Pop Operation on Stack in DSA Java - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to pop the top element from the stack.

DSA Java

int pop() {
    if (isEmpty()) {
        throw new EmptyStackException();
    }
    return stackArray[[1]--];
}

Drag options to blanks, or click blank then click option'

A0

Btop - 1

Ctop + 1

Dtop

Attempts:

3 left

💡 Hint

Common Mistakes

Using top - 1 returns the wrong element.

Using top + 1 causes index out of bounds.

The pop operation returns the element at the current top index and then decrements the top pointer.

2fill in blank

medium

Complete the code to check if the stack is empty before popping.

DSA Java

boolean isEmpty() {
    return [1] == -1;
}

Drag options to blanks, or click blank then click option'

AstackArray.length

Btop

C0

DstackArray[0]

Attempts:

3 left

💡 Hint

Common Mistakes

Checking if top == 0 instead of -1.

Using stackArray.length which is unrelated to emptiness.

The stack is empty when the top index is -1, meaning no elements are present.

3fill in blank

hard

Fix the error in the pop method to correctly update the top index.

DSA Java

int pop() {
    if (isEmpty()) {
        throw new EmptyStackException();
    }
    return stackArray[[1]];
    top--;
}

Drag options to blanks, or click blank then click option'

Atop

B--top

Ctop--

Dtop + 1

Attempts:

3 left

💡 Hint

Common Mistakes

Using pre-decrement causes the wrong element to be returned.

Not decrementing top at all.

Use post-decrement (top--) to return the element at the current top index and then decrement the top pointer.

4fill in blank

hard

Fill both blanks to pop the top element and update the top index correctly.

DSA Java

int pop() {
    if (isEmpty()) {
        throw new EmptyStackException();
    }
    int poppedElement = stackArray[[1]];
    [2];
    return poppedElement;
}

Drag options to blanks, or click blank then click option'

Atop

Btop--

C--top

Dtop = top - 1

Attempts:

3 left

💡 Hint

Common Mistakes

Decrementing top before accessing the element.

Not updating top after popping.

First, access the element at the current top index, then decrement the top index to update the stack.

5fill in blank

hard

Fill all three blanks to implement pop with empty check, element retrieval, and top update.

DSA Java

int pop() {
    if ([1]()) {
        throw new EmptyStackException();
    }
    int popped = stackArray[[2]];
    [3];
    return popped;
}

Drag options to blanks, or click blank then click option'

AisEmpty

Btop

Ctop--

DisFull

Attempts:

3 left

💡 Hint

Common Mistakes

Using isFull() instead of isEmpty() for the check.

Not decrementing top after popping.

Check if the stack is empty using isEmpty(), then get the element at top, and finally decrement top.

Practice

1. What does the pop() operation do in a stack?

easy

A. Removes and returns the top element of the stack

B. Adds an element to the bottom of the stack

C. Returns the bottom element without removing it

D. Checks if the stack is empty

Solution

1. Step 1: Understand the stack structure

   A stack follows Last In First Out (LIFO) principle, so operations affect the top element.

2. Step 2: Define pop operation

   The pop() removes and returns the top element from the stack.

3. Final Answer:

   Removes and returns the top element of the stack -> Option A

4. Quick Check:

   pop() removes top element [OK]

Hint: Pop always removes the last added item from stack [OK]

Common Mistakes:

• Thinking pop adds elements
• Confusing pop with peek
• Assuming pop removes bottom element

2. Which of the following is the correct syntax to pop an element from a stack named stack in Java?

easy

A. stack.pop();

B. pop(stack);

C. stack.remove();

D. pop();

Solution

1. Step 1: Recall Java Stack method syntax

   In Java, pop() is a method called on the stack object.

2. Step 2: Identify correct method call

   The correct syntax is calling pop() on the stack instance: stack.pop();

3. Final Answer:

   stack.pop(); -> Option A

4. Quick Check:

   Method call on object = stack.pop() [OK]

Hint: Pop is called as a method on the stack object [OK]

Common Mistakes:

• Using pop() without object
• Using remove() instead of pop()
• Passing stack as argument to pop

3. What will be the output after executing the following Java code?

Stack<Integer> stack = new Stack<>();
stack.push(10);
stack.push(20);
stack.push(30);
int popped = stack.pop();
System.out.println(popped);
System.out.println(stack);

medium

A. 30
[20, 10]

B. 10
[20, 30]

C. 20
[10, 30]

D. 30
[10, 20]

Solution

1. Step 1: Trace push operations

   Elements pushed in order: 10, then 20, then 30. Stack top is 30.

2. Step 2: Apply pop operation

   Pop removes top element 30, so popped = 30. Remaining stack is [10, 20].

3. Final Answer:

   30 [10, 20] -> Option D

4. Quick Check:

   pop() returns last pushed = 30 [OK]

Hint: Pop returns last pushed element, stack shrinks from top [OK]

Common Mistakes:

• Confusing order of elements after pop
• Printing popped element incorrectly
• Assuming pop removes bottom element

4. Consider this Java code snippet:

Stack<String> stack = new Stack<>();
stack.push("A");
stack.push("B");
stack.pop();
stack.pop();
stack.pop();

What error will occur when running this code?

medium

A. No error, code runs fine

B. EmptyStackException at third pop

C. NullPointerException at second pop

D. IndexOutOfBoundsException at first pop

Solution

1. Step 1: Count push and pop calls

   Two elements pushed: "A", "B". Then three pops called.

2. Step 2: Identify error on extra pop

   First two pops remove "B" and "A". Third pop tries to remove from empty stack causing EmptyStackException.

3. Final Answer:

   EmptyStackException at third pop -> Option B

4. Quick Check:

   Pop on empty stack throws EmptyStackException [OK]

Hint: Popping empty stack causes EmptyStackException [OK]

Common Mistakes:

• Assuming pop returns null on empty stack
• Confusing exception types
• Ignoring extra pop call

5. You have a stack with elements [5, 10, 15, 20] (5 at bottom, 20 at top). You perform two pop operations, then push 25, then pop once. What is the final stack content?

hard

A. [5, 10, 25]

B. [5, 10, 15]

C. [5, 10]

D. [5, 10, 15, 25]

Solution

1. Step 1: Perform two pop operations

   Initial stack: bottom 5,10,15, top 20. First pop removes 20, second pop removes 15. Stack now: [5, 10].

2. Step 2: Push 25 and pop once

   Push 25: stack becomes [5, 10, 25]. Pop removes 25, stack returns to [5, 10].

3. Step 3: Verify final stack

   Final stack is [5, 10].

4. Final Answer:

   [5, 10] -> Option C

5. Quick Check:

   Pop removes top, push adds top [OK]

Hint: Track pushes and pops stepwise to find final stack [OK]

Common Mistakes:

• Forgetting last pop removes pushed element
• Mixing order of elements
• Assuming push adds at bottom