Array Deletion at Beginning in DSA C#

Mental Model

Deleting the first item in an array means removing it and shifting all other items one place left.

Analogy: Imagine a row of chairs where the person in the first chair leaves, and everyone else moves one chair forward to fill the empty spot.

Index: 0   1   2   3   4
Array: [1]->[2]->[3]->[4]->[5]

Dry Run Walkthrough

Input: array: [1, 2, 3, 4, 5], delete element at beginning

Goal: Remove the first element and shift all others left to fill the gap

Step 1: Remove element at index 0 (value 1)

Index: 0   1   2   3   4
Array: [1]->[2]->[3]->[4]->[5]

Why: We identify the element to delete at the start

Step 2: Shift element at index 1 (value 2) to index 0

Index: 0   1   2   3   4
Array: [2]->[2]->[3]->[4]->[5]

Why: Move next element left to fill the empty spot

Step 3: Shift element at index 2 (value 3) to index 1

Index: 0   1   2   3   4
Array: [2]->[3]->[3]->[4]->[5]

Why: Continue shifting elements left

Step 4: Shift element at index 3 (value 4) to index 2

Index: 0   1   2   3   4
Array: [2]->[3]->[4]->[4]->[5]

Why: Continue shifting elements left

Step 5: Shift element at index 4 (value 5) to index 3

Index: 0   1   2   3   4
Array: [2]->[3]->[4]->[5]->[5]

Why: Continue shifting elements left

Step 6: Reduce array size by 1 to remove duplicate last element

Index: 0   1   2   3
Array: [2]->[3]->[4]->[5]

Why: Remove the last duplicate element after shifting

Result:

Index: 0   1   2   3
Array: [2]->[3]->[4]->[5]

Annotated Code

DSA C#

using System;

class ArrayDeletion
{
    public static int[] DeleteAtBeginning(int[] arr)
    {
        if (arr == null || arr.Length == 0)
            return arr; // handle empty array

        int n = arr.Length;
        for (int i = 1; i < n; i++)
        {
            arr[i - 1] = arr[i]; // shift element left
        }

        // create new array with size one less
        int[] newArr = new int[n - 1];
        for (int i = 0; i < n - 1; i++)
        {
            newArr[i] = arr[i]; // copy shifted elements
        }

        return newArr;
    }

    static void Main()
    {
        int[] arr = {1, 2, 3, 4, 5};
        int[] result = DeleteAtBeginning(arr);
        Console.WriteLine(string.Join(" -> ", result) + " -> null");
    }
}

for (int i = 1; i < n; i++)

shift each element one position left to overwrite the first element

arr[i - 1] = arr[i];

move element at i to position i-1

int[] newArr = new int[n - 1];

create new array smaller by one to remove last duplicate

for (int i = 0; i < n - 1; i++)

copy shifted elements to new smaller array

return newArr;

return the new array without the deleted first element

Output

2 -> 3 -> 4 -> 5 -> null

Complexity Analysis

Time: O(n) because we shift all elements left once

Space: O(n) because we create a new array smaller by one element

vs Alternative: Compared to linked lists where deletion at beginning is O(1), array deletion requires shifting elements making it O(n)

Edge Cases

empty array

returns the same empty array without error

DSA C#

if (arr == null || arr.Length == 0)
            return arr; // handle empty array

array with one element

returns an empty array after deletion

DSA C#

int[] newArr = new int[n - 1];

When to Use This Pattern

When you need to remove the first element from an array and maintain order, use the array deletion at beginning pattern that shifts elements left.

Common Mistakes

Mistake: Not shifting elements left and just setting first element to zero or null
Fix: Shift all elements left to fill the gap, then reduce array size

Mistake: Forgetting to create a new smaller array and returning original array with duplicate last element
Fix: Create a new array with size one less and copy shifted elements before returning

Summary

Deletes the first element of an array by shifting all other elements left and resizing the array.

Use when you need to remove the first item from an array but keep the order of remaining items.

The key insight is that arrays require shifting all elements left to fill the gap after deletion.

Practice

1. What happens to the elements of an array when you delete the first element (at the beginning) in C#?

easy

A. All elements shift one position to the left, and the last element becomes unused.

B. The array size decreases automatically by one.

C. The first element is replaced by null but others stay the same.

D. The array elements remain unchanged.

Solution

1. Step 1: Understand array fixed size in C#

   Arrays have fixed size; deleting an element does not reduce size automatically.

2. Step 2: Effect of deleting first element

   To delete the first element, all other elements shift left by one to fill the gap.

3. Final Answer:

   All elements shift one position to the left, and the last element becomes unused. -> Option A

4. Quick Check:

   Deleting first element shifts left [OK]

Hint: Deleting at start shifts all elements left by one [OK]

Common Mistakes:

• Thinking array size changes automatically
• Assuming elements stay in place
• Believing first element becomes null only

2. Which of the following is the correct syntax to delete the first element from an array named arr in C# by shifting elements?

easy

A. arr.RemoveAt(0);

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

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

D. arr.Delete(0);

Solution

1. Step 1: Understand array element shifting

   To delete first element, shift elements left starting from index 0 to length-2.

2. Step 2: Check syntax correctness

   for(int i = 0; i < arr.Length - 1; i++) arr[i] = arr[i + 1]; correctly loops from 0 to length-2 and assigns arr[i] = arr[i+1].

3. Final Answer:

   for(int i = 0; i < arr.Length - 1; i++) arr[i] = arr[i + 1]; -> Option B

4. Quick Check:

   Loop from 0 to length-2 shifting elements left [OK]

Hint: Shift elements from index 0 to length-2 left [OK]

Common Mistakes:

• Starting loop from 1 instead of 0
• Using non-existent array methods like RemoveAt
• Assigning elements in wrong direction

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

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

medium

A. 20 30 40 0

B. 10 20 30 40

C. 0 20 30 40

D. 20 30 40 40

Solution

1. Step 1: Trace the shifting loop

   Loop shifts elements left: arr[0]=20, arr[1]=30, arr[2]=40.

2. Step 2: Set last element to 0

   arr[3] = 0 replaces old last element 40.

3. Final Answer:

   20 30 40 0 -> Option A

4. Quick Check:

   Shift left + last zero = 20 30 40 0 [OK]

Hint: Shift left then zero last element for deletion [OK]

Common Mistakes:

• Not setting last element to zero
• Confusing loop range causing wrong shifts
• Printing old array without changes

4. Identify the error in the following code that tries to delete the first element of an array arr and fix it:

int[] arr = {5, 10, 15, 20};
for(int i = 1; i < arr.Length; i++) {
  arr[i] = arr[i - 1];
}
arr[arr.Length - 1] = 0;

medium

A. Last element should not be set to zero.

B. Array size must be reduced before shifting.

C. Loop direction is wrong; should shift elements left, not right.

D. The loop should start from 0 and go to arr.Length - 2.

Solution

1. Step 1: Analyze loop direction

   The loop copies arr[i-1] to arr[i], shifting elements right, which is incorrect for deletion.

2. Step 2: Correct loop to shift elements left

   Loop should assign arr[i] = arr[i+1] starting from i=0 to length-2.

3. Final Answer:

   Loop direction is wrong; should shift elements left, not right. -> Option C

4. Quick Check:

   Shift left to delete first element [OK]

Hint: Shift elements left, not right, when deleting at start [OK]

Common Mistakes:

• Shifting elements in wrong direction
• Trying to reduce array size directly
• Ignoring last element cleanup

5. You have an array int[] arr = {3, 6, 9, 12, 15}. Write a method to delete the first element and return a new array with the remaining elements only (size reduced by one). Which approach is correct?

hard

A. Use arr.RemoveAt(0) to delete the first element.

B. Shift elements left in original array and keep the same size.

C. Set arr[0] = 0 and return the original array.

D. Create a new array of size arr.Length - 1 and copy elements from index 1 onward.

Solution

1. Step 1: Understand fixed size arrays

   Arrays in C# have fixed size; to reduce size, create a new array.

2. Step 2: Copy elements except first

   Create new array of size one less and copy elements from index 1 onward.

3. Final Answer:

   Create a new array of size arr.Length - 1 and copy elements from index 1 onward. -> Option D

4. Quick Check:

   New smaller array with elements from index 1 [OK]

Hint: Create new smaller array and copy from index 1 [OK]

Common Mistakes:

• Trying to resize original array directly
• Only shifting elements without resizing
• Using non-existent RemoveAt method on array