Loops in C#

# CHAPTER 8

Loops in C# #

1. Introduction #

2. Learning Objectives #

• Use the for loop for iteration.

• Use the while loop for unknown iteration counts.

• Use the do-while loop for guaranteed first execution.

• Use the foreach loop to iterate over collections.

• Manipulate loop flow using break and continue.

3. The for Loop #

Syntax: for (initialization; condition; increment/decrement)

using System;

namespace LoopsApp
{
    class Program
    {
        static void Main(string[] args)
        {
            // Prints numbers 1 to 5
            for (int i = 1; i <= 5; i++) 
            {
                Console.WriteLine($"Count: {i}");
            }
        }
    }
}

How it works:

1. 1. int i = 1: Executes ONCE at the start.

1. 2. i <= 5: Checked before every loop. If true, run the code. If false, stop.

1. 3. i++: Executes after the code block runs, then goes back to step 2.

4. The while Loop #

int count = 1;

while (count <= 3) 
{
    Console.WriteLine("Hello!");
    count++; // CRITICAL: Don't forget to update the condition!
}

5. The do-while Loop #

int password;

do 
{
    Console.Write("Enter PIN (1234): ");
    password = int.Parse(Console.ReadLine());
    
} while (password != 1234); // Keeps looping until correct

Console.WriteLine("Access Granted!");

6. The foreach Loop #

string[] cars = { "Volvo", "BMW", "Ford", "Mazda" };

// Automatically grabs each item, one by one
foreach (string car in cars) 
{
    Console.WriteLine(car);
}

7. The break and continue Statements #

• break: Immediately terminates the loop completely.

• continue: Skips the rest of the current iteration and jumps to the next one.

for (int i = 1; i <= 10; i++) 
{
    if (i == 4) 
    {
        continue; // Skips printing 4
    }
    if (i == 8) 
    {
        break; // Stops the loop entirely at 8
    }
    Console.Write(i + " ");
}
// Output: 1 2 3 5 6 7

8. Mini Project: Number Guessing Game #

using System;

namespace GuessingGame
{
    class Program
    {
        static void Main(string[] args)
        {
            int secretNum = 7;
            int guess = 0;
            int attempts = 0;

            Console.WriteLine("Guess the secret number (1-10)!");

            while (guess != secretNum) 
            {
                Console.Write("Enter guess: ");
                guess = int.Parse(Console.ReadLine());
                attempts++;

                if (guess != secretNum) 
                {
                    Console.WriteLine("Wrong, try again.");
                }
            }

            Console.WriteLine($"You win! It took you {attempts} attempts.");
        }
    }
}

9. Common Mistakes #

• Infinite Loops: Forgetting count++ in a while loop means the condition never becomes false. The program will freeze and consume 100% of a CPU core until you force quit it.

• Off-by-One Errors: Using < 5 instead of <= 5 when you actually want it to run 5 times starting from 1.

10. Best Practices #

• Prefer foreach over for when iterating through collections. It makes your intent clearer and eliminates the risk of indexing errors.

• Always initialize your variables outside of while loops if you need to use their final values after the loop ends.

11. Exercises #

1. 1. Write a for loop that prints all even numbers from 2 to 20.

1. 2. Write a while loop that creates a countdown timer from 10 to 1, then prints "Liftoff!".

12. MCQs with Answers #

13. Interview Questions #

• Q: Explain the difference between a while loop and a do-while loop. Provide a scenario for each.

• Q: Why is foreach generally preferred over a standard for loop when dealing with List<T>?

14. Summary #

15. Next Chapter Recommendation #