Repeating Code with while Loops

In the previous lesson, we learned how for loops repeat code over a fixed sequence.

Now, let's explore how to repeat code when we don't know the exact count in advance!

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1. Why Do We Need while Loops?

A for loop is extremely useful when we know how many times we want to repeat or when we have a fixed sequence of items to process.

However, in many real-world programs, we don't know the exact number of repetitions in advance.

Real-World Examples:

• Password Retry: Keep asking a user for their password until they type it correctly.
• Video Game Lives: Keep running a game while the player still has lives remaining.

In these situations, repetition is not based on a fixed count. It is based on a rule or condition.

Key Rule: When repetition depends on a condition, we use a while loop.

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2. What is a while Loop?

Definition: A while loop repeats a block of code as long as a specified condition remains True.

How a while Loop Works Conceptually:

1. Python checks a condition.
2. If the condition is True, Python executes the block of code inside the loop.
3. After finishing the block, Python jumps back to check the condition again.
4. When the condition becomes False, Python stops the loop and moves to the next lines of code.

Click to enlarge

Visual Guide: Conceptual Execution Flowchart of a Python while Loop.

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3. while vs for Loop

Here is a quick way to decide which loop to use:

Feature	for Loop	while Loop
Primary Use	Used when we know the sequence or number of repetitions in advance.	Used when repetition depends on a condition that changes dynamically.
Example Scenario	Print numbers 1 to 10, or iterate through letters in "Python".	Ask for a password repeatedly until the correct one is entered.
Control Mechanism	Iterates over items in a sequence until the sequence ends.	Checks a boolean condition before every round until it becomes False.

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4. Understanding the Basic Structure of a while Loop

The syntax of a while loop is simple and clean:

while condition:
    block of code

Let's break down each part:

while keyword

• Starts the while loop instruction.
• Tells Python to keep repeating the block of code as long as the condition remains True.

condition

• A condition is a rule that evaluates to either True or False.
• Conditions are usually created using comparison operators:
  • > (greater than)
  • < (less than)
  • >= (greater than or equal to)
  • <= (less than or equal to)
  • == (equal to)
  • != (not equal to)

Simple Condition Example:

Consider the condition: lives > 0

• If lives = 3:
  • 3 > 0 → True → The loop continues!
• If lives = 0:
  • 0 > 0 → False → The loop stops!

block of code

• The indented lines of code (4 spaces) executed whenever the condition is True.

Visual Anatomy & The 3 Parts of a While Loop

1. STARTING VALUE

lives = 3   (Set variable before loop)

whilelives > 0:

INDENTATION(4 Spaces)

BLOCK OF CODE

Actions executed while condition is True

3. UPDATE STEP

lives = lives - 1   (Update variable to avoid Infinite Loop)

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5. How while Loop Works Step by Step

Let's look at a video game example:

lives = 3

while lives > 0:
    print(f"Playing game... Lives remaining: {lives}")
    lives = lives - 1

print("Game Over!")

Output:

Playing game... Lives remaining: 3
Playing game... Lives remaining: 2
Playing game... Lives remaining: 1
Game Over!

Detailed Round-by-Round Execution Trace:

• Starting State: lives = 3

Round 1:

• Condition Check: 3 > 0 → True
• Action: Prints "Playing game... Lives remaining: 3"
• Update Step: lives = 3 - 1 → lives becomes 2

Round 2:

• Condition Check: 2 > 0 → True
• Action: Prints "Playing game... Lives remaining: 2"
• Update Step: lives = 2 - 1 → lives becomes 1

Round 3:

• Condition Check: 1 > 0 → True
• Action: Prints "Playing game... Lives remaining: 1"
• Update Step: lives = 1 - 1 → lives becomes 0

Round 4:

• Condition Check: 0 > 0 → False
• Result: Loop stops! Python exits the loop and prints "Game Over!".

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Simple Mental Model:

Starting Value
      ↓
Check Condition
      ↓
Execute Block (if True)
      ↓
Update Value
      ↓
Check Condition Again

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6. The 3 Essential Parts of a while Loop

Notice that every working while loop requires 3 essential parts:

1. Starting Value: A variable initialized before the loop starts (e.g., lives = 3).
2. Condition: The rule checked before every iteration (e.g., while lives > 0:).
3. Update Step: A statement inside the loop that changes the variable (e.g., lives = lives - 1).

Click to enlarge

Visual Guide: The 3 Essential Parts of a Python while Loop (Starting Value, Condition, and Update Step).

Why the Update Step is Critical:

The Update Step changes the variable state so that the condition will eventually become False. Without an update step, the condition would stay True forever!

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7. Real-World Example: Retrying Password

Here is another classic use case: keeping a program running until a user types the correct password.

password = ""

while password != "secret123":
    password = input("Enter password: ")

print("Access Granted!")

Why a while Loop is Perfect Here:

We don't know how many wrong guesses the user will make.

• If they type "secret123" on attempt 1 → condition becomes False immediately, loop ends!
• If they take 5 attempts → loop runs 5 times until password != "secret123" evaluates to False.

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8. Infinite Loops (Important Warning!)

What happens if you forget Part 3 (Update Step)?

If the variable is never updated, the condition remains True forever! This is called an Infinite Loop, and it can freeze your program or crash your terminal.

# ⚠️ DANGER: Missing Update Step! Runs forever!
# counter = 1
# while counter <= 5:
#     print("Stuck inside this loop!")
#     # Forgotten: counter = counter + 1

How to Stop an Infinite Loop:

If your program gets stuck in an infinite loop: Press Ctrl + C in your terminal to force Python to stop immediately!

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Quick Summary

Concept	Meaning
while Loop	A condition-based repetition system that repeats a block of code as long as a condition is True.
Condition	A rule checked before every round that evaluates to True or False.
Starting Value	An initial variable set up before the loop starts to track the state.
Update Step	A statement inside the loop that changes the variable so the condition can eventually become False.
Block of Code	Indented lines executed repeatedly whenever the condition is True.
Infinite Loop	A loop that never stops because its condition stays True forever (stop using Ctrl + C).

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What's Next?

Now that we know both for loops and while loops, what if we want to stop a loop early or skip a specific round? Let's learn about break and continue in the next lesson!