Array Insertion at Middle Index in DSA C# - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output after inserting element in middle of array

What is the output of the following C# code that inserts an element in the middle of an array?

DSA C#

int[] arr = {1, 2, 4, 5};
int insertVal = 3;
int mid = arr.Length / 2;
int[] newArr = new int[arr.Length + 1];
for (int i = 0; i < mid; i++)
{
    newArr[i] = arr[i];
}
newArr[mid] = insertVal;
for (int i = mid; i < arr.Length; i++)
{
    newArr[i + 1] = arr[i];
}
foreach (int val in newArr)
{
    Console.Write(val + " ");
}

A 1 2 3 4 5

B 1 2 4 3 5

C 1 3 2 4 5

D 3 1 2 4 5

💡 Hint

Think about how the elements after the middle index are shifted to the right.

The code inserts 3 at the middle index (2) and shifts elements 4 and 5 to the right, resulting in 1 2 3 4 5.

❓ Predict Output

intermediate

Resulting array after middle insertion with even length

Given the code below, what will be printed after inserting 99 in the middle of the array?

DSA C#

int[] arr = {10, 20, 30, 40, 50, 60};
int insertVal = 99;
int mid = arr.Length / 2;
int[] newArr = new int[arr.Length + 1];
for (int i = 0; i < mid; i++)
{
    newArr[i] = arr[i];
}
newArr[mid] = insertVal;
for (int i = mid; i < arr.Length; i++)
{
    newArr[i + 1] = arr[i];
}
foreach (int val in newArr)
{
    Console.Write(val + " ");
}

A 10 20 30 99 40 50 60

B 10 20 99 30 40 50 60

C 10 20 30 40 99 50 60

D 99 10 20 30 40 50 60

💡 Hint

Remember that for even length arrays, middle is length/2.

The element 99 is inserted at index 3 (middle), shifting elements 40, 50, 60 right.

🔧 Debug

advanced

Identify the error in array insertion code

What error will this C# code produce when trying to insert an element in the middle of an array?

DSA C#

int[] arr = {1, 2, 3, 4};
int insertVal = 99;
int mid = arr.Length / 2;
int[] newArr = new int[arr.Length];
for (int i = 0; i < mid; i++)
{
    newArr[i] = arr[i];
}
newArr[mid] = insertVal;
for (int i = mid; i < arr.Length; i++)
{
    newArr[i + 1] = arr[i];
}
foreach (int val in newArr)
{
    Console.Write(val + " ");
}

A Compilation error due to array size

B No error, prints: 1 2 99 3 4

C IndexOutOfRangeException

D NullReferenceException

💡 Hint

Check the size of the new array and the indices used in the last loop.

The new array size is same as original, but code tries to assign at index i+1 which can exceed bounds.

🧠 Conceptual

advanced

Time complexity of inserting in middle of array

What is the time complexity of inserting an element in the middle of a fixed-size array by shifting elements?

A O(log n), due to binary search for middle

B O(n), because elements after the middle must be shifted

C O(1), because insertion is at a fixed index

D O(n^2), because shifting happens inside a nested loop

💡 Hint

Consider how many elements need to move when inserting in the middle.

Insertion requires shifting half the elements on average, so time is proportional to n.

🚀 Application

expert

Implement middle insertion in a dynamic array

You have a dynamic array (List) in C#. Which code snippet correctly inserts an element in the middle without manual shifting?

A list.RemoveAt(list.Count / 2); list.Add(value);

B list.Add(value); list.Sort();

C list[list.Count / 2] = value;

D list.Insert(list.Count / 2, value);

💡 Hint

Look for a built-in method that inserts at a specific index.

List.Insert inserts at index and shifts elements automatically.

Practice

1. What happens when you insert an element at the middle index of an array in C#?

easy

A. The array size decreases by one.

B. Elements after the middle index shift right to make space for the new element.

C. The new element replaces the element at the middle index without shifting.

D. The array elements remain unchanged.

Solution

1. Step 1: Understand array insertion behavior

   When inserting at the middle, elements from that index onward must move right to avoid overwriting.

2. Step 2: Effect on array size and elements

   The array size increases by one, and elements after the middle index shift right to make space.

3. Final Answer:

   Elements after the middle index shift right to make space for the new element. -> Option B

4. Quick Check:

   Insertion shifts elements right = A [OK]

Hint: Insertion shifts elements right from middle index [OK]

Common Mistakes:

• Thinking the array size decreases
• Replacing element without shifting
• Assuming no change in elements

2. Which of the following is the correct syntax to insert an element at the middle index of an array in C# (assuming you use a List<int> named 'numbers')?

easy

A. numbers.AddAtMiddle(10);

B. numbers.Add(10, numbers.Count / 2);

C. numbers.InsertAtMiddle(10);

D. numbers.Insert(numbers.Count / 2, 10);

Solution

1. Step 1: Identify List insertion method

   List<T> in C# uses Insert(index, value) to insert at any position.

2. Step 2: Calculate middle index and insert

   Middle index is numbers.Count / 2; so Insert(numbers.Count / 2, 10) inserts 10 at middle.

3. Final Answer:

   numbers.Insert(numbers.Count / 2, 10); -> Option D

4. Quick Check:

   Insert(index, value) syntax = B [OK]

Hint: Use Insert(index, value) with middle index [OK]

Common Mistakes:

• Using non-existent methods like AddAtMiddle
• Confusing Add and Insert parameters
• Wrong method names

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

List<int> nums = new List<int> {1, 2, 3, 4, 5};
nums.Insert(nums.Count / 2, 99);
Console.WriteLine(string.Join(", ", nums));

medium

A. 1, 2, 3, 99, 4, 5

B. 1, 2, 3, 4, 5, 99

C. 1, 2, 99, 3, 4, 5

D. 99, 1, 2, 3, 4, 5

Solution

1. Step 1: Calculate middle index

   nums.Count is 5, so middle index = 5 / 2 = 2.

2. Step 2: Insert 99 at index 2

   Original list: [1, 2, 3, 4, 5]. After insertion: [1, 2, 99, 3, 4, 5].

3. Final Answer:

   1, 2, 99, 3, 4, 5 -> Option C

4. Quick Check:

   Insert at index 2 shifts elements right = D [OK]

Hint: Middle index = Count/2; insert shifts elements right [OK]

Common Mistakes:

• Inserting at wrong index
• Appending instead of inserting
• Miscounting middle index

4. The following code tries to insert an element at the middle index of an array but does not insert properly. What is the issue and how to fix it?

int[] arr = {1, 2, 3, 4};
int middle = arr.Length / 2;
arr[middle] = 99;

medium

A. Error: Cannot insert in fixed-size array; use List<int> and Insert method.

B. No error; code works fine.

C. Error: Index out of range; middle index calculation is wrong.

D. Error: Assignment operator is invalid here.

Solution

1. Step 1: Understand array size and insertion

   Arrays in C# have fixed size; you cannot insert elements to increase size.

2. Step 2: Fix by using List<int> and Insert method

   Use List<int> which supports dynamic insertion with Insert(index, value).

3. Final Answer:

   Error: Cannot insert in fixed-size array; use List<int> and Insert method. -> Option A

4. Quick Check:

   Fixed-size arrays can't insert = A [OK]

Hint: Arrays fixed size; use List for insertion [OK]

Common Mistakes:

• Thinking assignment inserts element
• Misunderstanding array size limits
• Wrong index calculation assumption

5. You have an array of integers with an even length. You want to insert a new element exactly in the middle so that the array size increases by one and elements after the middle shift right. Which approach is best in C#?

hard

A. Use a List<int>, insert at middle index, then convert back to array if needed.

B. Directly assign the new element at middle index in the existing array.

C. Create a new array with size +1, copy elements before middle, insert new element, then copy remaining elements.

D. Use Array.Resize to increase size and then assign at middle index.

Solution

1. Step 1: Understand array limitations

   Arrays have fixed size; direct insertion is not possible without creating a new array.

2. Step 2: Use List for dynamic insertion

   List<int> allows insertion at any index easily; after insertion, convert back to array if needed.

3. Final Answer:

   Use a List<int>, insert at middle index, then convert back to array if needed. -> Option A

4. Quick Check:

   List supports insertion; arrays do not [OK]

Hint: Use List for easy insertion, then convert to array [OK]

Common Mistakes:

• Trying to assign directly in fixed array
• Using Array.Resize incorrectly
• Manually copying arrays without List