Lesson 10 of 20

5 mins read

Python While & Do-While Loops

In Plain English

Loops prevent repetitive copy-pasting. A while loop checks a condition first and repeats its body as long as the condition holds true. A do-while loop executes its body once before checking the condition.

The Mental Model

"A while loop is like checking if your phone battery has charge before turning on the screen. A do-while loop is taking at least one sip of coffee before checking if the mug is empty."

Deep Dive: How It Works

while (condition): Checks condition FIRST. If initially false, runs 0 times.

do { ... } while (condition);: Runs body FIRST, then checks condition. Always runs at least 1 time.

Loop Progression: The loop variable must be modified inside the loop (e.g. i++) to prevent infinite loops.

Syntax Blueprint

int i = 0;
while (i < 3) {
  Console.WriteLine(i);
  i++; // Progress counter!
}

Condition checked before every iteration; variable updated inside.

Core Rules to Remember

Condition First: while checks condition prior to entering the loop.

Guaranteed At Least Once: do-while executes at least once regardless of condition.

Always Update Counter: Ensure the loop progresses toward termination.

Common Beginner Traps & How to Fix Them

Mistake:Forgetting to increment the loop variable inside a while loop

Why it happens: Causes an infinite loop that locks up CPU execution.

How to fix: Ensure your counter variable increments (e.g. count++) before the loop block closes.

Pro Tip: Use while loops when waiting on unpredictable external conditions (like user input or file loading)!

Live Interactive Example

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Countdown Timer with While Loop

C# 12 / .NET

Output Console

Click "Run Code" to view the rendered output.

How it works: The while loop runs 3 times, decrementing countdown to 0.

Your Turn: Micro Challenge

No pressure! Edit the starter code below and test your solution with instant feedback.

Micro Exercise

Double a number with while loop

Declare int val = 1;
Write a while loop that prints val and doubles val *= 2 while val <= 8.

Sandbox Output

Click "Run & Check" to test your solution.

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