Stack vs Array Direct Use Why We Need Stack Abstraction in DSA Javascript - Interactive Comparison Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add an element to the top of the stack using array push method.

DSA Javascript

stack.[1](10);

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

Ashift

Bpop

Cpush

Dunshift

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop instead of push

Using shift or unshift which affect the start of the array

The push method adds an element to the end of the array, which represents the top of the stack.

2fill in blank

medium

Complete the code to remove the top element from the stack using array method.

DSA Javascript

let top = stack.[1]();

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

Apop

Bpush

Cshift

Dunshift

Attempts:

3 left

💡 Hint

Common Mistakes

Using push instead of pop

Using shift or unshift which remove from the start

The pop method removes and returns the last element of the array, which is the top of the stack.

3fill in blank

hard

Fix the error in the code that tries to access the top element of the stack without removing it.

DSA Javascript

let top = stack[1];

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

Astack.length - 1

B[stack.length - 1]

Cstack.pop()

Dstack.length

Attempts:

3 left

💡 Hint

Common Mistakes

Using stack.length - 1 directly

Using pop which removes the element

To access the top element without removing it, use stack[stack.length - 1].

4fill in blank

hard

Fill both blanks to create a stack class method that pushes an element and returns the new size.

DSA Javascript

class Stack {
  constructor() {
    this.items = [];
  }
  push(element) {
    this.items.[1](element);
    return this.items.[2];
  }
}

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

Apush

Bpop

Clength

Dsize

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop instead of push

Using size which is not a valid array property

The push method adds the element, and length returns the new size of the array.

5fill in blank

hard

Fill the two blanks to implement a stack pop method that removes and returns the top element safely.

DSA Javascript

pop() {
  if (this.items.[1] === 0) {
    return null;
  }
  return this.items.[2]();
}

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

Alength

Bpop

Csize

Dshift

Attempts:

3 left

💡 Hint

Common Mistakes

Using size which is not valid

Using shift which removes from the start

Check if length is zero to avoid errors, then use pop to remove and return the top element.

Practice

1. Why do we use a stack abstraction instead of directly using an array for stack operations?

easy

A. Because stacks use less memory than arrays

B. Because arrays cannot store multiple elements

C. To improve the speed of accessing elements randomly

D. To restrict operations to only push and pop, preventing misuse

Solution

1. Step 1: Understand stack operations

   A stack allows only push (add) and pop (remove) operations at one end (top).

2. Step 2: Compare with array capabilities

   Arrays allow many operations like random access, insertion, and deletion anywhere, which can lead to misuse when used as stacks.

3. Final Answer:

   To restrict operations to only push and pop, preventing misuse -> Option D

4. Quick Check:

   Stack abstraction limits operations = D [OK]

Hint: Stack abstraction limits operations to push/pop only [OK]

Common Mistakes:

• Thinking arrays cannot store multiple elements
• Believing stacks improve random access speed
• Assuming stacks use less memory than arrays

2. Which of the following is the correct way to push an element '5' onto a stack implemented using an array named stack in JavaScript?

easy

A. stack.insert(5);

B. stack.push(5);

C. stack.add(5);

D. stack.append(5);

Solution

1. Step 1: Recall JavaScript array methods

   The method to add an element at the end of an array is push().

2. Step 2: Match method to stack push operation

   Stack push means adding an element on top, which corresponds to stack.push(5);.

3. Final Answer:

   stack.push(5); -> Option B

4. Quick Check:

   Use push() to add element in array stack = C [OK]

Hint: Use push() to add elements to array stack [OK]

Common Mistakes:

• Using non-existent methods like add(), insert(), or append()
• Confusing push() with pop()
• Trying to use array indexing to add elements

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

const stack = [];
stack.push(10);
stack.push(20);
const popped = stack.pop();
console.log(popped);
console.log(stack);

medium

A. 10
[20]

B. 20
[20, 10]

C. 20
[10]

D. 10
[10]

Solution

1. Step 1: Trace push operations

   stack.push(10) adds 10: stack = [10]; stack.push(20) adds 20: stack = [10, 20]

2. Step 2: Trace pop operation and console logs

   stack.pop() removes and returns 20; popped = 20; stack now [10]. console.log(popped) prints 20; console.log(stack) prints [10].

3. Final Answer:

   20 [10] -> Option C

4. Quick Check:

   pop returns last pushed = 20, stack left with [10] [OK]

Hint: pop() returns last pushed element, stack shrinks [OK]

Common Mistakes:

• Confusing pop() return value with stack after pop
• Expecting stack to remain unchanged after pop
• Mixing order of elements in stack

4. Identify the error in the following code that tries to implement a stack using an array:

const stack = [];
stack.push(1);
stack.push(2);
const top = stack.pop;
console.log(top);

medium

A. pop is used without parentheses, so it returns the function, not the popped value

B. push method is incorrect, should be pushElement

C. stack array is not initialized properly

D. console.log cannot print variables

Solution

1. Step 1: Check pop usage

   pop is a function and must be called with parentheses to execute: stack.pop() returns the popped element.

2. Step 2: Analyze the code behavior

   stack.pop without () returns the function itself, so console.log(top) prints the function, not the value.

3. Final Answer:

   pop is used without parentheses, so it returns the function, not the popped value -> Option A

4. Quick Check:

   Call pop() with () to get value [OK]

Hint: Always call pop() with parentheses to execute [OK]

Common Mistakes:

• Forgetting parentheses when calling pop()
• Thinking push method name is wrong
• Assuming console.log cannot print variables

5. You want to implement a stack that only allows pushing even numbers and popping normally. Which approach best uses stack abstraction to enforce this rule?

hard

A. Create a Stack class with push method that only adds even numbers

B. Use an array directly and check even numbers before push externally

C. Use array and filter out odd numbers after pushing

D. Use a queue instead of a stack

Solution

1. Step 1: Understand the requirement

   We want to restrict push operation to only even numbers, enforcing a rule inside the stack.

2. Step 2: Compare approaches

   Using a Stack class with a custom push method enforces the rule internally, preventing misuse. External checks or filtering after push are less safe. Using a queue is unrelated.

3. Final Answer:

   Create a Stack class with push method that only adds even numbers -> Option A

4. Quick Check:

   Stack abstraction enforces rules internally = A [OK]

Hint: Use class with controlled push method to enforce rules [OK]

Common Mistakes:

• Relying on external checks instead of internal enforcement
• Filtering after push allows invalid data temporarily
• Confusing stack with queue