Arrays and multidimensional data

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Examples are independent C# snippets for modern .NET console projects unless noted otherwise. Class declarations and top-level code can live in Program.cs when top-level statements come first.

At a glance

An array stores a fixed number of elements of one element type. It gives constant-time access by index and is a fundamental building block for algorithms.

How it works

Create, access and update

Array indexes start at zero. Length is the number of elements. Arrays have fixed length, although the elements can normally be replaced.

int[] scores = { 7, 3, 9 };
scores[1] = 5;
Console.WriteLine(scores[0]); // 7
Console.WriteLine(scores.Length); // 3

Loop and sort

Use a for loop if you need both an index and a value, or foreach if you only need the values. Array.Sort modifies the array in place, so copy it first if the original order matters.

int[] original = { 38, 12, 27 };
int[] sorted = (int[])original.Clone();
Array.Sort(sorted);
foreach (int number in sorted)
    Console.WriteLine(number); // 12, 27, 38

Rectangular and jagged arrays

A rectangular array (int[,]) has a fixed grid shape. A jagged array (int[][]) is an array of arrays, so each row can have a different length.

int[,] grid = { { 1, 2 }, { 3, 4 } };
Console.WriteLine(grid[1, 0]); // 3
int[][] rows = { new[] { 1 }, new[] { 2, 3 } };
Console.WriteLine(rows[1][1]); // 3

Deep dive: index arithmetic, copying, and choosing collections

An array has a fixed length, not fixed values

new int[3] allocates three integer slots initialized to zero. You can replace their values, but you cannot resize the same array object. Indexes run from 0 to Length - 1. The expression numbers[^1] refers to the last element, while a range such as numbers[1..3] creates a new array containing the selected slice for ordinary arrays.

Copy an array when mutations must be isolated

An array is a reference type. Assigning int[] second = first; creates another reference to the same array. Changing second[0] changes what first[0] reads. Array.Clone() creates a shallow copy: for arrays of integers that gives separate values, but for arrays of class instances the elements still refer to the same objects.

int[] original = { 38, 12, 27 };
int[] copy = (int[])original.Clone();
Array.Sort(copy);
Console.WriteLine(string.Join(", ", original)); // 38, 12, 27
Console.WriteLine(string.Join(", ", copy));     // 12, 27, 38

Arrays, lists, and spans solve different needs

An array is useful for fixed-size indexed storage. List<T> grows when elements are added and exposes methods like Add and Remove; internally it manages storage capacity, which may be larger than its current Count. A Span<T> provides a bounded view over contiguous memory without necessarily making a new array, but it has usage and lifetime restrictions. Start with arrays or List<T> and reach for spans when a measured performance need justifies them.

Multidimensional versus jagged storage

int[,] grid describes a rectangular array with dimensions accessible through GetLength(dimension). int[][] rows is an array whose elements are arrays, so the rows can have different sizes or even be null. Choose based on your data rather than assuming one representation is automatically faster.

Complexity mental model

Indexing an array is normally O(1). Searching an unsorted array is usually O(n) in the worst case; sorting costs more and changes order. A binary search requires the data to already be sorted in the expected comparison order. Readability and correctness are more valuable than a theoretical improvement when data sets are tiny.

When and why

Use arrays for fixed-size ordered values and when tight indexed access is helpful; consider List<T> when size changes frequently.

Common mistake

array[array.Length] is out of range. Array.Sort mutates the array; it does not return a new sorted array.

Try it yourself

Sort a copy of an array without changing the original.

Reveal a solution

int[] copy = (int[])numbers.Clone(); Array.Sort(copy);.