Multi-dimensional arrays in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a 2D array element access

What is the output of this C# code snippet?

C Sharp (C#)

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

A Runtime error

B 5

C 3

D 6

💡 Hint

Remember that the first index is the row and the second is the column.

The matrix has two rows and three columns. matrix[1, 2] accesses the element in the second row and third column, which is 6.

❓ Predict Output

intermediate

Length properties of a 3D array

Given the following 3D array, what is the output of the code below?

C Sharp (C#)

int[,,] cube = new int[2,3,4];
Console.WriteLine(cube.GetLength(1));

A 3

B 2

C 4

D Runtime error

💡 Hint

GetLength(n) returns the size of the dimension n, starting from 0.

The array has dimensions 2 (dim 0), 3 (dim 1), and 4 (dim 2). cube.GetLength(1) returns 3.

🔧 Debug

advanced

Identify the error in multi-dimensional array initialization

What error will this code produce when compiled or run?

C Sharp (C#)

int[,] grid = new int[2, 2] { {1, 2}, {3, 4}, {5, 6} };

A Compile-time error: Too many initializers for the array

B Runtime error: Index out of range

C No error, compiles and runs fine

D Compile-time error: Missing semicolon

💡 Hint

Check the declared size versus the number of initializer rows.

The array is declared with size 2x2 but initialized with 3 rows, which causes a compile-time error.

❓ Predict Output

advanced

Output of nested loops over a 2D array

What will be printed by this code?

C Sharp (C#)

int[,] arr = { {1, 2}, {3, 4} };
for (int i = 0; i < arr.GetLength(0); i++)
{
    for (int j = 0; j < arr.GetLength(1); j++)
    {
        Console.Write(arr[i, j] + " ");
    }
    Console.WriteLine();
}

A 1 3 \n2 4 \n

B 1 2 3 4 \n

C 1 2 \n3 4 \n

D Runtime error

💡 Hint

The outer loop iterates rows, inner loop iterates columns.

The code prints each row's elements separated by spaces, then a newline after each row.

🧠 Conceptual

expert

Memory layout of multi-dimensional arrays in C#

Which statement best describes how multi-dimensional arrays are stored in memory in C#?

A Multi-dimensional arrays are stored as arrays of arrays (jagged arrays) with separate memory blocks for each sub-array.

B Multi-dimensional arrays are stored as a single contiguous block of memory in row-major order.

C Multi-dimensional arrays are stored in column-major order similar to Fortran arrays.

D Multi-dimensional arrays are stored randomly in memory without any specific order.

💡 Hint

Think about how C# handles rectangular arrays compared to jagged arrays.

C# rectangular multi-dimensional arrays are stored as a single contiguous block in row-major order, meaning rows are stored one after another.

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