Array Deletion at Beginning in DSA Python - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output after deleting the first element?

Consider the following Python code that deletes the first element from an array. What will be printed?

DSA Python

arr = [10, 20, 30, 40, 50]
del arr[0]
print(arr)

A [20, 30, 40, 50]

B IndexError

C [30, 40, 50]

D [10, 20, 30, 40, 50]

💡 Hint

Deleting the first element removes the element at index 0.

The del statement removes the element at index 0, so the array starts from the second element onward.

❓ Predict Output

intermediate

What is the output after popping the first element?

What will be the output of this code that removes the first element using pop?

DSA Python

arr = [5, 15, 25, 35]
removed = arr.pop(0)
print(arr)

A [5, 15, 25, 35]

B [15, 25, 35]

C [25, 35]

D IndexError

💡 Hint

pop(0) removes and returns the first element.

pop(0) removes the element at index 0, so the array starts from the second element.

🔧 Debug

advanced

Why does this code raise an error when deleting the first element?

Identify the error in this code and what error it raises.

DSA Python

arr = [1, 2, 3]
del arr[3]
print(arr)

A SyntaxError

B IndexError: list assignment index out of range

C IndexError: list deletion index out of range

D No error, prints [1, 2, 3]

💡 Hint

Index 3 does not exist in a list of length 3.

Deleting an element at an index outside the list length raises IndexError with message 'list deletion index out of range'.

❓ Predict Output

advanced

What is the output after removing the first element using slicing?

What will be printed after this slicing operation that removes the first element?

DSA Python

arr = [7, 14, 21, 28]
arr = arr[1:]
print(arr)

A [14, 21, 28]

B [7]

C [7, 14, 21, 28]

D TypeError

💡 Hint

Slicing from index 1 to end excludes the first element.

arr[1:] creates a new list starting from index 1 to the end, effectively removing the first element.

🧠 Conceptual

expert

What is the time complexity of deleting the first element in a Python list?

Consider a Python list with n elements. What is the time complexity of deleting the first element using del arr[0] or arr.pop(0)?

A O(1) - Constant time

B O(log n) - Logarithmic time

C O(n^2) - Quadratic time

D O(n) - Linear time

💡 Hint

Think about what happens to the rest of the elements after deletion.

Deleting the first element requires shifting all other elements one position left, which takes linear time O(n).

Practice

1. What happens to an array when you delete the first element in Python using del arr[0]?

easy

A. The first element is removed and the rest shift left by one position.

B. The last element is removed instead of the first.

C. The array becomes empty regardless of its size.

D. The array remains unchanged.

Solution

1. Step 1: Understand del arr[0] operation

   This command removes the element at index 0, which is the first element of the array.

2. Step 2: Effect on the array

   After deletion, all elements after the first shift one position to the left to fill the gap.

3. Final Answer:

   The first element is removed and the rest shift left by one position. -> Option A

4. Quick Check:

   Deleting first element shifts others left [OK]

Hint: Deleting index 0 removes first element and shifts left [OK]

Common Mistakes:

• Thinking last element is removed
• Assuming array becomes empty
• Believing array stays the same

2. Which of the following is the correct syntax to delete the first element of a list named arr in Python?

easy

A. del arr(0)

B. del arr[0]

C. arr.delete(0)

D. arr.remove(0)

Solution

1. Step 1: Check Python syntax for deleting by index

   Python uses del arr[index] to delete element at a specific index.

2. Step 2: Analyze options

   del arr[0] uses correct syntax del arr[0]. del arr(0) uses parentheses which is invalid. arr.delete(0) uses a method that does not exist. arr.remove(0) removes by value, not index.

3. Final Answer:

   del arr[0] -> Option B

4. Quick Check:

   Use del with square brackets for index deletion [OK]

Hint: Use del with square brackets to delete by index [OK]

Common Mistakes:

• Using parentheses instead of square brackets
• Confusing remove() with delete by index
• Trying to call a non-existent delete() method

3. What is the output of the following code?

arr = [10, 20, 30, 40]
del arr[0]
print(arr)

medium

A. [20, 30, 40]

B. [10, 20, 30]

C. [30, 40]

D. [10, 20, 30, 40]

Solution

1. Step 1: Delete first element from array

   The command del arr[0] removes the element 10 at index 0.

2. Step 2: Remaining elements shift left

   After deletion, the array becomes [20, 30, 40].

3. Final Answer:

   [20, 30, 40] -> Option A

4. Quick Check:

   First element removed, rest shifted left [OK]

Hint: Deleting index 0 removes first element, print rest [OK]

Common Mistakes:

• Expecting first element to remain
• Confusing deletion with removing last element
• Printing original array without deletion

4. Find the error in the code below that tries to delete the first element of the list arr:

arr = [5, 10, 15]
arr.delete(0)
print(arr)

medium

A. The print statement is incorrect.

B. The index 0 is out of range.

C. The list is empty, so deletion fails.

D. The method delete() does not exist for lists.

Solution

1. Step 1: Check list methods for deletion

   Python lists do not have a method called delete(). Deletion by index uses del keyword or pop() method.

2. Step 2: Identify correct deletion method

   To delete first element, use del arr[0] or arr.pop(0). Using arr.delete(0) causes AttributeError.

3. Final Answer:

   The method delete() does not exist for lists. -> Option D

4. Quick Check:

   Use del or pop, not delete() [OK]

Hint: Lists have no delete() method; use del or pop [OK]

Common Mistakes:

• Using delete() method on list
• Assuming delete() works like in other languages
• Ignoring AttributeError from wrong method

5. You have an array arr = [1, 2, 3, 4, 5]. You want to delete the first element and then add 0 at the beginning. Which code correctly does this in Python?

hard

A. arr.remove(0)
arr.insert(0, 0)
print(arr)

B. arr.pop(0)
arr.append(0)
print(arr)

C. del arr[0]
arr.insert(0, 0)
print(arr)

D. del arr[1]
arr.insert(0, 0)
print(arr)

Solution

1. Step 1: Delete first element correctly

   Using del arr[0] removes the first element (1) from the array.

2. Step 2: Insert 0 at the beginning

   Using arr.insert(0, 0) adds 0 at index 0, shifting others right.

3. Step 3: Check other options

   arr.pop(0) arr.append(0) print(arr) appends 0 at the end, not beginning. arr.remove(0) arr.insert(0, 0) print(arr) tries to remove value 0 which is not in array. del arr[1] arr.insert(0, 0) print(arr) deletes second element, not first.

4. Final Answer:

   del arr[0] arr.insert(0, 0) print(arr) -> Option C

5. Quick Check:

   Delete first, then insert at start [OK]

Hint: Delete index 0, then insert new element at index 0 [OK]

Common Mistakes:

• Appending instead of inserting at start
• Removing value 0 instead of index 0
• Deleting wrong index