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.