Array Deletion at End in DSA Java - Time & Space Complexity

Time Complexity: Array Deletion at End

O(1)

Understanding Time Complexity

We want to understand how long it takes to remove an item from the end of an array.

How does the time needed change as the array gets bigger?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

// Remove last element from array
int[] arr = {1, 2, 3, 4, 5};
int size = arr.length;

// Deleting last element
size = size - 1;
// Now size is 4, last element ignored
    

This code reduces the size count to ignore the last element, effectively deleting it.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Single step to reduce size by one.
• How many times: Exactly once per deletion.

How Execution Grows With Input

Removing the last element takes the same small step no matter how big the array is.

Input Size (n)	Approx. Operations
10	1
100	1
1000	1

Pattern observation: The work stays the same even if the array grows larger.

Final Time Complexity

Time Complexity: O(1)

This means deleting the last element takes a constant amount of time regardless of array size.

Common Mistake

[X] Wrong: "Deleting an element always takes time proportional to the array size because we have to move elements."

[OK] Correct: When deleting at the end, no elements need to be moved; we just reduce the size count, so it takes constant time.

Interview Connect

Knowing that deleting at the end is fast helps you choose the right data structure and operation in real problems.

Self-Check

"What if we changed deletion to remove the first element instead? How would the time complexity change?"

Practice

1. What happens when you delete an element from the end of a fixed-size array in Java?

easy

A. The last element is removed and the array size decreases by one

B. The last element is set to zero or null but the array size stays the same

C. All elements shift left by one position

D. The array is completely cleared

Solution

1. Step 1: Understand fixed-size array behavior

   Java arrays have a fixed size once created, so their length cannot change.

2. Step 2: Effect of deleting at end

   Deleting means logically ignoring the last element or setting it to default (0 or null), but the array length remains unchanged.

3. Final Answer:

   The last element is set to zero or null but the array size stays the same -> Option B

4. Quick Check:

   Fixed-size array length unchanged on deletion [OK]

Hint: Fixed arrays keep size; deletion just clears last element [OK]

Common Mistakes:

• Thinking array size shrinks automatically
• Assuming elements shift after deletion
• Believing array clears fully on one deletion

2. Which of the following is the correct syntax to delete the last element from an array named arr in Java by reducing its logical size stored in size?

easy

A. arr[size--] = 0;

B. arr[size] = 0; size--;

C. size--; arr[size] = 0;

D. arr[size] = null; size++;

Solution

1. Step 1: Understand logical deletion

   To delete last element, first reduce size by one, then clear that element.

2. Step 2: Check syntax correctness

   size--; arr[size] = 0; reduces size first (size--) then sets arr[size] to 0, correctly deleting last element.

3. Final Answer:

   size--; arr[size] = 0; -> Option C

4. Quick Check:

   Decrease size before clearing element [OK]

Hint: Decrease size first, then clear element [OK]

Common Mistakes:

• Clearing element before decreasing size
• Using post-decrement incorrectly
• Increasing size instead of decreasing

3. What will be the output of the following Java code?

int[] arr = {10, 20, 30, 40, 50};
int size = 5;
size--;
arr[size] = 0;
for(int i = 0; i < size; i++) {
    System.out.print(arr[i] + " ");
}

medium

A. 10 20 30 0 0

B. 10 20 30 40 0

C. 10 20 30 40 50

D. 10 20 30 40

Solution

1. Step 1: Understand size decrement and element clearing

   Size is reduced from 5 to 4, then arr[4] is set to 0, removing last element logically.

2. Step 2: Loop prints elements from index 0 to 3

   Loop runs i=0 to 3, printing arr[0]=10, arr[1]=20, arr[2]=30, arr[3]=40.

3. Final Answer:

   10 20 30 40 -> Option D

4. Quick Check:

   Last element removed, loop prints first 4 elements [OK]

Hint: Loop runs till size, last element skipped after decrement [OK]

Common Mistakes:

• Printing all 5 elements ignoring size
• Not updating last element to zero
• Confusing array index with size

4. Identify the error in this Java code snippet that tries to delete the last element from an array:

int[] arr = {1, 2, 3, 4};
int size = 4;
arr[size] = 0;
size--;

medium

A. Array index out of bounds error at arr[size] = 0;

B. Size variable is not decremented

C. Array elements are not shifted left

D. No error, code works fine

Solution

1. Step 1: Check array indexing

   Array indices go from 0 to size-1, so arr[size] is out of bounds.

2. Step 2: Identify correct order

   Must decrement size first, then access arr[size] to avoid error.

3. Final Answer:

   Array index out of bounds error at arr[size] = 0; -> Option A

4. Quick Check:

   Accessing arr[size] causes error [OK]

Hint: Decrement size before accessing arr[size] [OK]

Common Mistakes:

• Accessing arr[size] directly
• Not decrementing size before deletion
• Ignoring array index limits

5. You have an array int[] arr = {5, 10, 15, 20, 25} and a variable size = 5. You want to logically delete the last two elements without modifying the values in the array and print the remaining elements. Which code snippet correctly does this?

hard

A. size -= 2;
for(int i = 0; i < size; i++) System.out.print(arr[i] + " ");

B. arr[size-1] = 0; arr[size-2] = 0; size -= 2;
for(int i = 0; i < size; i++) System.out.print(arr[i] + " ");

C. size -= 2;
arr[size] = 0; arr[size+1] = 0;
for(int i = 0; i < size; i++) System.out.print(arr[i] + " ");

D. for(int i = 0; i < size-2; i++) System.out.print(arr[i] + " ");

Solution

1. Step 1: Reduce logical size by 2

   Decreasing size by 2 means last two elements are ignored logically.

2. Step 2: Print elements up to new size

   Loop from 0 to size-1 prints remaining elements correctly.

3. Final Answer:

   size -= 2; for(int i = 0; i < size; i++) System.out.print(arr[i] + " "); -> Option A

4. Quick Check:

   Logical size controls printed elements [OK]

Hint: Reduce size, then print up to size [OK]

Common Mistakes:

• Setting elements to zero before reducing size
• Printing without adjusting size
• Accessing invalid indices after deletion