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

Recall & Review

beginner

What does 'inserting at the beginning of an array' mean?

It means adding a new element at the first position (index 0) of the array, shifting all other elements one position to the right.

beginner

Why do we need to shift elements when inserting at the beginning of an array?

Because arrays have fixed positions, to insert at the start, all existing elements must move one step right to make space for the new element.

intermediate

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

The time complexity is O(n), where n is the number of elements, because all elements must be shifted one position to the right.

beginner

Show the array after inserting 5 at the beginning of [10, 20, 30].

The new array will be [5, 10, 20, 30]. The existing elements shift right to make space.

beginner

What Python list method can insert an element at the beginning?

The list method insert(0, value) inserts 'value' at index 0, which is the beginning of the list.

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

A They get deleted

B They shift one position to the left

C They stay in the same place

D They shift one position to the right

When inserting at the beginning, all elements move right to make space for the new element.

Which index is used to insert an element at the beginning of a Python list?

A Index 0

B Index 1

C Index -1

D Index length of list

Index 0 is the first position in a list, so inserting at index 0 adds the element at the beginning.

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

A O(log n)

B O(1)

C O(n)

D O(n^2)

Because all elements must shift, the operation takes linear time proportional to the number of elements.

Which Python list method inserts an element at a specific position?

A append()

B insert()

C extend()

D pop()

The insert() method adds an element at a given index, shifting others as needed.

After inserting 7 at the beginning of [3, 4, 5], what is the new array?

A [7, 3, 4, 5]

B [3, 7, 4, 5]

C [3, 4, 5, 7]

D [4, 5, 7, 3]

Inserting 7 at the start places it at index 0, pushing others right.

Explain step-by-step how to insert an element at the beginning of an array.

Think about moving elements to make space.

Describe the impact on performance when inserting at the beginning of a large array.

Consider how many elements move.

Practice

1. What does inserting an element at the beginning of an array do to the existing elements?

easy

A. Shifts all existing elements one position to the right

B. Deletes the last element of the array

C. Replaces the first element with the new element

D. Leaves the array unchanged

Solution

1. Step 1: Understand array insertion at beginning

   When we insert at the start, the new element takes index 0.

2. Step 2: Effect on existing elements

   All existing elements move one position right to make space.

3. Final Answer:

   Shifts all existing elements one position to the right -> Option A

4. Quick Check:

   Insert at start = shift right [OK]

Hint: New start means shift all right by one [OK]

Common Mistakes:

• Thinking insertion replaces first element
• Assuming array size shrinks
• Believing array stays unchanged

2. Which of the following Python statements correctly inserts the value 10 at the beginning of list arr?

easy

A. arr.add(10)

B. arr.append(10)

C. arr[0] = 10

D. arr.insert(0, 10)

Solution

1. Step 1: Identify correct method for insertion at index

   Python list method insert(index, value) inserts at given index.

2. Step 2: Check each option

   arr.insert(0, 10) uses insert(0, 10) which inserts 10 at start. Others either append at end, replace element, or invalid method.

3. Final Answer:

   arr.insert(0, 10) -> Option D

4. Quick Check:

   Use insert(0, value) to add at start [OK]

Hint: Use insert(0, value) to add at beginning [OK]

Common Mistakes:

• Using append() adds at end, not start
• Assigning arr[0] replaces element, no shift
• Using non-existent method add()

3. What is the output of the following code?

arr = [2, 3, 4]
arr.insert(0, 1)
print(arr)

medium

A. [1, 3, 4]

B. [2, 3, 4, 1]

C. [1, 2, 3, 4]

D. [2, 3, 4]

Solution

1. Step 1: Initial array and insertion

   Original array is [2, 3, 4]. Using insert(0, 1) adds 1 at index 0.

2. Step 2: Resulting array after insertion

   All elements shift right. New array is [1, 2, 3, 4].

3. Final Answer:

   [1, 2, 3, 4] -> Option C

4. Quick Check:

   Insert at start shifts elements right [OK]

Hint: Insert at 0 puts element first, shifts others [OK]

Common Mistakes:

• Thinking insert adds at end
• Replacing first element instead of shifting
• Ignoring shift effect on elements

4. Find the error in the code below that tries to insert 5 at the beginning of the list nums:

nums = [10, 20, 30]
nums.add(5)
print(nums)

medium

A. Method add() does not exist for lists

B. The list nums is empty

C. The print statement is incorrect

D. The value 5 cannot be inserted into a list

Solution

1. Step 1: Check method used for insertion

   Python lists do not have an add() method; this causes an error.

2. Step 2: Correct method to insert at beginning

   Use insert(0, 5) to add 5 at start of list.

3. Final Answer:

   Method add() does not exist for lists -> Option A

4. Quick Check:

   Use insert() not add() for list insertion [OK]

Hint: Use insert() for lists, add() is invalid [OK]

Common Mistakes:

• Assuming add() works like append()
• Thinking print statement causes error
• Believing list cannot hold new values

5. Given the list data = [4, 5, 6], which code snippet correctly inserts the elements of another list new_items = [1, 2, 3] at the beginning so that the final list is [1, 2, 3, 4, 5, 6]?

hard

A. for item in new_items:
data.insert(0, item)

B. for item in reversed(new_items):
data.insert(0, item)

C. data.extend(new_items)

D. data.append(new_items)

Solution

1. Step 1: Understand insertion order impact

   Inserting each item at index 0 shifts existing elements right. To keep order, insert reversed list.

2. Step 2: Analyze each option

   for item in new_items: data.insert(0, item) inserts 1, then 2 at front, reversing order. for item in reversed(new_items): data.insert(0, item) inserts reversed list, preserving order. Options C and D add at end or nested list.

3. Final Answer:

   for item in reversed(new_items): data.insert(0, item) -> Option B

4. Quick Check:

   Insert reversed to keep order at start [OK]

Hint: Insert reversed list at 0 to keep order [OK]

Common Mistakes:

• Inserting forward order reverses new items
• Using extend() adds at end, not start
• Appending list nests it inside