Lesson 12 of 60 13 minBeginner

Multi-dimensional and jagged arrays

Store tables and grids with 2D arrays, rows of different lengths with jagged arrays, and loop through both.

A normal array is one row of values. Many things are tables: seats in a theatre, pixels in an image, a chess board, the floors and grid lines of a building. C# has two ways to store them.

Rectangular (2D) arrays: int[,]

One comma inside the brackets means two dimensions: rows and columns. Every row has the same number of columns.

Program.cs

1int[,] grid = new int[3, 4];     // 3 rows, 4 columns, all 02 3int[,] seats =4{5    { 1, 2, 3, 4 },6    { 5, 6, 7, 8 },7    { 9, 10, 11, 12 }8};9 10Console.WriteLine(seats[1, 2]);          // row 1, column 2 → 711seats[0, 0] = 100;12Console.WriteLine(seats.GetLength(0));   // rows: 313Console.WriteLine(seats.GetLength(1));   // columns: 414Console.WriteLine(seats.Length);         // all cells: 12

Output

Looping through a 2D array

RowTotals.cs

1int[,] sales =2{3    { 5, 3, 8 },    // shop 0: Mon, Tue, Wed4    { 2, 7, 4 },    // shop 15};6 7for (int r = 0; r < sales.GetLength(0); r++)8{9    int rowTotal = 0;10    for (int c = 0; c < sales.GetLength(1); c++)11    {12        rowTotal += sales[r, c];13    }14    Console.WriteLine($"Shop {r}: {rowTotal}");15}

Output

Shop 0: 16
Shop 1: 13

The outer loop walks the rows and the inner loop walks the columns of the current row. That is called row-major order; press “Nested loop order” in the diagram to watch it.

Jagged arrays: int[][]

A jagged array is an array of arrays. Each row is its own separate array, so rows can have different lengths, like floors of a building with different numbers of rooms.

Program.cs

1int[][] rooms = new int[3][];   // 3 rows, not created yet (null)2rooms[0] = new int[] { 101, 102 };3rooms[1] = new int[] { 201, 202, 203, 204 };4rooms[2] = new int[] { 301 };5 6Console.WriteLine(rooms[1][3]);       // 204 (note: [row][col])7Console.WriteLine(rooms.Length);      // 3 rows8Console.WriteLine(rooms[1].Length);   // 4 in row 19 10foreach (int[] floor in rooms)11{12    Console.WriteLine(string.Join(" ", floor));13}

Output

204
3
4
101 102
201 202 203 204
301

	2D array `int[,]`	Jagged array `int[][]`
Shape	Perfect rectangle	Each row any length
Access	`a[r, c]`	`a[r][c]`
Rows / columns	`GetLength(0)` / `GetLength(1)`	`a.Length` / `a[r].Length`
Created	All at once	Outer array first, then each row
Typical use	Grids, matrices, boards	Uneven data, lines of text

A matrix example

In BIM and 3D software, positions and rotations are stored in matrices: 2D arrays of numbers. Here is a 3×3 identity matrix, which leaves points unchanged:

Identity.cs

1int size = 3;2double[,] identity = new double[size, size];3for (int i = 0; i < size; i++)4    identity[i, i] = 1;      // 1 on the diagonal, 0 elsewhere5 6for (int r = 0; r < size; r++)7{8    for (int c = 0; c < size; c++)9        Console.Write(identity[r, c] + " ");10    Console.WriteLine();11}

Output

1 0 0 
0 1 0 
0 0 1

Quick check

For int[,] m = new int[4, 5];, what is m.Length?

Quick check

How do you read row 2, column 1 of a jagged array j?

Practice

Create a 3×3 int[,] with the values 1 to 9 and print the sum of the main diagonal (top-left to bottom-right) and the sum of each column.

Show the solution

Solution.cs

1int[,] m =2{3    { 1, 2, 3 },4    { 4, 5, 6 },5    { 7, 8, 9 }6};7 8int diagonal = 0;9for (int i = 0; i < 3; i++) diagonal += m[i, i];10Console.WriteLine($"Diagonal: {diagonal}");11 12for (int c = 0; c < m.GetLength(1); c++)13{14    int colSum = 0;15    for (int r = 0; r < m.GetLength(0); r++) colSum += m[r, c];16    Console.WriteLine($"Column {c}: {colSum}");17}

Output

Diagonal: 15
Column 0: 12
Column 1: 15
Column 2: 18

Remember

int[,] is a rectangular grid accessed with a[row, col].
GetLength(0) = rows, GetLength(1) = columns, Length = all cells.
int[][] is an array of arrays; each row can be a different length.
Use nested loops: outer over rows, inner over columns.

Getting started

Control flow

Arrays and methods

Object-oriented programming

Working with data

Functional C# and LINQ

Advanced C#

Master level

Multi-dimensional and jagged arrays

Rows and columns

Rectangular (2D) arrays: int[,]

Looping through a 2D array

Jagged arrays: int[][]

A matrix example

Remember