Lesson 3: Repetition with Loops

Imagine you need to print the numbers 1 through 100. You could write 100 separate print() statements—but that would be tedious, error-prone, and wasteful. Or imagine checking a password: you want to give the user three attempts to enter the correct password. You don't know how many tries they'll need, but you know the loop should stop after three failures.

This is where loops shine. Loops automate repetition, letting you run the same code multiple times without copying it. Python gives you two fundamental loop types: for loops for definite iteration (when you know how many times to repeat) and while loops for indefinite iteration (when repetition depends on a changing condition). In this lesson, you'll learn both, understand when to use each, and recognize how to avoid common pitfalls like infinite loops.

Why Automate Repetition?

In real-world programs, repetition is everywhere:

• Processing data: Apply the same transformation to every item in a list
• User input validation: Keep asking until the user provides valid input
• Countdown timers: Count down from 10 to 1 for a game or app
• Batch operations: Convert 100 files from one format to another

Without loops, you'd copy-paste code hundreds of times. With loops, you write the logic once and let Python handle the repetition. This makes your code shorter, clearer, and easier to maintain.

For Loops: Definite Iteration

A for loop repeats code a specific number of times. You use it when you know how many iterations you need—or when you're iterating over a sequence (like a list of items).

Basic For Loop with range()

The simplest for loop uses range() to generate a sequence of numbers:

# Print numbers 1 through 10
for i in range(1, 11):
    print(i)

Expected Output:

1
2
3
4
5
6
7
8
9
10

How it works:

1. range(1, 11) generates numbers from 1 up to (but not including) 11
2. The loop variable i takes each value in sequence: 1, then 2, then 3, ... up to 10
3. The indented print(i) runs once for each value of i
4. When range() runs out of numbers, the loop ends

💬 AI Colearning Prompt

"Explain how for loops work under the hood with range() and iteration in Python."

Understanding range()

The range() function generates number sequences. It has three forms:

1. One argument (stop)

for i in range(5):
    print(i)

Output:

0
1
2
3
4

Generates: 0, 1, 2, 3, 4 (5 numbers total, starting from 0)

2. Two arguments (start, stop)

for i in range(2, 6):
    print(i)

Output:

2
3
4
5

Generates: 2, 3, 4, 5 (starts at 2, stops before 6)

3. Three arguments (start, stop, step)

# Print even numbers from 2 to 10
for i in range(2, 11, 2):
    print(i)

Output:

2
4
6
8
10

Generates: 2, 4, 6, 8, 10 (starts at 2, stops before 11, increments by 2)

Key Insight: range(start, stop, step) NEVER includes the stop value. This is Python's convention: range(1, 11) gives you 1-10, not 1-11.

🎓 Instructor Commentary

In AI-native development, you don't memorize range() parameters—you understand WHEN you need a number sequence. Syntax is cheap; recognizing "I need to repeat N times" is gold.

Practical Example: Countdown Timer

# Countdown from 10 to 1
for i in range(10, 0, -1):
    print(f"T-minus {i} seconds")

print("Liftoff!")

Expected Output:

T-minus 10 seconds
T-minus 9 seconds
T-minus 8 seconds
T-minus 7 seconds
T-minus 6 seconds
T-minus 5 seconds
T-minus 4 seconds
T-minus 3 seconds
T-minus 2 seconds
T-minus 1 seconds
Liftoff!

How it works:

• range(10, 0, -1) counts DOWN from 10 to 1 (step = -1)
• Loop runs 10 times (i = 10, 9, 8, ..., 1)
• After the loop finishes, print("Liftoff!") runs once

While Loops: Indefinite Iteration

A while loop repeats code as long as a condition remains True. You use it when you don't know in advance how many iterations you'll need—the loop continues until some condition changes.

Basic While Loop Syntax

# Count from 1 to 5 using while
count: int = 1

while count <= 5:
    print(count)
    count += 1  # CRITICAL: Update the loop variable

Expected Output:

1
2
3
4
5

How it works:

1. Before each iteration, Python checks: Is count <= 5?
2. If True, the indented code runs
3. count += 1 increases count by 1
4. Loop repeats until count becomes 6 (then count <= 5 is False)
5. Loop ends

CRITICAL: The loop variable (count) MUST change inside the loop. If it doesn't, the condition stays True forever, creating an infinite loop.

Practical Example: User Input Validation

# Keep asking until user enters valid age
age: int = 0

while age < 1 or age > 120:
    age = int(input("Enter your age (1-120): "))

    if age < 1 or age > 120:
        print("Invalid age. Please try again.")

print(f"Thank you! You are {age} years old.")

How it works:

• Loop continues while age is outside valid range (less than 1 OR greater than 120)
• User is prompted repeatedly until they enter a valid age (1-120)
• Once a valid age is entered, condition becomes False and loop exits

✨ Teaching Tip

Use your AI companion to explore edge cases: "What happens if the user enters a negative number? What if they type 'abc' instead of a number? Show me the error and explain it."

Loop Termination Conditions: How to Avoid Infinite Loops

An infinite loop runs forever because its condition never becomes False. This is one of the most common beginner errors.

Example of Infinite Loop (DON'T DO THIS)

# ⚠️ INFINITE LOOP - DO NOT RUN
count: int = 1

while count <= 5:
    print(count)
    # ERROR: Forgot to update count!

What happens:

• count starts at 1
• Condition count <= 5 is True
• Loop prints 1, then checks condition again
• count is still 1, so condition is still True
• Loop prints 1 again... and again... forever

How to avoid infinite loops:

1. Always update the loop variable inside the loop body
2. Ensure the condition can eventually become False
3. Test with small values to verify loop exits correctly
4. Use Ctrl+C to stop a runaway loop if you accidentally create one

Fixed Version

# ✅ CORRECT - Loop terminates
count: int = 1

while count <= 5:
    print(count)
    count += 1  # ✅ Updates count each iteration

For vs While: When to Use Each

Both loop types can accomplish many of the same tasks, but each has its ideal use cases:

Loop Type	Use When	Example Scenarios
for	You know how many iterations you need	Print numbers 1-100, process each item in a list, run code exactly N times
while	Repetition depends on a changing condition	User input validation, retry logic until success, game loop until player quits

Rule of Thumb:

• Known count → for loop
• Condition-based → while loop

🚀 CoLearning Challenge

Ask your AI Co-Teacher:

"Show me the same task (print numbers 1-5) implemented with BOTH a for loop and a while loop. Then explain when each approach is more appropriate."

Expected Outcome: You'll see both implementations side-by-side and understand the tradeoffs.

AI Companion Section: Comparing For and While

Let's solve the same problem both ways to see the differences:

Task: Print numbers 1 through 5

Using for loop:

for i in range(1, 6):
    print(i)

Using while loop:

i: int = 1

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

Comparison:

Aspect	For Loop	While Loop
Clarity	More concise: one line manages iteration	Requires explicit variable initialization and update
Safety	Can't accidentally create infinite loop with range()	Risk of infinite loop if you forget to update variable
Use Case	When you know iteration count	When iteration depends on condition

Which is better? For this task, for loop is clearer and safer. Use while when you truly need condition-based repetition (like user input validation).

Infinite Loop Demonstration + Fix

Infinite Loop (⚠️ Warning: Don't run this):

# ⚠️ INFINITE LOOP
counter: int = 0

while counter < 10:
    print(f"Counter: {counter}")
    # ERROR: Forgot to increment counter

Fixed Version:

# ✅ CORRECT
counter: int = 0

while counter < 10:
    print(f"Counter: {counter}")
    counter += 1  # ✅ Increments counter each iteration

The fix: Added counter += 1 so the loop variable changes and the condition eventually becomes False.

Red Flags to Watch For

When working with loops, watch for these common errors:

🚩 Infinite Loops

Symptom: Program hangs, repeats output forever, or becomes unresponsive

Causes:

• Forgot to update loop variable in while loop
• Condition can never become False
• Update logic is inside an unreachable if block

Fix: Ensure loop variable changes each iteration and condition can become False

AI Troubleshooting Prompt:

"I have a while loop that runs forever. Here's my code: [paste code]. What's wrong and how do I fix it?"

🚩 Off-by-One Errors

Symptom: Loop runs one too many or one too few times

Example:

# Want to print 1-10, but this prints 1-9
for i in range(1, 10):  # ❌ Stops at 9, not 10
    print(i)

# Correct version
for i in range(1, 11):  # ✅ Stops at 10
    print(i)

Fix: Remember range(start, stop) goes UP TO but DOES NOT INCLUDE stop

AI Troubleshooting Prompt:

"My for loop with range(1, 10) only prints up to 9 instead of 10. Why?"

🚩 Loop Variable Scope Confusion

Symptom: Trying to use loop variable outside loop or getting unexpected value

Example:

for i in range(5):
    print(i)

print(f"Final value: {i}")  # i = 4 (last value from loop)

What happens: Loop variable i persists after loop ends with its last value (4)

Best Practice: Don't rely on loop variable value after loop; use meaningful names (for item in items)

AI Troubleshooting Prompt:

"Why does my loop variable still exist after the loop ends? Is that expected?"

🚩 Step Zero Error

Symptom: ValueError: range() arg 3 must not be zero

Example:

for i in range(0, 10, 0):  # ❌ Step can't be zero
    print(i)

Fix: Step must be non-zero (positive to count up, negative to count down)

AI Troubleshooting Prompt:

"I got 'range() arg 3 must not be zero' error. What does this mean?"

Try With AI

Now that you understand for and while loops, test your knowledge with your AI companion. Use ChatGPT web (or your AI companion tool if you've already set one up from earlier chapters).

1. Recall: Understand the Difference

"What's the difference between for loops and while loops in Python? When should I use each?"

Expected Outcome: Your AI will explain that for is for known iteration counts, while is for condition-based repetition. You'll see examples of ideal use cases for each.

2. Understand: Trace Execution Step-by-Step

"Trace this for loop step-by-step: for i in range(2, 6, 1): print(i). What values does i take, and what is the output?"

Expected Outcome: Your AI will walk through each iteration: i=2 (prints 2), i=3 (prints 3), i=4 (prints 4), i=5 (prints 5). Loop stops before 6. You'll understand how range(start, stop, step) works.

3. Apply: Generate Countdown Loop

"Generate a while loop that counts down from 10 to 1, printing each number. Include type hints and proper termination logic."

Expected Outcome: Your AI will provide code with:

• Type-hinted variable initialization (count: int = 10)
• While condition (while count >= 1)
• Print statement
• Decrement logic (count -= 1)

Review the code and verify it would print: 10, 9, 8, ..., 1

4. Analyze: Diagnose Infinite Loop

"What happens if I forget to update the loop counter in this while loop? Show me an example of the error and how to fix it:

i = 0
while i < 5:
    print(i)

"

Expected Outcome: Your AI will explain that the loop runs forever because i never changes. It will show the fix: adding i += 1 inside the loop. You'll understand why loop variable updates are critical.

Safety Note: AI-generated code may contain infinite loops. Always review loop termination logic before running code. If your program hangs, use Ctrl+C to stop execution.