While & Do-While Loops

In Plain English

Loops prevent repetitive copy-pasted code. A while loop repeatedly executes its body as long as its condition evaluates to true. A do-while loop guarantees the loop body runs at least once before checking the condition.

The Mental Model

"A while loop is like checking if you have gas before starting a road trip. A do-while loop is taking at least one bite of food before deciding whether you want to keep eating."

Deep Dive: How It Works

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

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

The Infinite Loop Trap: If you forget to increment your counter (e.g. i++), the condition stays true forever and your program freezes!

Syntax Blueprint

int i = 0;
while (i < 3) {
  std::cout << i << " ";
  i++; // Update step!
}

Condition checked before every iteration; variable must be updated inside.

Core Rules to Remember

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

Guaranteed Once: do-while always executes at least once.

Update Loop Variable: Always ensure the loop variable progresses toward termination.

Common Beginner Traps & How to Fix Them

Mistake:Putting a semicolon immediately after while(condition);

Why it happens: Forgets that while takes a block { }, creating an empty infinite loop.

How to fix: Never put a semicolon after while(condition) header.

Pro Tip: Use while loops when you don't know beforehand how many times the loop will repeat (e.g. waiting for network packets).

Live Interactive Example

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Countdown Timer with While Loop

C++ 20

Output Console

Click "Run Code" to view the rendered output.

How it works: The loop executes 3 times until countdown reaches 0.

Your Turn: Micro Challenge

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

Micro Exercise

Print powers of 2

Declare int value = 1;
Write a while loop that prints value and multiplies value *= 2 as long as value <= 8.

Sandbox Output

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Deep Dive: How It Works

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

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

The Infinite Loop Trap: If you forget to increment your counter (e.g. i++), the condition stays true forever and your program freezes!