C# — Arrays

Array declaration

using System;

// Array initialization
int[] nums = { 1, 2, 3, 4, 5 };
string[] names = new string[3];
int[] zeros = new int[10];

// Array with size
int[] sized = new int[5]; // All zeros

// Access elements
Console.WriteLine(nums[0]);    // 1
Console.WriteLine(nums.Length); // 5

// Modify
nums[0] = 100;
Console.WriteLine(nums[0]); // 100

Array operations

using System;
using System.Linq;

int[] nums = { 5, 2, 8, 1, 9, 3 };

// Sort
Array.Sort(nums);
Console.WriteLine(string.Join(", ", nums)); // 1, 2, 3, 5, 8, 9

// Reverse
Array.Reverse(nums);
Console.WriteLine(string.Join(", ", nums)); // 9, 8, 5, 3, 2, 1

// Find
int idx = Array.IndexOf(nums, 5);
Console.WriteLine($"Index of 5: {idx}");

// Resize
Array.Resize(ref nums, 8);
Console.WriteLine($"New length: {nums.Length}");

// Clear
int[] temp = { 1, 2, 3 };
Array.Clear(temp, 0, 2); // Clear first 2 elements

Multi-dimensional arrays

using System;

// 2D array
int[,] matrix = {
    { 1, 2, 3 },
    { 4, 5, 6 },
    { 7, 8, 9 }
};

Console.WriteLine(matrix[1, 2]); // 6

// Iterate
for (int i = 0; i < 3; i++)
{
    for (int j = 0; j < 3; j++)
    {
        Console.Write($"{matrix[i, j]} ");
    }
    Console.WriteLine();
}

// Jagged array (array of arrays)
int[][] jagged = {
    new[] { 1, 2 },
    new[] { 3, 4, 5 },
    new[] { 6 }
};

foreach (var row in jagged)
{
    Console.WriteLine(string.Join(", ", row));
}

List<T>

using System;
using System.Collections.Generic;

var numbers = new List<int> { 1, 2, 3, 4, 5 };

numbers.Add(6);                  // Add to end
numbers.Insert(0, 0);            // Insert at index
numbers.Remove(3);               // Remove first occurrence
numbers.RemoveAt(0);             // Remove at index

Console.WriteLine($"Count: {numbers.Count}");
Console.WriteLine($"Contains 4: {numbers.Contains(4)}");

// Iterate
foreach (int n in numbers)
{
    Console.Write($"{n} ");
}
Console.WriteLine();

LINQ basics

using System;
using System.Linq;

int[] nums = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

// Filter
var evens = nums.Where(n => n % 2 == 0);
Console.WriteLine($"Evens: {string.Join(", ", evens)}");

// Transform
var doubled = nums.Select(n => n * 2);
Console.WriteLine($"Doubled: {string.Join(", ", doubled)}");

// Aggregate
Console.WriteLine($"Sum: {nums.Sum()}");
Console.WriteLine($"Average: {nums.Average()}");
Console.WriteLine($"Min: {nums.Min()}");
Console.WriteLine($"Max: {nums.Max()}");

// Order
var desc = nums.OrderByDescending(n => n);
Console.WriteLine($"Desc: {string.Join(", ", desc)}");

// First / Last
Console.WriteLine($"First: {nums.First()}");
Console.WriteLine($"Last: {nums.Last()}");
Console.WriteLine($"First > 5: {nums.First(n => n > 5)}");

Dictionary<TKey, TValue>

using System;
using System.Collections.Generic;

var ages = new Dictionary<string, int>
{
    ["Alice"] = 30,
    ["Bob"] = 25,
    ["Charlie"] = 35
};

// Access
Console.WriteLine($"Alice: {ages["Alice"]}");

// TryGetValue (safe)
if (ages.TryGetValue("David", out int age))
{
    Console.WriteLine($"David: {age}");
}
else
{
    Console.WriteLine("David not found");
}

// Add and remove
ages["Eve"] = 28;
ages.Remove("Charlie");

// Iterate
foreach (var (name, a) in ages)
{
    Console.WriteLine($"{name}: {a}");
}

Array vs List

Feature	Array	List<T>
Size	Fixed	Dynamic
Performance	Faster	Slightly slower
Memory	Less overhead	More overhead
Use when	Size known at compile time	Size changes at runtime

Mini Practice

Write C# code that:

1. Creates a jagged array and prints each row
2. Uses List<T> to build a dynamic collection
3. Uses LINQ to filter and transform an array
4. Creates a dictionary and iterates with foreach

Up Next

In the next lesson, you'll learn about Methods — defining and using methods in C#.