Array Deletion at Beginning in DSA Python - Execution Trace

Concept Flow - Array Deletion at Beginning

Start with array

↓

Delete first element

↓

Shift all elements left by 1

↓

Reduce array size by 1

↓

End with updated array

This flow shows deleting the first element of an array by removing it and shifting all other elements left to fill the gap.

Execution Sample

DSA Python

arr = [10, 20, 30, 40]
# Delete first element
for i in range(1, len(arr)):
    arr[i-1] = arr[i]
arr.pop()
print(arr)

This code deletes the first element of the array by shifting elements left and removing the last duplicate.

Execution Table

Step	Operation	Array State	Pointer Changes	Visual State
0	Start	[10, 20, 30, 40]	i = None	10 -> 20 -> 30 -> 40 -> null
1	i=1: arr[0] = arr[1]	[20, 20, 30, 40]	i=1	20 -> 20 -> 30 -> 40 -> null
2	i=2: arr[1] = arr[2]	[20, 30, 30, 40]	i=2	20 -> 30 -> 30 -> 40 -> null
3	i=3: arr[2] = arr[3]	[20, 30, 40, 40]	i=3	20 -> 30 -> 40 -> 40 -> null
4	pop last element	[20, 30, 40]	i=None	20 -> 30 -> 40 -> null
5	End	[20, 30, 40]	i=None	20 -> 30 -> 40 -> null

💡 All elements shifted left, last duplicate removed, array size reduced by 1

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
arr	[10, 20, 30, 40]	[20, 20, 30, 40]	[20, 30, 30, 40]	[20, 30, 40, 40]	[20, 30, 40]	[20, 30, 40]
i	None	1	2	3	None	None

Key Moments - 3 Insights

Why do we shift elements left instead of just removing the first element?

Because arrays have fixed positions, removing the first element leaves a gap. Shifting left fills that gap, as shown in steps 1 to 3 in the execution_table.

Why do we pop the last element after shifting?

After shifting, the last element is duplicated. Popping removes this duplicate, reducing the array size, as shown in step 4.

What happens if the array is empty before deletion?

The loop does not run and pop would cause an error. We must check array size before deletion to avoid this, which is not shown here but important.

Visual Quiz - 3 Questions

Look at the execution_table, what is the array state after step 2?

A [20, 20, 30, 40]

B [20, 30, 30, 40]

C [20, 30, 40, 40]

D [10, 20, 30, 40]

💡 Hint

Check the 'Array State' column at step 2 in the execution_table.

At which step is the last element removed from the array?

A Step 4

B Step 5

C Step 3

D Step 1

💡 Hint

Look for the 'pop last element' operation in the execution_table.

If the initial array was [5, 15, 25], what would be the final array after deletion?

A [5, 15]

B [25]

C [15, 25]

D [5, 15, 25]

💡 Hint

Deletion removes first element and shifts left, check variable_tracker logic.

Concept Snapshot

Array Deletion at Beginning:
- Remove first element by shifting all elements left
- Last element becomes duplicate after shift
- Remove last element to reduce size
- Array size decreases by 1
- Must check array not empty before deletion

Full Transcript

This concept shows how to delete the first element of an array. We start with the full array. Then, we shift every element one position to the left to fill the gap left by the removed element. This causes the last element to be duplicated. To fix this, we remove the last element, reducing the array size by one. The final array no longer contains the original first element. This process is shown step-by-step in the execution table and variable tracker. Beginners often wonder why shifting is needed and why the last element is removed. Also, care must be taken if the array is empty before deletion.

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