Arrays and Collections in C#

# CHAPTER 9

Arrays and Collections #

1. Introduction #

2. Learning Objectives #

• Create, initialize, and access Arrays.

• Understand the limitations of Arrays.

• Use List<T> for dynamic data storage.

• Explain the difference between ArrayList and List<T>.

• Traverse collections using loops.

3. Arrays (Fixed Size) #

Declaration and Initialization:

// Method 1: Specify size, then add values using the Index
string[] cars = new string[3]; // Creates an array for 3 strings
cars[0] = "Volvo";  // Arrays are zero-indexed!
cars[1] = "BMW";
cars[2] = "Ford";
// cars[3] = "Mazda"; // ERROR: Index out of bounds!

// Method 2: Initialize directly
int[] scores = { 95, 80, 100, 75 };

Console.WriteLine(scores[0]); // Prints 95
Console.WriteLine(scores.Length); // Prints 4 (Total elements)

4. Dynamic Collections: List<T> #

*(Requires using System.Collections.Generic;)*

using System;
using System.Collections.Generic;

namespace CollectionsDemo
{
    class Program
    {
        static void Main(string[] args)
        {
            List<string> users = new List<string>();

            users.Add("Alice"); // Size becomes 1
            users.Add("Bob");   // Size becomes 2
            users.Add("Charlie");

            users.Remove("Bob"); // "Charlie" shifts down to take Bob's spot

            Console.WriteLine(users[1]); // Prints "Charlie"
            Console.WriteLine(users.Count); // Prints 2
        }
    }
}

5. ArrayList (The Old Way - Avoid) #

6. Traversing Collections #

List<int> numbers = new List<int> { 10, 20, 30, 40 };

// Using for loop
for (int i = 0; i < numbers.Count; i++) 
{
    Console.WriteLine(numbers[i]);
}

// Using foreach loop (Preferred)
foreach (int num in numbers) 
{
    Console.WriteLine(num);
}

7. Mini Project: Student Marks Manager #

using System;
using System.Collections.Generic;

namespace MarksManager
{
    class Program
    {
        static void Main(string[] args)
        {
            List<int> marks = new List<int>();
            
            while (true)
            {
                Console.Write("Enter a mark (or -1 to finish): ");
                int input = int.Parse(Console.ReadLine());
                
                if (input == -1) break;
                
                marks.Add(input);
            }

            int sum = 0;
            foreach (int mark in marks)
            {
                sum += mark;
            }

            Console.WriteLine($"\nTotal Marks: {sum}");
            Console.WriteLine($"Average: {(double)sum / marks.Count}");
        }
    }
}

8. OOP and Memory Explanation #

9. Common Mistakes #

• IndexOutOfRangeException: Remembering that arrays and lists start at Index 0. If an array has 5 items, asking for array[5] will crash the program. The last item is array[4].

• Using .Length vs .Count: Arrays use the property .Length. Collections like List<T> use the property .Count.

10. Best Practices #

• Default to using List<T> for data storage. Only use raw arrays if you are strictly optimizing for memory or performance in algorithms or game engines.

11. Exercises #

1. 1. Create an array of 5 integers. Write a for loop that prints them in reverse order.

1. 2. Create a List<string> for groceries. Add 3 items, remove 1 item, and print the remaining items using a foreach loop.

12. MCQs with Answers #

13. Interview Questions #

• Q: Explain the structural difference between an Array and a List<T> under the hood. (Answer: List<T> is actually just a wrapper around an Array. When the internal array fills up, it creates a new, larger array, copies the data over, and deletes the old one).

• Q: What is Boxing and Unboxing, and why makes ArrayList inefficient?

14. Summary #

15. Next Chapter Recommendation #