03 - Control Flow

📌 What You'll Learn

• •Conditional statements (if, elif, else)
• •Comparison and logical operators in conditions
• •Loops (for, while)
• •Loop control (break, continue, pass)
• •List comprehensions
• •Match-case statements (Python 3.10+)

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🔀 Conditional Statements

Conditional statements allow your program to make decisions and execute different code based on conditions.

if Statement

age = 18

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

if-else Statement

age = 15

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

if-elif-else Statement

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"Your grade is: {grade}")

Nested if Statements

age = 25
has_license = True

if age >= 18:
    if has_license:
        print("You can drive")
    else:
        print("Get a license first")
else:
    print("Too young to drive")

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🎯 Conditions and Operators

Comparison Operators in Conditions

x = 10

if x == 10:     # Equal
    print("x is 10")

if x != 5:      # Not equal
    print("x is not 5")

if x > 5:       # Greater than
    print("x is greater than 5")

if x >= 10:     # Greater or equal
    print("x is 10 or more")

Logical Operators

age = 25
has_id = True

# AND - both conditions must be True
if age >= 18 and has_id:
    print("Can enter")

# OR - at least one condition must be True
if age < 18 or not has_id:
    print("Cannot enter")

# NOT - inverts the condition
if not (age < 18):
    print("Adult")

Combining Multiple Conditions

score = 85
attendance = 90

if score >= 80 and attendance >= 80:
    print("Excellent student")
elif score >= 60 or attendance >= 90:
    print("Good student")
else:
    print("Needs improvement")

Membership Operators

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

if "apple" in fruits:
    print("Apple is available")

if "mango" not in fruits:
    print("Mango is not available")

Identity Operators

a = [1, 2, 3]
b = a
c = [1, 2, 3]

if a is b:      # Same object in memory
    print("a and b are the same object")

if a is not c:  # Different objects
    print("a and c are different objects")

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📐 Ternary Operator (Conditional Expression)

A one-line way to write simple if-else statements.

# Syntax: value_if_true if condition else value_if_false

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

# Multiple conditions
score = 85
grade = "A" if score >= 90 else "B" if score >= 80 else "C"
print(grade)  # B

# In function calls
x = 5
print("positive" if x > 0 else "zero or negative")

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🔄 Truthy and Falsy Values

In Python, values have a boolean context (truthy or falsy).

# Falsy values (evaluate to False):
# - False
# - None
# - 0 (integer zero)
# - 0.0 (float zero)
# - "" (empty string)
# - [] (empty list)
# - {} (empty dict)
# - () (empty tuple)
# - set() (empty set)

# Examples:
if []:
    print("This won't print")  # Empty list is falsy

if [1, 2, 3]:
    print("This will print")   # Non-empty list is truthy

# Common pattern: check if list has items
items = []
if items:
    print("Has items")
else:
    print("Empty list")

# Check for None
value = None
if value is None:
    print("No value provided")

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🔁 For Loop

The for loop iterates over a sequence (list, tuple, string, range, etc.).

Basic for Loop

# Iterate over a list
fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit)

# Iterate over a string
for char in "Python":
    print(char)

# Iterate over a range
for i in range(5):      # 0, 1, 2, 3, 4
    print(i)

for i in range(1, 6):   # 1, 2, 3, 4, 5
    print(i)

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

for i in range(10, 0, -1):  # 10, 9, 8... 1 (countdown)
    print(i)

enumerate() - Get Index and Value

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

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

# Output:
# 0: apple
# 1: banana
# 2: cherry

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

zip() - Iterate Multiple Sequences

names = ["Alice", "Bob", "Charlie"]
ages = [25, 30, 35]

for name, age in zip(names, ages):
    print(f"{name} is {age} years old")

# With three lists
scores = [90, 85, 95]
for name, age, score in zip(names, ages, scores):
    print(f"{name}, {age}, scored {score}")

Nested for Loops

# Multiplication table
for i in range(1, 4):
    for j in range(1, 4):
        print(f"{i} x {j} = {i*j}")
    print()  # Empty line between tables

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🔄 While Loop

The while loop repeats as long as a condition is True.

Basic while Loop

count = 0
while count < 5:
    print(count)
    count += 1

# Output: 0, 1, 2, 3, 4

while with User Input

# Keep asking until valid input
while True:
    answer = input("Enter 'quit' to exit: ")
    if answer.lower() == 'quit':
        break
    print(f"You entered: {answer}")

while with else

count = 0
while count < 3:
    print(count)
    count += 1
else:
    print("Loop completed normally")

# The else block runs when the loop completes without break

Avoiding Infinite Loops

# BAD - Infinite loop!
# while True:
#     print("Forever...")

# GOOD - Has exit condition
count = 0
while count < 10:
    print(count)
    count += 1  # Don't forget to update!

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🛑 Loop Control Statements

break - Exit the Loop

# Stop when we find what we're looking for
for num in range(10):
    if num == 5:
        print("Found 5!")
        break
    print(num)
# Output: 0, 1, 2, 3, 4, Found 5!

continue - Skip to Next Iteration

# Skip odd numbers
for num in range(10):
    if num % 2 != 0:
        continue
    print(num)
# Output: 0, 2, 4, 6, 8

pass - Do Nothing (Placeholder)

# Placeholder for future code
for num in range(5):
    if num == 3:
        pass  # TODO: handle this case later
    else:
        print(num)

# Empty function placeholder
def my_function():
    pass  # Will implement later

else with Loops

# else runs if loop completes WITHOUT break
for num in range(5):
    if num == 10:  # Never true
        break
else:
    print("Loop completed without finding 10")

# else does NOT run if break is executed
for num in range(5):
    if num == 3:
        break
else:
    print("This won't print")  # break was executed

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📋 List Comprehensions

A concise way to create lists based on existing sequences.

Basic Syntax

# [expression for item in iterable]

# Traditional way
squares = []
for x in range(5):
    squares.append(x ** 2)

# List comprehension
squares = [x ** 2 for x in range(5)]
print(squares)  # [0, 1, 4, 9, 16]

With Condition

# [expression for item in iterable if condition]

# Even numbers only
evens = [x for x in range(10) if x % 2 == 0]
print(evens)  # [0, 2, 4, 6, 8]

# Filter and transform
words = ["hello", "world", "python", "hi"]
long_words_upper = [w.upper() for w in words if len(w) > 3]
print(long_words_upper)  # ['HELLO', 'WORLD', 'PYTHON']

With if-else

# [expr_if_true if condition else expr_if_false for item in iterable]

numbers = [1, 2, 3, 4, 5]
labels = ["even" if n % 2 == 0 else "odd" for n in numbers]
print(labels)  # ['odd', 'even', 'odd', 'even', 'odd']

Nested Comprehensions

# Flatten a 2D list
matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
flat = [num for row in matrix for num in row]
print(flat)  # [1, 2, 3, 4, 5, 6, 7, 8, 9]

# Create a 2D list
grid = [[i * j for j in range(1, 4)] for i in range(1, 4)]
print(grid)  # [[1, 2, 3], [2, 4, 6], [3, 6, 9]]

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📦 Dictionary and Set Comprehensions

Dictionary Comprehension

# {key: value for item in iterable}

# Create dict from two lists
names = ["a", "b", "c"]
values = [1, 2, 3]
my_dict = {k: v for k, v in zip(names, values)}
print(my_dict)  # {'a': 1, 'b': 2, 'c': 3}

# Square numbers dict
squares = {x: x**2 for x in range(5)}
print(squares)  # {0: 0, 1: 1, 2: 4, 3: 9, 4: 16}

# Filter
even_squares = {x: x**2 for x in range(10) if x % 2 == 0}
print(even_squares)  # {0: 0, 2: 4, 4: 16, 6: 36, 8: 64}

Set Comprehension

# {expression for item in iterable}

# Unique squared values
nums = [1, 2, 2, 3, 3, 3]
unique_squares = {x**2 for x in nums}
print(unique_squares)  # {1, 4, 9}

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🎭 Match-Case (Python 3.10+)

Pattern matching similar to switch-case in other languages.

Basic Match

status = 404

match status:
    case 200:
        print("OK")
    case 404:
        print("Not Found")
    case 500:
        print("Server Error")
    case _:
        print("Unknown status")  # _ is the default case

Match with Patterns

# Match with multiple values
def get_day_type(day):
    match day.lower():
        case "saturday" | "sunday":
            return "Weekend"
        case "monday" | "tuesday" | "wednesday" | "thursday" | "friday":
            return "Weekday"
        case _:
            return "Invalid day"

# Match with guards
def classify_number(n):
    match n:
        case n if n < 0:
            return "negative"
        case 0:
            return "zero"
        case n if n > 0:
            return "positive"

# Match with sequences
def analyze_point(point):
    match point:
        case (0, 0):
            return "Origin"
        case (0, y):
            return f"On Y-axis at y={y}"
        case (x, 0):
            return f"On X-axis at x={x}"
        case (x, y):
            return f"Point at ({x}, {y})"

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📝 Summary

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🎯 Next Steps

After mastering control flow, proceed to 04_data_structures to learn about lists, tuples, dicts, and sets!