While and Do-While Loops

Loops allow you to repeat code multiple times. This lesson covers while and do-while loops for condition-based iteration.

The while Loop

The while loop repeats code as long as a condition is true:


csharp
1int count = 1;
2
3while (count <= 5)
4{
5    Console.WriteLine($"Count: {count}");
6    count++;
7}

Output


1Count: 1
2Count: 2
3Count: 3
4Count: 4
5Count: 5

Syntax


csharp
1while (condition)
2{
3    // Code to repeat
4}

The condition is checked before each iteration
If the condition is false initially, the loop body never executes
The loop continues until the condition becomes false

Avoiding Infinite Loops

If the condition never becomes false, you get an infinite loop:


csharp
1// DANGER: Infinite loop!
2int x = 1;
3while (x > 0)  // Always true!
4{
5    Console.WriteLine(x);
6    x++;  // x keeps growing, never becomes <= 0
7}
8
9// Correct version:
10int x = 1;
11while (x <= 10)
12{
13    Console.WriteLine(x);
14    x++;  // Eventually x becomes 11, loop stops
15}

while Loop Examples

Countdown


csharp
1int seconds = 5;
2
3while (seconds > 0)
4{
5    Console.WriteLine($"{seconds}...");
6    seconds--;
7    Thread.Sleep(1000);  // Wait 1 second
8}
9
10Console.WriteLine("Liftoff!");

Sum Numbers


csharp
1int sum = 0;
2int number = 1;
3
4while (number <= 100)
5{
6    sum += number;
7    number++;
8}
9
10Console.WriteLine($"Sum of 1 to 100: {sum}");  // 5050

User Input Validation


csharp
1int age = -1;
2
3while (age < 0 || age > 150)
4{
5    Console.Write("Enter your age (0-150): ");
6    string input = Console.ReadLine() ?? "";
7
8    if (!int.TryParse(input, out age) || age < 0 || age > 150)
9    {
10        Console.WriteLine("Invalid age. Try again.");
11        age = -1;  // Reset to continue loop
12    }
13}
14
15Console.WriteLine($"Your age: {age}");

The do-while Loop

The do-while loop is similar but checks the condition after each iteration:


csharp
1int count = 1;
2
3do
4{
5    Console.WriteLine($"Count: {count}");
6    count++;
7} while (count <= 5);

Key Difference

The do-while loop always executes at least once:


csharp
1// while: may not execute at all
2int x = 10;
3while (x < 5)  // False immediately
4{
5    Console.WriteLine("This never prints");
6}
7
8// do-while: always executes at least once
9int y = 10;
10do
11{
12    Console.WriteLine("This prints once");  // Prints!
13} while (y < 5);  // Then checks condition

do-while Use Cases

Menu System


csharp
1string choice;
2
3do
4{
5    Console.WriteLine("\n=== Menu ===");
6    Console.WriteLine("1. Play Game");
7    Console.WriteLine("2. Settings");
8    Console.WriteLine("3. Exit");
9    Console.Write("Choice: ");
10
11    choice = Console.ReadLine() ?? "";
12
13    switch (choice)
14    {
15        case "1":
16            Console.WriteLine("Starting game...");
17            break;
18        case "2":
19            Console.WriteLine("Opening settings...");
20            break;
21        case "3":
22            Console.WriteLine("Goodbye!");
23            break;
24        default:
25            Console.WriteLine("Invalid choice!");
26            break;
27    }
28} while (choice != "3");

Input with Retry


csharp
1int number;
2bool valid;
3
4do
5{
6    Console.Write("Enter a positive number: ");
7    string input = Console.ReadLine() ?? "";
8    valid = int.TryParse(input, out number) && number > 0;
9
10    if (!valid)
11    {
12        Console.WriteLine("Invalid! Try again.");
13    }
14} while (!valid);
15
16Console.WriteLine($"You entered: {number}");

Practical Examples

Password Attempts


csharp
1const string correctPassword = "secret123";
2const int maxAttempts = 3;
3int attempts = 0;
4bool isLoggedIn = false;
5
6while (attempts < maxAttempts && !isLoggedIn)
7{
8    Console.Write("Enter password: ");
9    string password = Console.ReadLine() ?? "";
10    attempts++;
11
12    if (password == correctPassword)
13    {
14        isLoggedIn = true;
15        Console.WriteLine("Login successful!");
16    }
17    else
18    {
19        int remaining = maxAttempts - attempts;
20        if (remaining > 0)
21        {
22            Console.WriteLine($"Wrong password. {remaining} attempts left.");
23        }
24    }
25}
26
27if (!isLoggedIn)
28{
29    Console.WriteLine("Account locked. Too many failed attempts.");
30}

Reading Until Empty Line


csharp
1Console.WriteLine("Enter items (empty line to finish):");
2List<string> items = new List<string>();
3string input;
4
5while ((input = Console.ReadLine() ?? "") != "")
6{
7    items.Add(input);
8}
9
10Console.WriteLine($"\nYou entered {items.Count} items:");
11foreach (string item in items)
12{
13    Console.WriteLine($"  - {item}");
14}

Guessing Game Core Loop


csharp
1Random random = new Random();
2int secret = random.Next(1, 11);
3int guess = 0;
4
5Console.WriteLine("Guess a number between 1 and 10!");
6
7while (guess != secret)
8{
9    Console.Write("Your guess: ");
10    if (int.TryParse(Console.ReadLine(), out guess))
11    {
12        if (guess < secret)
13            Console.WriteLine("Too low!");
14        else if (guess > secret)
15            Console.WriteLine("Too high!");
16    }
17}
18
19Console.WriteLine("Correct!");

Summary

In this lesson, you learned:

while loops check the condition before each iteration
do-while loops check after, guaranteeing at least one execution
Always ensure the loop condition will eventually become false
Use while for "zero or more" iterations
Use do-while for "one or more" iterations

Next, we'll explore for loops for counter-controlled iteration.