C# Arrays

int[] scores = new int[5];                  // five zeros
string[] days = { "Mon", "Tue", "Wed" };     // initialised with values
double[] temps = new double[] { 18.5, 21.0 };
int[] primes = [2, 3, 5, 7];                 // collection expression (C# 12)

• The type is written with square brackets: int[] means "array of int".
• A new array is filled with default values: 0 for numbers, false for bools, and null for strings and other reference types.
• scores.Length gives the number of items.

Indexes start at 0, so the last item is at Length - 1:

string first = days[0];      // "Mon"
days[1] = "Tuesday";         // change an item
string last = days[^1];      // "Wed", one from the end
string[] middle = days[1..3];  // items 1 and 2

• Reading or writing outside the array throws an IndexOutOfRangeException.
• ^1 is the last item, ^2 the second to last.
• A range start..end includes start and excludes end; ..2 means the first two, 2.. means from index 2 to the end.
• Use foreach to read every item, and for when you need the index or want to change items.

• Array.Sort(arr) sorts in place; Array.Reverse(arr) reverses.
• Array.IndexOf(arr, value) returns the position or -1; arr.Contains(value) returns a bool.
• arr.Max(), arr.Min(), arr.Sum() and arr.Average() come from LINQ.

Arrays are reference types. int[] b = a; doesn't copy the items; both names point to the same array, so changing b[0] changes a[0] too. Use (int[])a.Clone() or a.ToArray() for a real copy. Array.Resize(ref arr, 10) creates a bigger copy when you really need one.

A two-dimensional array is a grid with rows and columns:

char[,] board = new char[3, 3];
board[1, 1] = 'X';
int rows = board.GetLength(0);    // 3

A jagged array is an array of arrays, where each row can have a different length: int[][] weeks = new int[4][];. Jagged arrays are more common in everyday code because each row is a normal array you can loop over with foreach.

Because arrays are reference types, == checks whether two variables point to the same array, not whether they hold the same values:

int[] a = { 1, 2, 3 };
int[] b = { 1, 2, 3 };
Console.WriteLine(a == b);              // False: two separate arrays
Console.WriteLine(a.SequenceEqual(b));  // True: same items, same order

SequenceEqual comes from LINQ and compares item by item.

Printing an array directly gives you its type name rather than its contents: Console.WriteLine(a) prints System.Int32[]. Join the items into one string instead:

Console.WriteLine(string.Join(", ", a));     // 1, 2, 3
Console.WriteLine($"[{string.Join(", ", a)}]");  // [1, 2, 3]

To fill an existing array with the same value, use Array.Fill(arr, 7) rather than writing a loop.