Array Insertion at Beginning in DSA C# - Cheat Sheet & Quick Revision

Recall & Review

beginner

What happens to the existing elements when you insert a new element at the beginning of an array?

All existing elements are shifted one position to the right to make space for the new element at index 0.

beginner

Why can't we directly insert an element at the beginning of a fixed-size array without shifting?

Because arrays have fixed size and continuous memory, inserting at the beginning requires moving existing elements to avoid overwriting data.

intermediate

In C#, what is a common approach to insert an element at the beginning of an array?

Create a new array with size +1, place the new element at index 0, then copy existing elements starting from index 1.

beginner

What is the time complexity of inserting an element at the beginning of an array?

O(n), because all existing elements must be shifted or copied to new positions.

intermediate

How does inserting at the beginning of an array differ from inserting at the end?

Inserting at the end can be done directly if space is available, often O(1), but inserting at the beginning requires shifting elements, O(n).

What must you do to existing elements when inserting at the beginning of a fixed-size array?

A Shift all elements one position to the right

B Shift all elements one position to the left

C Replace the first element directly

D No action needed

To insert at the beginning, all elements must move right to avoid overwriting.

What is the time complexity of inserting an element at the beginning of an array?

A O(n^2)

B O(1)

C O(log n)

D O(n)

Insertion at the beginning requires shifting all elements, which takes O(n) time.

In C#, how can you insert an element at the beginning of an array if the array size is fixed?

A Overwrite the first element

B Create a new larger array and copy elements

C Use Array.Insert method

D Use List.Insert method

Arrays have fixed size, so you create a new array and copy elements to insert at the beginning.

Which data structure is more efficient than arrays for frequent insertions at the beginning?

A Linked List

B Stack

C Queue

D Hash Table

Linked lists allow O(1) insertion at the beginning without shifting elements.

If you have an array [10, 20, 30] and insert 5 at the beginning, what is the resulting array?

A [10, 5, 20, 30]

B [10, 20, 30, 5]

C [5, 10, 20, 30]

D [5, 20, 30]

The new element 5 is placed at index 0, shifting others right.

Explain step-by-step how to insert an element at the beginning of a fixed-size array in C#.

Think about how to avoid overwriting existing elements.

Why is inserting at the beginning of an array less efficient than inserting at the end?

Consider how elements move in memory.

Practice

1. What happens when you insert an element at the beginning of a fixed-size array in C#?

easy

A. The last element is removed and the new element is added at the start.

B. The new element replaces the first element without shifting others.

C. The array size automatically increases to fit the new element.

D. All existing elements shift one position to the right to make space.

Solution

1. Step 1: Understand fixed-size array behavior

   In C#, arrays have fixed size, so to insert at the beginning, elements must shift right.

2. Step 2: Effect of insertion at index 0

   The element at index 0 is replaced by the new element, and all others move one index higher.

3. Final Answer:

   All existing elements shift one position to the right to make space. -> Option D

4. Quick Check:

   Insert at start = shift right [OK]

Hint: Insert at start means shift all elements right [OK]

Common Mistakes:

• Thinking array size grows automatically
• Replacing first element without shifting
• Removing last element silently

2. Which of the following C# code snippets correctly inserts an integer 5 at the beginning of an array arr of size 5, shifting elements right?

easy

A. for (int i = arr.Length - 1; i > 0; i--) { arr[i] = arr[i - 1]; } arr[0] = 5;

B. for (int i = 0; i < arr.Length - 1; i++) { arr[i] = arr[i + 1]; } arr[arr.Length - 1] = 5;

C. arr[0] = 5; // No shifting needed

D. arr[arr.Length] = 5; // Add at end

Solution

1. Step 1: Identify correct loop for shifting right

   Loop must start from the end and move elements one step right to avoid overwriting.

2. Step 2: Assign new value at index 0

   After shifting, assign 5 to arr[0].

3. Final Answer:

   for (int i = arr.Length - 1; i > 0; i--) { arr[i] = arr[i - 1]; } arr[0] = 5; -> Option A

4. Quick Check:

   Shift right from end, then insert [OK]

Hint: Shift from end to start to avoid overwrite [OK]

Common Mistakes:

• Shifting left instead of right
• Assigning without shifting
• Using invalid index arr.Length

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

int[] arr = {10, 20, 30, 40, 50};
for (int i = arr.Length - 1; i > 0; i--) {
    arr[i] = arr[i - 1];
}
arr[0] = 5;
foreach (int num in arr) {
    Console.Write(num + " ");
}

medium

A. 5 10 20 30 50

B. 5 10 20 30 40

C. 5 20 30 40 50

D. 10 5 20 30 40

Solution

1. Step 1: Shift elements right

   Elements move right: arr[4]=arr[3]=40, arr[3]=arr[2]=30, arr[2]=arr[1]=20, arr[1]=arr[0]=10

2. Step 2: Insert 5 at index 0

   arr[0] = 5

3. Final Answer:

   5 10 20 30 40 -> Option B

4. Quick Check:

   Shift right + insert 5 at start = 5 10 20 30 40 [OK]

Hint: Trace shifts from end, then insert new value [OK]

Common Mistakes:

• Not shifting elements correctly
• Overwriting elements before shifting
• Incorrect final array print

4. The following code attempts to insert 99 at the beginning of an array arr but produces incorrect output. What is the error?

int[] arr = {1, 2, 3, 4, 5};
for (int i = 0; i < arr.Length - 1; i++) {
    arr[i + 1] = arr[i];
}
arr[0] = 99;

medium

A. The loop shifts elements left instead of right, overwriting data.

B. The assignment to arr[0] should be inside the loop.

C. The loop index range is incorrect; it should start from the end.

D. The array size is too small to insert a new element.

Solution

1. Step 1: Analyze loop direction and range

   The loop starts from 0 and moves forward, causing overwriting of elements.

2. Step 2: Correct loop should start from end

   To shift right safely, loop must start from last index and move backward.

3. Final Answer:

   The loop index range is incorrect; it should start from the end. -> Option C

4. Quick Check:

   Shift right requires backward loop [OK]

Hint: Shift right needs backward loop from end [OK]

Common Mistakes:

• Looping forward overwrites elements
• Assigning new value inside loop
• Ignoring fixed array size

5. You have an array arr = new int[6] {0, 1, 2, 3, 4, 5} and want to insert a new element 99 at the beginning without losing any data. Which approach correctly handles this?

hard

A. Create a new array of size 7, set arr2[0] = 99, then copy all elements of arr to arr2 starting at index 1.

B. Shift all elements right by one and assign 99 to arr[0], but last element is lost.

C. Replace arr[0] with 99 directly without shifting elements.

D. Use a List<int> instead of array to insert at beginning easily.

Solution

1. Step 1: Understand fixed array size limitation

   Original array size is 6, so inserting without losing data requires a bigger array.

2. Step 2: Create new larger array and copy elements

   Create new array size 7, assign 99 at index 0, then copy original elements starting at index 1.

3. Final Answer:

   Create a new array of size 7, set arr2[0] = 99, then copy all elements of arr to arr2 starting at index 1. -> Option A

4. Quick Check:

   New bigger array + copy elements = safe insertion [OK]

Hint: Use bigger array and copy to insert without loss [OK]

Common Mistakes:

• Trying to insert in fixed-size array losing last element
• Replacing without shifting
• Ignoring better data structures like List