Day 3: Control Flow

What You'll Learn Today

• Conditional statements (if, elif, else)
• Comparison and logical operators
• Match statement (Python 3.10+)
• Loops (while, for)
• break, continue, and else

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Conditional Statements

Programs can execute different code based on conditions. This is called "branching."

flowchart TB
    A["Check condition"] --> B{Is it true?}
    B -->|Yes| C["Execute action A"]
    B -->|No| D["Execute action B"]
    C --> E["Continue"]
    D --> E

    style A fill:#3b82f6,color:#fff
    style B fill:#f59e0b,color:#fff
    style C fill:#22c55e,color:#fff
    style D fill:#ef4444,color:#fff

Basic if Statement

age = 20

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

Important: In Python, indentation is part of the syntax. The block after if must be indented.

if-else Statement

age = 15

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

if-elif-else Statement

For checking multiple conditions:

score = 75

if score >= 90:
    print("Grade: A")
elif score >= 80:
    print("Grade: B")
elif score >= 70:
    print("Grade: C")
elif score >= 60:
    print("Grade: D")
else:
    print("Grade: F")

flowchart TB
    A["score = 75"] --> B{score >= 90?}
    B -->|No| C{score >= 80?}
    C -->|No| D{score >= 70?}
    D -->|Yes| E["Grade: C"]

    style A fill:#3b82f6,color:#fff
    style E fill:#22c55e,color:#fff

Nested Conditionals

You can nest conditionals inside each other:

age = 25
has_license = True

if age >= 18:
    if has_license:
        print("You can drive")
    else:
        print("Please get a license")
else:
    print("Wait until you're 18")

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Comparison Operators

Operators used in conditional expressions:

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

x = 10

print(x == 10)  # True
print(x != 5)   # True
print(x > 5)    # True
print(x < 15)   # True

String Comparison

name = "Python"

print(name == "Python")  # True
print(name == "python")  # False (case-sensitive)
print(name.lower() == "python")  # True

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Logical Operators

Combine multiple conditions:

Operator	Meaning	Example
and	Both must be True	x > 0 and x < 10
or	Either can be True	x < 0 or x > 10
not	Negation	not x > 5

age = 25
income = 500000

# and: Both conditions must be met
if age >= 20 and income >= 300000:
    print("Eligible for loan application")

# or: Either condition must be met
day = "Saturday"
if day == "Saturday" or day == "Sunday":
    print("It's the weekend!")

# not: Reverse the condition
is_raining = False
if not is_raining:
    print("You can go outside")

Chained Comparisons

Python allows mathematical notation:

x = 5

# Traditional way
if x >= 1 and x <= 10:
    print("Between 1 and 10")

# Pythonic way
if 1 <= x <= 10:
    print("Between 1 and 10")

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The in Operator

Check if a value is in a sequence:

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

if "banana" in fruits:
    print("We have bananas")

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

# Works with strings too
text = "Hello, Python!"
if "Python" in text:
    print("Contains Python")

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Truthiness and Falsiness

In Python, various values evaluate as True or False:

Falsy Values	Truthy Values
False	True
None	Everything else
0, 0.0	Non-zero numbers
"" (empty string)	Non-empty strings
[] (empty list)	Non-empty lists
{} (empty dict)	Non-empty dicts

# Empty list is Falsy
items = []
if items:
    print("Has items")
else:
    print("List is empty")

# Zero is Falsy
count = 0
if count:
    print("Count is greater than 0")
else:
    print("Count is 0")

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Match Statement (Python 3.10+)

Structural pattern matching introduced in Python 3.10:

command = "start"

match command:
    case "start":
        print("Starting program")
    case "stop":
        print("Stopping program")
    case "restart":
        print("Restarting program")
    case _:
        print("Unknown command")

Pattern Matching Examples

def describe_point(point):
    match point:
        case (0, 0):
            return "Origin"
        case (0, y):
            return f"On Y-axis: y={y}"
        case (x, 0):
            return f"On X-axis: x={x}"
        case (x, y):
            return f"Point: ({x}, {y})"

print(describe_point((0, 0)))   # Origin
print(describe_point((0, 5)))   # On Y-axis: y=5
print(describe_point((3, 4)))   # Point: (3, 4)

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while Loop

Executes code repeatedly while a condition is True:

flowchart TB
    A["Start"] --> B{Condition true?}
    B -->|Yes| C["Execute code"]
    C --> D["Update variable"]
    D --> B
    B -->|No| E["End"]

    style A fill:#3b82f6,color:#fff
    style B fill:#f59e0b,color:#fff
    style C fill:#22c55e,color:#fff
    style E fill:#ef4444,color:#fff

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

# Output: 1 2 3 4 5

Beware of Infinite Loops

# Bad: Infinite loop (forgot to update count)
# count = 1
# while count <= 5:
#     print(count)  # Loops forever since count never changes

# Good: Always progress toward the exit condition
count = 1
while count <= 5:
    print(count)
    count += 1  # Don't forget this!

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for Loop

Iterates through elements of a sequence (list, string, etc.):

# Process list elements
fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print(fruit)

# Process each character in a string
for char in "Python":
    print(char)

The range() Function

Generates a sequence of numbers:

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

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

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

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

The enumerate() Function

Get both index and value:

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

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

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

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The break Statement

Exits the loop early:

for i in range(10):
    if i == 5:
        break
    print(i)

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

Practical Example: Input Validation

while True:
    password = input("Enter password: ")
    if password == "secret123":
        print("Login successful!")
        break
    print("Wrong password. Try again.")

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The continue Statement

Skips to the next iteration:

for i in range(10):
    if i % 2 == 0:  # Skip even numbers
        continue
    print(i)

# Output: 1, 3, 5, 7, 9 (odd numbers only)

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Loop else Clause

Executes when the loop completes without break:

# When break doesn't occur
for i in range(5):
    print(i)
else:
    print("Loop completed normally")

# When break occurs
for i in range(5):
    if i == 3:
        break
    print(i)
else:
    print("This won't print")

Practical Example: Search

numbers = [1, 3, 5, 7, 9]
target = 4

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

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Nested Loops

Loops can be placed inside other loops:

# Multiplication table
for i in range(1, 10):
    for j in range(1, 10):
        print(f"{i * j:2}", end=" ")
    print()  # New line

Output:

 1  2  3  4  5  6  7  8  9
 2  4  6  8 10 12 14 16 18
 3  6  9 12 15 18 21 24 27
...

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Summary

Syntax	Description	Example
if	Execute if condition is true	if x > 0:
elif	Execute if previous false and this true	elif x < 0:
else	Execute if all conditions false	else:
match	Pattern matching (3.10+)	match value:
while	Loop while condition is true	while count < 10:
for	Iterate over sequence	for item in items:
break	Exit the loop	break
continue	Skip to next iteration	continue

Key Takeaways

1. Python uses indentation to define code blocks
2. if-elif-else efficiently checks multiple conditions
3. while is condition-based, for is sequence-based
4. break exits the loop, continue skips to next iteration
5. Loop else runs when no break occurred

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Practice Exercises

Exercise 1: Basics

Write a program that asks for a number and determines if it's positive, negative, or zero.

Exercise 2: FizzBuzz

Display numbers 1-30, but print "Fizz" for multiples of 3, "Buzz" for multiples of 5, and "FizzBuzz" for multiples of both.

Challenge

Write a program that asks for a positive integer and determines if it's a prime number.

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References

• Python Docs - if Statements
• Python Docs - for Statements
• Python Docs - match Statements

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Next Up: In Day 4, you'll learn about "Data Structures." Master lists, tuples, dictionaries, and sets!