Array bounds checking behavior in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of accessing array element within bounds

What is the output of this C# code snippet?

C Sharp (C#)

int[] numbers = {10, 20, 30};
Console.WriteLine(numbers[1]);

A 20

B 10

C 30

D IndexOutOfRangeException

💡 Hint

Remember that array indexing in C# starts at 0.

The array 'numbers' has elements 10 at index 0, 20 at index 1, and 30 at index 2. Accessing numbers[1] returns 20.

❓ Predict Output

intermediate

Output when accessing array element out of bounds

What happens when this C# code runs?

C Sharp (C#)

int[] arr = new int[3] {1, 2, 3};
Console.WriteLine(arr[3]);

A Throws IndexOutOfRangeException

B Prints 3

C Prints null

D Prints 0

💡 Hint

Check if the index 3 is valid for an array of length 3.

Array indices go from 0 to length-1. Index 3 is outside the valid range, so an IndexOutOfRangeException is thrown.

🔧 Debug

advanced

Identify the cause of runtime error in array access

Why does this code throw an exception at runtime?

C Sharp (C#)

string[] fruits = {"apple", "banana", "cherry"};
for (int i = 0; i < fruits.Length; i++)
{
    Console.WriteLine(fruits[i]);
}

A The array is not initialized properly

B The loop condition should be i < fruits.Length, not i <= fruits.Length

C The Console.WriteLine statement is incorrect

D The variable i is not declared

💡 Hint

Check the loop boundary condition carefully.

The loop runs from i = 0 to i = fruits.Length inclusive. Since fruits.Length is 3, accessing fruits[3] causes an IndexOutOfRangeException.

🧠 Conceptual

advanced

Array bounds checking behavior in C#

Which statement best describes how C# handles array bounds checking?

A C# checks array bounds only in debug mode but not in release mode

B C# does not check array bounds and accessing invalid indices causes silent memory corruption

C C# automatically checks array bounds at runtime and throws an exception if an invalid index is accessed

D C# requires the programmer to manually check array bounds before accessing elements

💡 Hint

Think about safety features of C# compared to languages like C or C++.

C# performs automatic bounds checking on arrays at runtime and throws IndexOutOfRangeException if an invalid index is accessed.

❓ Predict Output

expert

Output of multi-dimensional array access with invalid index

What is the output or result of running this C# code?

C Sharp (C#)

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

A Prints 4

B Prints 0

C Compilation error

D Throws IndexOutOfRangeException

💡 Hint

Check the valid indices for a 2x2 array.

The valid indices for a 2x2 array are 0 and 1 for both dimensions. Accessing matrix[1, 2] is out of bounds and throws an IndexOutOfRangeException.

Practice

1. What happens if you try to access an array element with an index less than 0 in C#?

easy

A. It throws an IndexOutOfRangeException.

B. It returns the first element of the array.

C. It returns null.

D. It automatically adjusts the index to 0.

Solution

1. Step 1: Understand array index rules in C#

   Array indexes must be between 0 and array length - 1.

2. Step 2: Behavior on invalid index

   Accessing an index less than 0 is outside valid range and causes an exception.

3. Final Answer:

   It throws an IndexOutOfRangeException. -> Option A

4. Quick Check:

   Invalid index access = IndexOutOfRangeException [OK]

Hint: Negative indexes always cause IndexOutOfRangeException [OK]

Common Mistakes:

• Thinking negative index returns first element
• Assuming negative index returns null
• Believing index auto-corrects to 0

2. Which of the following is the correct way to declare and access the third element of an integer array named numbers in C#?

easy

A. int value = numbers[3];

B. int value = numbers[0];

C. int value = numbers[1];

D. int value = numbers[2];

Solution

1. Step 1: Recall zero-based indexing in C# arrays

   Array indexes start at 0, so the first element is numbers[0].

2. Step 2: Identify the third element index

   The third element is at index 2 (0,1,2).

3. Final Answer:

   int value = numbers[2]; -> Option D

4. Quick Check:

   Third element index = 2 [OK]

Hint: Remember array indexing starts at zero [OK]

Common Mistakes:

• Using index 3 for third element
• Confusing element position with index
• Accessing index 1 or 0 for third element

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

int[] arr = {10, 20, 30};
Console.WriteLine(arr[3]);

medium

A. 0

B. IndexOutOfRangeException

C. 30

D. 20

Solution

1. Step 1: Check array length and valid indexes

   Array has 3 elements with indexes 0, 1, 2.

2. Step 2: Accessing index 3

   Index 3 is outside the array bounds, causing an exception.

3. Final Answer:

   IndexOutOfRangeException -> Option B

4. Quick Check:

   Accessing arr[3] = IndexOutOfRangeException [OK]

Hint: Indexes must be less than array length [OK]

Common Mistakes:

• Expecting last element value instead of error
• Assuming default zero value returned
• Confusing index 3 with valid index

4. Consider this code snippet:

int[] data = new int[5];
data[5] = 100;

What is the issue and how to fix it?

medium

A. Syntax error due to array declaration.

B. No issue, code runs fine.

C. IndexOutOfRangeException because index 5 is invalid; fix by using index 4.

D. NullReferenceException because array is not initialized.

Solution

1. Step 1: Analyze array size and valid indexes

   Array size is 5, valid indexes are 0 to 4.

2. Step 2: Identify invalid index usage

   Accessing data[5] is outside bounds, causing IndexOutOfRangeException.

3. Step 3: Correct the index

   Use data[4] to access the last element safely.

4. Final Answer:

   IndexOutOfRangeException because index 5 is invalid; fix by using index 4. -> Option C

5. Quick Check:

   Index 5 invalid, max index 4 [OK]

Hint: Last index is length minus one [OK]

Common Mistakes:

• Assuming index 5 is valid for length 5
• Confusing syntax error with runtime error
• Thinking array is uninitialized

5. You have an array int[] scores = {5, 10, 15};. You want to safely print the element at index i without causing an exception. Which code snippet correctly checks bounds before accessing?

hard

A. if (i >= 0 && i < scores.Length) Console.WriteLine(scores[i]); else Console.WriteLine("Invalid index");

B. if (i > 0 && i <= scores.Length) Console.WriteLine(scores[i]); else Console.WriteLine("Invalid index");

C. if (i >= 1 && i < scores.Length) Console.WriteLine(scores[i]); else Console.WriteLine("Invalid index");

D. if (i > 0 && i < scores.Length - 1) Console.WriteLine(scores[i]); else Console.WriteLine("Invalid index");

Solution

1. Step 1: Understand valid index range for array

   Valid indexes are from 0 up to Length - 1.

2. Step 2: Check each condition

   if (i >= 0 && i < scores.Length) Console.WriteLine(scores[i]); else Console.WriteLine("Invalid index"); correctly checks i >= 0 and i < Length, ensuring safe access.

3. Step 3: Identify incorrect conditions

   Other options exclude index 0 or allow invalid upper bounds.

4. Final Answer:

   if (i >= 0 && i < scores.Length) Console.WriteLine(scores[i]); else Console.WriteLine("Invalid index"); -> Option A

5. Quick Check:

   Bounds check: 0 ≤ i < Length [OK]

Hint: Check index >= 0 and < array length [OK]

Common Mistakes:

• Using > 0 instead of >= 0
• Allowing index equal to length
• Ignoring zero index as valid