Array Insertion at Beginning in DSA Python - Execution Trace

Concept Flow - Array Insertion at Beginning

Start with array

↓

Create space at index 0

↓

Shift all elements right by 1

↓

Insert new element at index 0

↓

Array updated with new element at start

We start with an array, shift all elements one step right to make space, then insert the new element at the beginning.

Execution Sample

DSA Python

arr = [10, 20, 30, None]
new_element = 5
for i in range(len(arr) - 1, 0, -1):
    arr[i] = arr[i-1]
arr[0] = new_element
print(arr)

This code inserts 5 at the beginning of the array by shifting elements right.

Execution Table

Step	Operation	Index i	Element Shifted	Array State	Pointer Changes	Visual State
1	Start	-	-	[10, 20, 30, _]	head=0	10 -> 20 -> 30 -> _
2	Shift element right	3	-	[10, 20, 30, 30]	arr[3]=arr[2]	10 -> 20 -> 30 -> 30
3	Shift element right	2	30	[10, 20, 20, 30]	arr[2]=arr[1]	10 -> 20 -> 20 -> 30
4	Shift element right	1	20	[10, 10, 20, 30]	arr[1]=arr[0]	10 -> 10 -> 20 -> 30
5	Insert new element	0	-	[5, 10, 20, 30]	arr[0]=5	5 -> 10 -> 20 -> 30
6	End	-	-	[5, 10, 20, 30]	-	5 -> 10 -> 20 -> 30

💡 All elements shifted right, new element inserted at index 0, array updated.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	Final
arr	[10, 20, 30, _]	[10, 20, 30, 30]	[10, 20, 20, 30]	[10, 10, 20, 30]	[5, 10, 20, 30]	[5, 10, 20, 30]
i	-	3	2	1	0	-
new_element	5	5	5	5	5	5

Key Moments - 3 Insights

Why do we shift elements from the end towards the beginning instead of the other way?

Shifting from the end (step 2 to 4) prevents overwriting elements before they are moved, as shown in execution_table rows 2-4.

What happens if the array has no extra space for the new element?

The code assumes space at the end (index 3). Without space, shifting would overwrite data or cause errors, so array resizing is needed before insertion.

Why do we insert the new element at index 0 after shifting?

After shifting all elements right, index 0 is free to place the new element safely, as shown in step 5 of the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what is the array state after shifting the element at index 2?

A [10, 20, 30, 30]

B [10, 10, 20, 30]

C [10, 20, 20, 30]

D [5, 10, 20, 30]

💡 Hint

Check row 3 in the execution_table under 'Array State'.

At which step does the new element get inserted at the beginning?

A Step 2

B Step 5

C Step 4

D Step 6

💡 Hint

Look for 'Insert new element' operation in the execution_table.

If we did not shift elements from the end, what would happen to the original first element?

A It would be overwritten and lost

B It would remain unchanged

C It would move to the end automatically

D It would duplicate

💡 Hint

Refer to key_moments explanation about shifting direction.

Concept Snapshot

Array Insertion at Beginning:
- Shift all elements right starting from the last index
- Insert new element at index 0
- Requires extra space at array end
- Prevents overwriting by shifting backwards
- Final array has new element at start

Full Transcript

This visual trace shows how to insert an element at the beginning of an array. We start with the array [10, 20, 30] and want to insert 5 at the start. To do this, we shift elements one by one from the end towards the beginning to make space. After shifting, the first position is free, so we place 5 there. The array becomes [5, 10, 20, 30]. The key is shifting from the end to avoid overwriting elements before moving them. This process assumes there is extra space at the end of the array to hold the shifted elements.