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

Recall & Review

beginner

What happens when you delete the first element of an array?

The first element is removed, and all other elements shift one position to the left to fill the gap.

beginner

Why do we need to shift elements after deleting the first element in an array?

Because arrays have fixed positions, shifting keeps the array continuous without empty spaces.

beginner

Show the array after deleting the first element from [10, 20, 30, 40].

After deletion, the array becomes [20, 30, 40]. The elements 20, 30, and 40 shift left by one position.

intermediate

What is the time complexity of deleting the first element in an array?

It is O(n) because all elements after the first must be shifted one position to the left.

intermediate

Can we delete the first element of an array without shifting elements?

No, because arrays store elements in continuous memory. To keep order, shifting is necessary.

What must be done after deleting the first element of an array?

A Do nothing

B Shift all elements to the left by one position

C Replace the first element with zero

D Shift all elements to the right by one position

After deleting the first element, all other elements must shift left to fill the empty space.

What is the time complexity of deleting the first element in an array of size n?

A O(1)

B O(log n)

C O(n)

D O(n^2)

Deleting the first element requires shifting all other n-1 elements, so time complexity is O(n).

If the array is [5, 10, 15], what is the array after deleting the first element?

A []

B [5, 10, 15]

C [15, 10]

D [10, 15]

The first element 5 is removed, and the rest shift left, resulting in [10, 15].

Why can't we just mark the first element as deleted without shifting?

A Because arrays require continuous memory without gaps

B Because marking is not allowed in arrays

C Because shifting is faster

D Because arrays are linked lists

Arrays store elements in continuous memory, so gaps are not allowed; shifting maintains continuity.

What happens to the size of the array after deleting the first element?

A It decreases by one logically

B It increases by one

C It stays the same

D It doubles

Logically, the array size decreases by one because one element is removed.

Explain step-by-step how to delete the first element of an array and update the array.

Think about what happens to each element after deletion.

Describe the time complexity of deleting the first element in an array and why it is that way.

Consider how many elements move after deletion.

Practice

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

easy

A. All elements shift one position to the right, and the last element is removed.

B. All elements shift one position to the left, and the first element is removed.

C. The array size increases by one.

D. The array remains unchanged.

Solution

1. Step 1: Understand deletion at beginning

   Deleting the first element means removing the element at index 0.

2. Step 2: Effect on remaining elements

   All other elements move one position left to fill the gap.

3. Final Answer:

   All elements shift one position to the left, and the first element is removed. -> Option B

4. Quick Check:

   Deletion at start shifts elements left [OK]

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

Common Mistakes:

• Thinking elements shift right instead of left
• Assuming array size increases
• Believing array stays unchanged

2. Which of the following C code snippets correctly deletes the first element of an integer array arr of size n by shifting elements left?

easy

A. for(int i = 0; i < n; i++) { arr[i] = arr[i]; } n--;

B. for(int i = n - 1; i > 0; i--) { arr[i] = arr[i - 1]; } n++;

C. for(int i = 0; i < n - 1; i++) { arr[i] = arr[i + 1]; } n--;

D. for(int i = 1; i < n; i++) { arr[i] = arr[i - 1]; } n--;

Solution

1. Step 1: Identify correct loop for shifting left

   The loop must start from index 0 and assign arr[i] = arr[i+1] up to n-2.

2. Step 2: Check array size update

   After shifting, size decreases by 1 (n--).

3. Final Answer:

   for(int i = 0; i < n - 1; i++) { arr[i] = arr[i + 1]; } n--; -> Option C

4. Quick Check:

   Shift left from 0 to n-2, then reduce size [OK]

Hint: Shift elements left from 0 to n-2, then decrease size [OK]

Common Mistakes:

• Looping backwards instead of forwards
• Increasing size instead of decreasing
• Incorrect loop bounds causing overwrite

3. What is the output array after deleting the first element from int arr[] = {10, 20, 30, 40, 50}; using the correct deletion method?

medium

A. [50, 40, 30, 20]

B. [10, 20, 30, 40]

C. [30, 40, 50]

D. [20, 30, 40, 50]

Solution

1. Step 1: Remove first element 10

   Deleting first element removes 10 from the array.

2. Step 2: Shift remaining elements left

   Elements 20, 30, 40, 50 shift left to fill index 0 to 3.

3. Final Answer:

   [20, 30, 40, 50] -> Option D

4. Quick Check:

   First element removed, others shifted left [OK]

Hint: After deletion, array starts from second element [OK]

Common Mistakes:

• Leaving original first element in array
• Removing last element instead
• Reversing array order

4. Consider this code snippet for deleting the first element of an array:

for(int i = 1; i < n; i++) { arr[i-1] = arr[i]; } n--;

What is the error in this code?

medium

A. The loop correctly shifts elements; no error present.

B. The loop should start from i = 0, not 1.

C. The array size should be increased, not decreased.

D. The assignment should be arr[i] = arr[i-1], not arr[i-1] = arr[i].

Solution

1. Step 1: Analyze loop start and assignments

   The loop starts at i=1 and assigns arr[i-1] = arr[i], shifting elements left correctly.

2. Step 2: Check array size update

   Decreasing n by 1 after shifting is correct to reflect deletion.

3. Final Answer:

   The loop correctly shifts elements; no error present. -> Option A

4. Quick Check:

   Loop from 1 with arr[i-1]=arr[i] shifts left [OK]

Hint: Loop from 1 with arr[i-1]=arr[i] shifts left correctly [OK]

Common Mistakes:

• Thinking loop must start at 0 for shifting left
• Confusing assignment direction
• Incorrectly changing array size

5. You have an array int arr[] = {5, 10, 15, 20, 25, 30}; and size n = 6. You delete the first element twice using the correct deletion method. What is the resulting array and size?

hard

A. Array: [15, 20, 25, 30], Size: 4

B. Array: [10, 15, 20, 25, 30], Size: 5

C. Array: [5, 10, 15, 20], Size: 4

D. Array: [20, 25, 30], Size: 3

Solution

1. Step 1: First deletion removes 5

   After first deletion, array becomes [10, 15, 20, 25, 30], size 5.

2. Step 2: Second deletion removes 10

   After second deletion, array becomes [15, 20, 25, 30], size 4.

3. Final Answer:

   Array: [15, 20, 25, 30], Size: 4 -> Option A

4. Quick Check:

   Two deletions remove first two elements, size reduces by 2 [OK]

Hint: Each deletion removes first element and reduces size by one [OK]

Common Mistakes:

• Not updating size after each deletion
• Deleting wrong elements
• Confusing array indexes after shifts