Multi-dimensional arrays in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a multi-dimensional array in C#?

A multi-dimensional array is an array with more than one dimension, like a table with rows and columns. It stores data in a grid-like structure.

beginner

How do you declare a 2D array of integers with 3 rows and 4 columns in C#?

You declare it like this:
int[,] array = new int[3,4];
This creates a grid with 3 rows and 4 columns.

beginner

How do you access the element in the 2nd row and 3rd column of a 2D array named array?

Use array[1, 2] because indexing starts at 0. So row 2 is index 1, column 3 is index 2.

intermediate

What is the difference between a rectangular array and a jagged array in C#?

A rectangular array has rows and columns all the same size (like a grid). A jagged array is an array of arrays where each row can have different lengths.

beginner

How can you loop through all elements of a 2D array in C#?

Use two nested loops: one for rows and one for columns. For example:
for (int i = 0; i < rows; i++) {
for (int j = 0; j < cols; j++) {
// access array[i, j]
}
}

How do you declare a 3D array of doubles with dimensions 2x3x4 in C#?

A double arr = new double[2,3,4];

B double[][][] arr = new double[2][3][4];

C double[] arr = new double[2,3,4];

D double[,,] arr = new double[2,3,4];

In C#, multi-dimensional arrays use commas inside square brackets. So double[,,] declares a 3D array.

What is the index of the first element in any dimension of a C# array?

A -1

B 1

C 0

D Depends on the array

C# arrays are zero-indexed, so the first element is always at index 0.

Which of these is a jagged array declaration in C#?

A int[] jagged = new int[3,4];

B int[][] jagged = new int[3][];

C int[,] jagged = new int[3,4,5];

D int jagged = new int[3][];

Jagged arrays are arrays of arrays, declared with multiple square brackets like int[][].

How do you find the number of rows in a 2D array named arr?

A arr.GetLength(0)

B arr.GetLength(1)

C arr.Length

D arr.Rank

GetLength(0) returns the size of the first dimension (rows) in a multi-dimensional array.

What will happen if you try to access array[3,0] in a 3x4 array?

A Throws an IndexOutOfRangeException

B Returns the first element

C Returns null

D Returns default value 0

Index 3 is out of range for rows (0 to 2), so it throws an exception.

Explain how to declare, initialize, and access elements in a 2D array in C#.

Think of a table with rows and columns.

Describe the difference between rectangular and jagged arrays and when you might use each.

Imagine a perfect grid vs. uneven shelves.

Practice

1. What is a multi-dimensional array in C#?

easy

A. An array that can only hold one value

B. An array that stores only strings

C. An array with more than one dimension, like rows and columns

D. A list of unrelated variables

Solution

1. Step 1: Understand array dimensions

   A multi-dimensional array stores data in multiple dimensions, such as rows and columns, unlike a simple one-dimensional array.

2. Step 2: Recognize the definition

   An array with more than one dimension, like rows and columns correctly describes a multi-dimensional array as having more than one dimension, like a table with rows and columns.

3. Final Answer:

   An array with more than one dimension, like rows and columns -> Option C

4. Quick Check:

   Multi-dimensional array = rows and columns [OK]

Hint: Think of a table with rows and columns [OK]

Common Mistakes:

• Confusing multi-dimensional arrays with simple lists
• Thinking arrays can only hold one type of data
• Believing multi-dimensional arrays are unrelated variables

2. Which of the following is the correct way to declare a 2x3 multi-dimensional array of integers in C#?

easy

A. int[,] arr = new int[2,3];

B. int[][] arr = new int[2][3];

C. int arr[2,3];

D. int arr = new int[2,3];

Solution

1. Step 1: Recall C# multi-dimensional array syntax

   In C#, a 2D array is declared with commas inside square brackets: int[,] arr = new int[rows, columns];

2. Step 2: Check each option

   int[,] arr = new int[2,3]; uses correct syntax. int[][] arr = new int[2][3]; is jagged array syntax, not multi-dimensional. Options A and C are invalid syntax.

3. Final Answer:

   int[,] arr = new int[2,3]; -> Option A

4. Quick Check:

   Use comma inside brackets for multi-dimensional arrays [OK]

Hint: Use comma inside brackets for multi-dimensional arrays [OK]

Common Mistakes:

• Using jagged array syntax instead of multi-dimensional
• Missing 'new' keyword or wrong brackets
• Confusing array declaration with other languages

3. What will be the output of the following C# code?

int[,] matrix = { {1, 2, 3}, {4, 5, 6} };
Console.WriteLine(matrix[1, 2]);

medium

A. 3

B. 5

C. Runtime error

D. 6

Solution

1. Step 1: Understand array indexing

   matrix[1, 2] means row 1, column 2. Rows and columns start at 0.

2. Step 2: Locate the value at matrix[1, 2]

   Row 1 is {4, 5, 6}, column 2 is the third element, which is 6.

3. Final Answer:

   6 -> Option D

4. Quick Check:

   matrix[1,2] = 6 [OK]

Hint: Remember indexing starts at zero for rows and columns [OK]

Common Mistakes:

• Confusing row and column indexes
• Starting count from 1 instead of 0
• Expecting the value 3 or 5 instead of 6

4. Identify the error in this C# code snippet:

int[,] grid = new int[3, 3];
grid[3, 0] = 10;

medium

A. Index out of range error at grid[3, 0]

B. Syntax error in array declaration

C. Missing initialization of array

D. No error, code runs fine

Solution

1. Step 1: Check array size and valid indexes

   The array grid has size 3x3, so valid row indexes are 0, 1, 2.

2. Step 2: Analyze the assignment grid[3, 0]

   Index 3 for row is out of range, causing an IndexOutOfRangeException at runtime.

3. Final Answer:

   Index out of range error at grid[3, 0] -> Option A

4. Quick Check:

   Index 3 invalid for size 3 array [OK]

Hint: Indexes start at 0, max index is size-1 [OK]

Common Mistakes:

• Using index equal to array size
• Confusing syntax error with runtime error
• Assuming arrays auto-expand on invalid index

5. Given a 3x3 multi-dimensional array int[,] board representing a tic-tac-toe game, which code snippet correctly counts how many cells contain the value 1?

int count = 0;
// Which loop is correct?
A) for (int i = 0; i < 3; i++)
   for (int j = 0; j < 3; j++)
       if (board[i, j] == 1) count++;

B) for (int i = 1; i <= 3; i++)
   for (int j = 1; j <= 3; j++)
       if (board[i, j] == 1) count++;

C) for (int i = 0; i < 3; i++)
   for (int j = 0; j < 2; j++)
       if (board[i, j] == 1) count++;

D) for (int i = 0; i < 3; i++)
   for (int j = 0; j < 3; j++)
       if (board[j, i] == 1) count++;

hard

A. rorre egnar fo tuo xedni sesuaC

B. Causes index out of range error

C. Misses last column in counting

D. Counts transposed cells, not original positions

Solution

1. Step 1: Understand array indexing and loop ranges

   Indexes start at 0 and go up to size-1, so loops must run from 0 to less than 3.

2. Step 2: Analyze each option's loops

   Causes index out of range error correctly counts all cells with value 1 loops correctly over all rows and columns. Causes index out of range error causes index out of range error uses 1 to 3, causing out of range. Misses last column in counting misses last column by looping j to 2. Counts transposed cells, not original positions swaps indexes, counting transposed cells.

3. Final Answer:

   Causes index out of range error -> Option B

4. Quick Check:

   Loop from 0 to less than size for rows and columns [OK]

Hint: Loop indexes from 0 to size-1 for all dimensions [OK]

Common Mistakes:

• Starting loops at 1 instead of 0
• Using wrong loop limits causing missed elements
• Swapping row and column indexes accidentally