Array bounds checking behavior in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an array of 5 integers.

C Sharp (C#)

int[] numbers = new int[[1]];

Drag options to blanks, or click blank then click option'

A5

B0

C10

D-1

Attempts:

3 left

💡 Hint

Common Mistakes

Using zero or negative numbers for array size causes errors.

The array size must be a positive integer. Here, 5 creates an array with 5 elements.

2fill in blank

medium

Complete the code to access the first element of the array.

C Sharp (C#)

int first = numbers[[1]];

Drag options to blanks, or click blank then click option'

A0

B1

C5

D-1

Attempts:

3 left

💡 Hint

Common Mistakes

Using 1 as the first index instead of 0.

Array indexing in C# starts at 0, so the first element is at index 0.

3fill in blank

hard

Fix the error in the code to avoid an IndexOutOfRangeException.

C Sharp (C#)

int value = numbers[[1]];

Drag options to blanks, or click blank then click option'

A5

B4

C6

D-1

Attempts:

3 left

💡 Hint

Common Mistakes

Using the array length as an index, which is out of bounds.

The last valid index in an array of size 5 is 4 (since indexing starts at 0). Using 5 or more causes an exception.

4fill in blank

hard

Fill both blanks to create a loop that safely accesses all elements of the array.

C Sharp (C#)

for (int i = [1]; i [2] numbers.Length; i++) {
    Console.WriteLine(numbers[i]);
}

Drag options to blanks, or click blank then click option'

A0

B<=

C<

D1

Attempts:

3 left

💡 Hint

Common Mistakes

Using <= causes the loop to access an invalid index equal to the array length.

The loop starts at index 0 and runs while i is less than numbers.Length to avoid going out of bounds.

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension that maps each number to its square if the number is less than 5.

C Sharp (C#)

var squares = numbers.Where(n => n [1] 5).ToDictionary(n => n, n => n [2] [3]);

Drag options to blanks, or click blank then click option'

A<

B*

Cn

D+

Attempts:

3 left

💡 Hint

Common Mistakes

Using <= instead of < changes the filter condition.

Using + instead of * does not compute the square.

The code filters numbers less than 5, then creates a dictionary mapping each number to its square (n * n).

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