Python Chapter 3: Flow Control

Master Conditional Logic, Loops, and Program Flow

Introduction to Flow Control

Flow control statements allow your program to make decisions and repeat actions. They are the building blocks of program logic, enabling your code to respond to different conditions and process data efficiently.

Key Concepts Covered

Conditional Statements: if, elif, else - Make decisions based on conditions
Comparison Operators: ==, !=, <, >, <=, >= - Compare values
Logical Operators: and, or, not - Combine conditions
While Loops: Repeat code while a condition is true
For Loops: Iterate over sequences with range()
Loop Control: break, continue, and pass statements
Loop-Else: Execute code when loops complete normally
Truthy/Falsy: Understanding boolean evaluation

Conditional Statements (if/elif/else)

Conditional statements allow your program to execute different code blocks based on whether conditions are true or false.

# Basic if statement
if temperature > 30:
    print("It's hot outside!")

# if-else statement
if age >= 18:
    print("You can vote")
else:
    print("You cannot vote yet")

# if-elif-else statement
if score >= 90:
    grade = "A"
elif score >= 80:
    grade = "B"
elif score >= 70:
    grade = "C"
else:
    grade = "F"

Key Point: Python uses indentation (usually 4 spaces) to define code blocks. All statements at the same indentation level belong to the same block.

Comparison and Logical Operators

Comparison Operators

==	Equal to
!=	Not equal to
<	Less than
>	Greater than
<=	Less than or equal
>=	Greater than or equal

Logical Operators

and	Both conditions must be True
or	At least one condition must be True
not	Reverses the boolean value

# Using logical operators
if age >= 18 and has_license:
    print("You can drive")

if is_weekend or is_holiday:
    print("Time to relax!")

if not is_raining:
    print("Let's go for a walk")

Loops: While and For

While Loops

Execute code repeatedly while a condition is true.

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

# While with break
while True:
    user_input = input("Enter 'quit' to exit: ")
    if user_input == "quit":
        break

# While with continue
i = 0
while i < 10:
    i += 1
    if i % 2 == 0:
        continue  # Skip even numbers
    print(i)

For Loops

Iterate over sequences using the range() function.

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

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

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

# Nested loops
for i in range(3):
    for j in range(3):
        print(f"i={i}, j={j}")

Special Loop Features

Loop-Else Clause

The else clause executes when a loop completes normally (without hitting a break statement).

# For-else example
for n in range(2, 10):
    for x in range(2, n):
        if n % x == 0:
            print(f"{n} is not prime")
            break
    else:
        print(f"{n} is prime")

The Pass Statement

A null operation that does nothing. Used as a placeholder.

# Using pass as placeholder
if condition:
    pass  # TODO: implement this later
else:
    handle_error()

Truthy and Falsy Values

In Python, many values can be evaluated as True or False in boolean contexts.

# Falsy values: False, None, 0, 0.0, "", [], {}, ()
# Everything else is truthy

items = []
if not items:  # Empty list is falsy
    print("No items")

name = "Alice"
if name:  # Non-empty string is truthy
    print(f"Hello, {name}")

Interactive Flowchart Visualizer

Visualize how conditional statements flow through different execution paths. Select a code example and step through the execution.

Select Example Code:

Code Example

Enter Age:

Loop Simulator

Step through loop execution to see how variables change and understand loop behavior.

Variables

Output

Condition Builder

Build complex conditions by dragging and dropping operators and values. See how they evaluate in real-time.

Components

Click to add to workspace

Variables

age

score

temp

Comparison

Logical

and

not

Values

100

True

Build Your Condition

Click components from the palette to build a condition

Set Variable Values:

age:

score:

temp:

Result

Quick Reference Guide

if Statement Syntax

if condition:
    # code block
elif condition:
    # code block
else:
    # code block

while Loop Syntax

while condition:
    # code block
    if exit_condition:
        break
    if skip_condition:
        continue

for Loop Syntax

for var in range(start, stop, step):
    # code block

# range(5) → 0,1,2,3,4
# range(2,7) → 2,3,4,5,6
# range(0,10,2) → 0,2,4,6,8

Comparison Operators

==	Equal to
!=	Not equal to
<	Less than
>	Greater than
<=	Less than or equal to
>=	Greater than or equal to

Logical Operators

and	True if both are True
or	True if at least one is True
not	Reverses the boolean

Truthy and Falsy Values

Falsy Values:

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

Truthy Values:

Everything else!

Loop Control

break	Exit the loop immediately
continue	Skip to next iteration
pass	Do nothing (placeholder)

Loop-Else Clause

for item in items:
    if found:
        break
else:
    # Runs if no break

Common Patterns

Input Validation Loop

while True:
    user_input = input("Enter a positive number: ")
    if user_input.isdigit() and int(user_input) > 0:
        number = int(user_input)
        break
    print("Invalid input. Try again.")

Counting Loop

count = 0
for item in items:
    if item > 10:
        count += 1
print(f"Found {count} items greater than 10")

Finding an Item

for item in items:
    if item == target:
        print(f"Found {target}!")
        break
else:
    print(f"{target} not found")

Test Your Knowledge

Answer the following questions to test your understanding of Python flow control.

Your Score

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