Array Access and Update at Index in DSA C# - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Array Update and Access

What is the output of the following C# code snippet?

DSA C#

int[] arr = {1, 2, 3, 4, 5};
arr[2] = arr[4] + arr[0];
Console.WriteLine(string.Join(", ", arr));

A 1, 2, 6, 4, 5

B 1, 2, 8, 4, 5

C 1, 2, 9, 4, 5

D 1, 2, 7, 4, 5

💡 Hint

Remember array indexing starts at 0 and addition is done before assignment.

arr[2] is updated to arr[4] + arr[0] which is 5 + 1 = 6. The array becomes [1, 2, 6, 4, 5].

❓ Predict Output

intermediate

Array Element After Multiple Updates

What is the value of arr[1] after running this code?

DSA C#

int[] arr = {10, 20, 30, 40};
arr[1] = arr[3] - arr[0];
arr[3] = arr[1] + 5;

A 35

B 40

C 20

D 30

💡 Hint

Update arr[1] first, then use its new value to update arr[3].

arr[1] = arr[3] - arr[0] = 40 - 10 = 30. Then arr[3] = arr[1] + 5 = 30 + 5 = 35. So arr[1] is 30.

🔧 Debug

advanced

Identify the Error in Array Update

What error will this C# code produce when run?

DSA C#

int[] arr = new int[3];
arr[0] = 5;
arr[3] = 10;
Console.WriteLine(arr[0]);

A NullReferenceException

B No error, outputs 5

C IndexOutOfRangeException

D SyntaxError

💡 Hint

Check the valid index range for the array.

The array has size 3, so valid indices are 0, 1, and 2. Accessing arr[3] causes IndexOutOfRangeException.

❓ Predict Output

advanced

Final Array State After Multiple Updates

What is the final state of the array after executing this code?

DSA C#

int[] arr = {2, 4, 6, 8};
arr[1] = arr[2] + arr[3];
arr[2] = arr[0] * 2;
arr[3] = arr[1] - arr[2];

A 2, 14, 12, 2

B 2, 14, 4, 10

C 2, 14, 4, 4

D 2, 14, 12, 10

💡 Hint

Update step by step and track each change carefully.

Step 1: arr[1] = 6 + 8 = 14 Step 2: arr[2] = 2 * 2 = 4 Step 3: arr[3] = 14 - 4 = 10 Final array: [2, 14, 4, 10]

❓ Predict Output

expert

Output After Complex Array Index Updates

What is printed by this C# code?

DSA C#

int[] arr = {1, 3, 5, 7, 9};
int i = 1;
arr[i] = arr[i + 1] + arr[i - 1];
i = 3;
arr[i] = arr[i] + arr[i - 2];
Console.WriteLine(string.Join(", ", arr));

A 1, 6, 5, 13, 9

B 1, 8, 5, 7, 9

C 1, 8, 5, 10, 9

D 1, 8, 5, 14, 9

💡 Hint

Update arr[1] first, then arr[3], carefully using updated values.

First, i=1: arr[1] = arr[2] + arr[0] = 5 + 1 = 6. Array: [1, 6, 5, 7, 9] Then i=3: arr[3] = arr[3] + arr[1] = 7 + 6 = 13. Array: [1, 6, 5, 13, 9] Output: 1, 6, 5, 13, 9

Practice

1. What does the expression arr[2] do in an array arr?

easy

A. Returns the length of the array

B. Accesses the element at index 2 in the array

C. Deletes the element at index 2

D. Adds a new element at index 2

Solution

1. Step 1: Understand array indexing

   Array elements are accessed by their index, starting from 0.

2. Step 2: Access element at index 2

   The expression arr[2] retrieves the element stored at the third position (index 2) in the array.

3. Final Answer:

   Accesses the element at index 2 in the array -> Option B

4. Quick Check:

   arr[2] means get element at index 2 [OK]

Hint: Remember array indexing starts at zero [OK]

Common Mistakes:

• Thinking index starts at 1
• Confusing access with insertion
• Assuming arr[2] returns array length

2. Which of the following is the correct way to update the element at index 1 in an integer array numbers to 10 in C#?

easy

A. numbers(1) = 10;

B. numbers{1} = 10;

C. numbers[1] = 10;

D. numbers<1> = 10;

Solution

1. Step 1: Identify correct syntax for array indexing in C#

   C# uses square brackets [] to access or update array elements by index.

2. Step 2: Apply update syntax

   To update the element at index 1, use numbers[1] = 10;.

3. Final Answer:

   numbers[1] = 10; -> Option C

4. Quick Check:

   Use square brackets for array index update [OK]

Hint: Use square brackets [] to access or update arrays [OK]

Common Mistakes:

• Using parentheses () instead of brackets []
• Using curly braces {} which are invalid for arrays
• Using angle brackets <> which are for generics

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

int[] arr = {5, 10, 15};
arr[1] = 20;
Console.WriteLine(arr[1]);

medium

A. 20

B. 15

C. 10

D. 5

Solution

1. Step 1: Initialize array and update element

   The array arr starts as {5, 10, 15}. Then arr[1] = 20; changes the element at index 1 from 10 to 20.

2. Step 2: Print updated element

   Console.WriteLine(arr[1]); prints the updated value at index 1, which is now 20.

3. Final Answer:

   20 -> Option A

4. Quick Check:

   arr[1] updated to 20, so output is 20 [OK]

Hint: Remember updates change the element value at given index [OK]

Common Mistakes:

• Printing old value before update
• Confusing index 1 with index 0
• Assuming array is unchanged

4. Identify the error in the following code snippet:

int[] data = new int[3];
data[3] = 100;

medium

A. Index out of range error at data[3]

B. No error, code runs fine

C. Syntax error in array declaration

D. Cannot assign value to array element

Solution

1. Step 1: Check array size and valid indices

   The array data has size 3, so valid indices are 0, 1, and 2.

2. Step 2: Analyze assignment at index 3

   Accessing data[3] is invalid and causes an index out of range error at runtime.

3. Final Answer:

   Index out of range error at data[3] -> Option A

4. Quick Check:

   Array indices go from 0 to length-1 [OK]

Hint: Array index must be less than array length [OK]

Common Mistakes:

• Using index equal to array length
• Assuming array indices start at 1
• Ignoring runtime exceptions

5. Given an array int[] nums = {1, 2, 3, 4, 5};, which code snippet correctly doubles the value of each element in the array?

hard

A. foreach (int i in nums) { i = i * 2; }

B. for (int i = 0; i <= nums.Length; i++) { nums[i] = nums[i] * 2; }

C. for (int i = 1; i < nums.Length; i++) { nums[i] = nums[i] * 2; }

D. for (int i = 0; i < nums.Length; i++) { nums[i] = nums[i] * 2; }

Solution

1. Step 1: Understand array traversal for update

   To update each element, we must loop from index 0 to Length-1 using i < nums.Length.

2. Step 2: Analyze each option

   for (int i = 0; i <= nums.Length; i++) { nums[i] = nums[i] * 2; } uses i <= nums.Length which causes index out of range error.
   for (int i = 0; i < nums.Length; i++) { nums[i] = nums[i] * 2; } correctly loops from 0 to Length-1 and updates each element.
   for (int i = 1; i < nums.Length; i++) { nums[i] = nums[i] * 2; } starts from 1, skipping index 0.
   foreach (int i in nums) { i = i * 2; } uses foreach which does not allow modifying elements directly.

3. Final Answer:

   for (int i = 0; i < nums.Length; i++) { nums[i] = nums[i] * 2; } -> Option D

4. Quick Check:

   Use for loop with i < Length to update array [OK]

Hint: Use for loop with i < Length to safely update all elements [OK]

Common Mistakes:

• Using <= instead of < causing errors
• Starting loop at 1 skipping first element
• Using foreach which cannot modify array elements