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

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output after inserting at the beginning?

Consider the following C# code that inserts a value at the beginning of an array by shifting elements. What will be printed?

DSA C#

int[] arr = {2, 3, 4, 5};
int newValue = 1;
int n = arr.Length;
int[] newArr = new int[n + 1];
for (int i = 0; i < n; i++) {
    newArr[i + 1] = arr[i];
}
newArr[0] = newValue;
for (int i = 0; i < newArr.Length; i++) {
    Console.Write(newArr[i] + " ");
}

A 0 2 3 4 5

B 2 3 4 5 1

C 1 2 3 4 5

D 1 3 4 5 0

💡 Hint

Think about how elements are shifted to the right to make space at the start.

The code creates a new array with one extra space. It shifts all original elements one position right and inserts the new value at index 0. So the output is the new value followed by the original elements.

❓ Predict Output

intermediate

What error occurs when inserting without resizing?

What happens if you try to insert an element at the beginning of a fixed-size array without creating a new array or resizing it?

DSA C#

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

A 1 3 4 5

B 1 2 3 4

C 1 2 3 4 5

D Runtime error due to array size overflow

💡 Hint

Arrays in C# have fixed size. Shifting elements overwrites the last element.

The code shifts elements right inside the same array, overwriting the last element (5). So the last element is lost and the array becomes {1, 2, 3, 4}. No error occurs because the array size is fixed.

🔧 Debug

advanced

Why does this insertion code cause an error?

This code tries to insert a value at the beginning of an array but causes a runtime error. What is the cause?

DSA C#

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

A IndexOutOfRangeException because loop goes beyond array length

B NullReferenceException due to uninitialized array

C Compile-time error due to invalid syntax

D No error, code runs fine

💡 Hint

Check the loop condition and array indexing carefully.

The loop runs from 0 to arr.Length exclusive, but arr[i + 1] accesses beyond the array size when i = arr.Length - 1, causing IndexOutOfRangeException.

🧠 Conceptual

advanced

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

If you insert an element at the beginning of an array by shifting all elements one position to the right, what is the time complexity in terms of n (number of elements)?

A O(1) - constant time

B O(n^2) - quadratic time

C O(log n) - logarithmic time

D O(n) - linear time

💡 Hint

Consider how many elements need to be moved for each insertion.

Each insertion at the beginning requires shifting all existing elements one position to the right, which takes time proportional to n.

🚀 Application

expert

How to efficiently insert multiple elements at the beginning?

You need to insert multiple elements at the beginning of a large array repeatedly. Which data structure or approach is best to optimize insertion performance?

A Use a linked list instead of an array

B Use a stack data structure

C Use a queue data structure

D Use a fixed-size array and shift elements each time

💡 Hint

Think about which data structure allows fast insertion at the start without shifting.

Linked lists allow insertion at the beginning in constant time without shifting elements, unlike arrays which require shifting.

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