Sabin Sim

Posted on Feb 19

22. C# (do-while Loop)

#csharp #programming #learning #career

Goal

Keep asking the user:

“Enter a word longer than 10 letters.”

Repeat until the condition is satisfied.

1. The do-while Version (The Standard Structure)

Full Runnable Code

using System;

class Program
{
    static void Main()
    {
        string word;

        do
        {
            Console.WriteLine("Enter a word longer than 10 letters:");
            word = Console.ReadLine();

        } while (word.Length <= 10);

        Console.WriteLine("Accepted!");
        Console.ReadKey();
    }
}

Thinking Flow

do-while Structure

do
{
    // code
}
while (condition);

Execution Order

Execute the do block first
Check the condition
If the condition is true → repeat
If the condition is false → exit

That is the core meaning of “runs at least once.”

The Key Condition Here

while (word.Length <= 10);

Meaning:

If the word length is 10 or less, keep repeating.

Why do-while Is Cleaner Here

Because we must collect input at least once.

You cannot evaluate word.Length before the user types something.

That is exactly the scenario where do-while is structurally correct.

2. Implementing the Same Thing With while

using System;

class Program
{
    static void Main()
    {
        string word;

        while (word.Length <= 10)  // compile-time error
        {
            Console.WriteLine("Enter a word longer than 10 letters:");
            word = Console.ReadLine();
        }
    }
}

Why This Fails

Error:

Use of unassigned local variable 'word'

Reason:

word has not been assigned yet
But you are trying to read word.Length

C# is statically typed and enforces definite assignment.

Uninitialized local variables cannot be used.

Fixing while by Initializing

using System;

class Program
{
    static void Main()
    {
        string word = "";

        while (word.Length <= 10)
        {
            Console.WriteLine("Enter a word longer than 10 letters:");
            word = Console.ReadLine();
        }

        Console.WriteLine("Accepted!");
        Console.ReadKey();
    }
}

This compiles.

But it changes the model:

You are creating an artificial initial state just to make the loop start.

Sometimes that’s fine.

Sometimes it creates hidden bugs.

A Real Pitfall With while

If the requirement changes to:

“Enter a word shorter than 3 letters.”

using System;

class Program
{
    static void Main()
    {
        string word = "";

        while (word.Length >= 3)
        {
            Console.WriteLine("Enter a word shorter than 3 letters:");
            word = Console.ReadLine();
        }
    }
}

What happens?

Initial word is ""
Length is 0
0 >= 3 is false

So:

The loop runs zero times
The user is never asked
No input is collected

This is not a syntax problem.

This is a structural mismatch.

The Core Difference

Structure	When condition is checked	Minimum executions
while	Before execution	Can be 0
do-while	After execution	At least 1

The Mental Model You Must Build

Use while when:

The state already exists, and you repeat while a condition holds.

Example:

int number = 0;

while (number < 10)
{
    number++;
}

Use do-while when:

You must perform the action once before you can evaluate the condition.

Typical scenarios:

Login input
Re-enter password
Menu selection loops
Validating user input

Visual Difference

while

Check condition
   ↓
True → execute
False → exit

do-while

Execute once
   ↓
Check condition
True → repeat
False → exit

A Common Real-World Pattern

string choice;

do
{
    Console.WriteLine("Choose A, B or C:");
    choice = Console.ReadLine();

} while (choice != "A" && choice != "B" && choice != "C");

This is the basic menu validation pattern.

Practice Task (Important)

Build this:

Ask the user for a number.
If it is not between 1 and 5, keep asking.

Hint:

while (number < 1 || number > 5)

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