Array Deletion at Middle Index in DSA C# - Cheat Sheet & Quick Revision

Recall & Review

beginner

What happens to the elements in an array when you delete an element at a middle index?

All elements after the deleted index shift one position to the left to fill the gap, reducing the array's effective size by one.

beginner

Why can't we physically reduce the size of a fixed-size array in C# after deletion?

Because arrays in C# have a fixed size once created, so we can only shift elements and track the new logical size, but the physical size remains the same.

intermediate

In the context of array deletion, what is the 'logical size' of an array?

The number of valid elements currently stored in the array, which may be less than the physical size after deletions.

intermediate

What is the time complexity of deleting an element at the middle index of an array?

O(n) because all elements after the deleted index must be shifted one position to the left.

beginner

Show the array state after deleting the element at index 2 from [10, 20, 30, 40, 50].

After deletion, the array elements shift left: [10, 20, 40, 50]. The logical size is now 4.

What must you do after deleting an element at a middle index in a fixed-size array?

A Shift all elements after the index one position to the left

B Increase the array size by one

C Replace the deleted element with zero without shifting

D Create a new array with one less element automatically

In fixed-size arrays, elements after the deleted index must be shifted left to fill the gap.

What is the time complexity of deleting an element at the middle of an array of size n?

A O(n)

B O(1)

C O(log n)

D O(n log n)

Deletion requires shifting elements, which takes linear time proportional to n.

After deleting an element at index 1 from [5, 10, 15, 20], what is the new logical size?

A 4

B 2

C 3

D 1

One element is removed, so logical size decreases by one.

In C#, can you reduce the physical size of an array after deletion?

A Only if the array is empty

B Yes, by calling a built-in method

C Yes, automatically

D No, arrays have fixed size

Arrays in C# have fixed size; you cannot shrink them after creation.

What is the correct way to delete an element at index 0 in an array [1, 2, 3, 4]?

A Set element at index 0 to zero

B Shift elements at indexes 1, 2, 3 left by one

C Swap element at index 0 with last element

D Do nothing

Shifting elements left fills the gap created by deletion.

Explain step-by-step how to delete an element at a middle index in a fixed-size array and update its logical size.

Think about how to fill the gap left by the deleted element.

Describe why deleting an element in the middle of an array is less efficient than deleting the last element.

Consider the number of elements moved after deletion.

Practice

1. What happens when you delete the middle element from an array in C#?

easy

A. The element is removed and all elements after it shift left by one position.

B. The array size automatically decreases by one without shifting elements.

C. The middle element is replaced by zero but array size stays the same.

D. The array throws an error because arrays have fixed size.

Solution

1. Step 1: Understand array behavior on deletion

   In C#, arrays have fixed size, so to delete an element, we shift elements after it left to overwrite it.

2. Step 2: Effect on array elements

   After shifting, the last element is duplicated or ignored logically, but the middle element is effectively removed from the sequence.

3. Final Answer:

   The element is removed and all elements after it shift left by one position. -> Option A

4. Quick Check:

   Deletion shifts elements left [OK]

Hint: Deleting means shifting elements left to fill the gap [OK]

Common Mistakes:

• Thinking array size changes automatically
• Assuming deletion replaces element with zero
• Believing arrays throw error on deletion

2. Which of the following C# code snippets correctly deletes the middle element from an integer array arr of odd length?

easy

A. int mid = arr.Length / 2; arr[mid] = 0;

B. int mid = arr.Length / 2; for(int i = mid; i < arr.Length - 1; i++) arr[i] = arr[i + 1];

C. int mid = arr.Length / 2; arr.RemoveAt(mid);

D. int mid = arr.Length / 2; Array.Delete(arr, mid);

Solution

1. Step 1: Identify valid deletion method in arrays

   Arrays in C# do not have RemoveAt or Delete methods; we must shift elements manually.

2. Step 2: Check code correctness

   int mid = arr.Length / 2; for(int i = mid; i < arr.Length - 1; i++) arr[i] = arr[i + 1]; calculates middle index and shifts elements left from middle to end-1, correctly deleting the middle element.

3. Final Answer:

   int mid = arr.Length / 2; for(int i = mid; i < arr.Length - 1; i++) arr[i] = arr[i + 1]; -> Option B

4. Quick Check:

   Manual shift after mid index [OK]

Hint: Use loop to shift elements left from middle index [OK]

Common Mistakes:

• Using non-existent array methods like RemoveAt
• Just setting middle element to zero without shifting
• Trying to delete without shifting elements

3. What is the output of the following C# code?

int[] arr = {10, 20, 30, 40, 50};
int mid = arr.Length / 2;
for(int i = mid; i < arr.Length - 1; i++)
    arr[i] = arr[i + 1];
for(int i = 0; i < arr.Length - 1; i++)
    Console.Write(arr[i] + " ");

medium

A. 10 20 30 50

B. 10 20 30 40 50

C. 10 20 40 50

D. 10 20 40 50 50

Solution

1. Step 1: Calculate middle index and shift elements

   Array length is 5, mid = 2. Loop shifts arr[2] = arr[3] (40), arr[3] = arr[4] (50).

2. Step 2: Print elements except last

   Loop prints arr[0]=10, arr[1]=20, arr[2]=40, arr[3]=50. Last element ignored.

3. Final Answer:

   10 20 40 50 -> Option C

4. Quick Check:

   Shifted middle element out = 10 20 40 50 [OK]

Hint: Shift elements left, print all but last [OK]

Common Mistakes:

• Printing full array including last duplicate
• Not shifting elements correctly
• Confusing middle index calculation

4. Find the error in this code that tries to delete the middle element from an array arr of length 6:

int mid = arr.Length / 2;
for(int i = mid; i <= arr.Length - 1; i++)
    arr[i] = arr[i + 1];

medium

A. Array elements are not shifted correctly.

B. Middle index calculation is wrong for even length arrays.

C. No error; code works fine.

D. Loop condition causes index out of range error.

Solution

1. Step 1: Analyze loop range and array indexing

   Loop runs from mid to arr.Length - 1 inclusive, accessing arr[i + 1]. For i = arr.Length - 1, arr[i + 1] is out of bounds.

2. Step 2: Identify cause of error

   Accessing arr[arr.Length] causes IndexOutOfRangeException.

3. Final Answer:

   Loop condition causes index out of range error. -> Option D

4. Quick Check:

   Loop index out of bounds [OK]

Hint: Loop must stop before last index to avoid out of range [OK]

Common Mistakes:

• Using <= instead of < in loop condition
• Ignoring array bounds when accessing i+1
• Misunderstanding middle index for even length

5. You have an array arr = {5, 10, 15, 20, 25, 30, 35}. Write the steps to delete the middle element and print the resulting array with correct size handling.

hard

A. Create a list from arr; remove element at mid; convert back to array; print.

B. Calculate mid = arr.Length / 2; shift elements left from mid; print first arr.Length - 1 elements.

C. Use Array.RemoveAt(mid); print arr.

D. Set arr[mid] = 0; print arr as is.

Solution

1. Step 1: Convert array to list for flexible size

   Arrays have fixed size, so convert to List<int> to remove element easily.

2. Step 2: Remove middle element and convert back

   Remove element at mid index, then convert list back to array for printing.

3. Step 3: Print resulting array

   Print elements of new array which has one less element.

4. Final Answer:

   Create a list from arr; remove element at mid; convert back to array; print. -> Option A

5. Quick Check:

   Use List for deletion, then convert back [OK]

Hint: Use List to delete middle element easily, then convert back [OK]

Common Mistakes:

• Trying to resize arrays directly
• Only shifting elements without resizing
• Using non-existent Array.RemoveAt method