i == 1:
        pass  # TODO: implement this later
    else:
        print(f"Processing item {i}")

Nested Loops

# Multiplication table
print("Multiplication Table:")
for i in range(1, 4):
    for j in range(1, 4):
        result = i * j
        print(f"{i} × {j} = {result}")
    print()  # Empty line after each row

# Creating a grid pattern
for row in range(3):
    for col in range(3):
        print(f"({row},{col})", end=" ")
    print()  # New line after each row

# Output:
# (0,0) (0,1) (0,2) 
# (1,0) (1,1) (1,2) 
# (2,0) (2,1) (2,2)

Real-World Example: Grade Calculator

def calculate_grade(scores):
    if not scores:  # Check if list is empty
        return "No scores provided"
    
    average = sum(scores) / len(scores)
    
    # Determine letter grade
    if average >= 90:
        letter = "A"
        message = "Excellent work!"
    elif average >= 80:
        letter = "B"
        message = "Good job!"
    elif average >= 70:
        letter = "C"
        message = "Keep working!"
    elif average >= 60:
        letter = "D"
        message = "Need improvement"
    else:
        letter = "F"
        message = "Must retake"
    
    return f"Average: {average:.1f}, Grade: {letter} - {message}"

# Test the function
student_scores = [85, 92, 78, 96, 88]
result = calculate_grade(student_scores)
print(result)

Pattern Printing Challenge

# Print a triangle pattern
def print_triangle(height):
    for i in range(1, height + 1):
        # Print spaces for centering
        spaces = " " * (height - i)
        # Print stars
        stars = "*" * (2 * i - 1)
        print(spaces + stars)

print_triangle(5)

Output:

    *
   ***
  *****
 *******
*********

FizzBuzz - Classic Programming Challenge

# Print numbers 1-30, but:
# - "Fizz" for multiples of 3
# - "Buzz" for multiples of 5  
# - "FizzBuzz" for multiples of both

for i in range(1, 31):
    if i % 15 == 0:        # Divisible by both 3 and 5
        print("FizzBuzz")
    elif i % 3 == 0:       # Divisible by 3
        print("Fizz")
    elif i % 5 == 0:       # Divisible by 5
        print("Buzz")
    else:
        print(i)

Prime Number Finder

def is_prime(n):
    """Check if a number is prime."""
    if n < 2:
        return False
    
    for i in range(2, int(n ** 0.5) + 1):
        if n % i == 0:
            return False
    
    return True

# Find all prime numbers up to 30
print("Prime numbers up to 30:")
primes = []
for num in range(2, 31):
    if is_prime(num):
        primes.append(num)

print(primes)  # [2, 3, 5, 7, 11, 13, 17, 19, 23, 29]

List Comprehensions - Pythonic Loops

# Traditional loop approach
squares = []
for x in range(1, 6):
    squares.append(x**2)

# List comprehension - more Pythonic!
squares = [x**2 for x in range(1, 6)]

# With conditions
even_numbers = [x for x in range(20) if x % 2 == 0]
words = ["hello", "world", "python", "code"]
long_words = [word for word in words if len(word) > 4]

# Nested comprehensions
matrix = [[i*j for j in range(1, 4)] for i in range(1, 4)]
# Result: [[1, 2, 3], [2, 4, 6], [3, 6, 9]]

Error Handling with try-except

# Handle potential errors gracefully
while True:
    try:
        user_input = input("Enter a number (or 'quit'): ")
        
        if user_input.lower() == 'quit':
            break
            
        number = int(user_input)  # This might raise ValueError
        result = 100 / number     # This might raise ZeroDivisionError
        
        print(f"100 divided by {number} = {result}")
        
    except ValueError:
        print("Please enter a valid number!")
    except ZeroDivisionError:
        print("Cannot divide by zero!")
    except Exception as e:
        print(f"An unexpected error occurred: {e}")

Best Practices & Tips

Use descriptive condition names - if is_valid_age: vs if x > 0:
Avoid deeply nested conditions - use early returns
Use elif for mutually exclusive conditions
Be careful with infinite loops in while statements
Use for loops for known iterations, while for conditional
List comprehensions for simple transformations

Common Pitfalls

Assignment vs Comparison: = vs ==
Indentation errors - Python is strict about indentation
Infinite loops - always ensure loop conditions can become False
Off-by-one errors - remember ranges are exclusive of end
Modifying lists while iterating - can cause unexpected behavior

Practice Exercises

Number Classifier: Categorize numbers as positive, negative, or zero
Password Validator: Check if password meets security requirements
Simple Calculator: Use if-elif for different operations
Pattern Generator: Create different ASCII patterns with loops
Word Game: Build a simple word guessing game

Summary

Conditionals: if, elif, else for decision making
Loops: for for iterations, while for conditions
Control: break, continue, pass for flow control
Operators: Comparison, logical, membership for conditions
Patterns: List comprehensions and nested structures

What’s Next?

Functions - Organizing code into reusable blocks
Scope and Variables - Understanding variable lifetime
Advanced Patterns - Decorators and functional programming
Real Projects - Building complete applications

Thank You!

Questions?

Ready to continue with Functions?

Keep practicing: - Solve coding challenges on HackerRank - Build a simple text-based game - Create a calculator program

Control Flow

Control Flow

Programming Logic

if Statements - Making Decisions

if-elif-else Chains

Comparison Operators

Logical Operators

Membership & Identity Operators

for Loops - Iterating Collections

range() - Generating Number Sequences

while Loops - Condition-Based Repetition

Loop Control - break, continue, pass

Nested Loops

Real-World Example: Grade Calculator

Pattern Printing Challenge

FizzBuzz - Classic Programming Challenge

Prime Number Finder

List Comprehensions - Pythonic Loops

Error Handling with try-except

Best Practices & Tips

Common Pitfalls

Practice Exercises

Summary

What’s Next?

Navigation

Thank You!

Questions?