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

Practice - 5 Tasks

1fill in blank

easy

Complete the code to push an element onto the stack using array direct use.

DSA Go

stack := []int{1, 2, 3}
stack = append(stack, [1])
fmt.Println(stack)

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

A4

B5

Cappend

Dstack

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'append' as the element instead of a value.

Trying to append the whole stack again.

Using a number not intended to be pushed.

To add an element to the stack (array), we use append with the new value, here 4.

2fill in blank

medium

Complete the code to pop the top element from the stack using array direct use.

DSA Go

stack := []int{1, 2, 3, 4}
top := stack[len(stack)[1]]
stack = stack[:len(stack)-1]
fmt.Println(top, stack)

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A+1

B-2

C-1

D+2

Attempts:

3 left

💡 Hint

Common Mistakes

Using len(stack)+1 which causes index out of range error.

Using len(stack) which is out of bounds.

Using -2 which gets the wrong element.

To get the last element, we use len(stack)-1 as index because slices are zero-based.

3fill in blank

hard

Fix the error in the stack pop function that uses array direct use.

DSA Go

func pop(stack []int) ([]int, int) {
    if len(stack) == 0 {
        return stack, 0
    }
    top := stack[len(stack)[1]]
    stack = stack[:len(stack)-1]
    return stack, top
}

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A+1

B+2

C-2

D-1

Attempts:

3 left

💡 Hint

Common Mistakes

Using +1 causes index out of range error.

Using -2 gets wrong element.

Not checking for empty stack before pop.

The last element index is len(stack)-1, so use -1 to get the top element.

4fill in blank

hard

Fill both blanks to implement a stack push method using struct abstraction.

DSA Go

type Stack struct {
    items []int
}

func (s *Stack) Push([1] int) {
    s.items = append(s.items, [2])
}

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Avalue

Bval

Ditem

Attempts:

3 left

💡 Hint

Common Mistakes

Using different names for parameter and append argument.

Using undefined variable in append.

Not using pointer receiver for method.

The method takes a parameter named 'value' and appends it to the items slice.

5fill in blank

hard

Fill all three blanks to implement a stack pop method using struct abstraction.

DSA Go

func (s *Stack) Pop() (int, bool) {
    if len(s.items) == 0 {
        return 0, false
    }
    top := s.items[len(s.items)[1]]
    s.items = s.items[:len(s.items)[2]]
    return top, [3]
}

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

A-1

Ctrue

Dfalse

Attempts:

3 left

💡 Hint

Common Mistakes

Using +1 causes index error.

Returning false on successful pop.

Incorrect slicing removing wrong elements.

Use len(s.items)-1 to get last element and slice to remove it. Return true to indicate success.

Practice

1. Why do we prefer using a stack abstraction over directly using an array for stack operations?

easy

A. Stack abstraction hides implementation details and enforces LIFO behavior

B. Arrays are slower than stacks in all operations

C. Stacks use less memory than arrays

D. Arrays cannot store multiple data types

Solution

1. Step 1: Understand stack abstraction purpose

   Stack abstraction provides a clear interface for push/pop operations and hides how data is stored internally.

2. Step 2: Compare with direct array use

   Direct array use requires manual management of indexes and does not prevent misuse like accessing elements out of stack order.

3. Final Answer:

   Stack abstraction hides implementation details and enforces LIFO behavior -> Option A

4. Quick Check:

   Stack abstraction = hides details + LIFO [OK]

Hint: Stack abstraction controls access and order, unlike raw arrays [OK]

Common Mistakes:

• Thinking arrays are always slower than stacks
• Believing stacks use less memory than arrays
• Confusing data type storage capabilities

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

easy

A. stack = append(stack, element)

B. stack.push(element)

C. stack.add(element)

D. stack[element] = element

Solution

1. Step 1: Recall Go slice append syntax

   In Go, to add an element to a slice (array), use append(slice, element).

2. Step 2: Check other options

   Methods like push or add do not exist for slices; direct index assignment requires index, not element.

3. Final Answer:

   stack = append(stack, element) -> Option A

4. Quick Check:

   Go slice append = stack push [OK]

Hint: Use append(slice, val) to add elements in Go [OK]

Common Mistakes:

• Using push method which does not exist in Go slices
• Trying to assign element directly without index
• Confusing array with other language stack methods

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

stack := []int{}
stack = append(stack, 10)
stack = append(stack, 20)
last := stack[len(stack)-1]
stack = stack[:len(stack)-1]
fmt.Println(last, stack)

medium

A. 10 [20]

B. 20 [10 20]

C. 20 [10]

D. 10 []

Solution

1. Step 1: Trace stack after appends

   After appending 10 and 20, stack = [10, 20].

2. Step 2: Extract last element and pop it

   last = stack[1] = 20; stack sliced to stack[:1] = [10].

3. Final Answer:

   20 [10] -> Option C

4. Quick Check:

   Pop returns 20, stack left with 10 [OK]

Hint: Last element is popped, stack shrinks by one [OK]

Common Mistakes:

• Printing full stack without popping
• Confusing index for last element
• Not slicing stack to remove last element

4. Identify the error in this Go code that tries to pop an element from a stack implemented as a slice:

func pop(stack []int) int {
    val := stack[len(stack)]
    stack = stack[:len(stack)-1]
    return val
}

medium

A. Using wrong slice syntax for popping element

B. Missing return type in function signature

C. Stack slice is not updated outside the function

D. Index out of range when accessing stack[len(stack)]

Solution

1. Step 1: Check index used for accessing element

   stack[len(stack)] is out of range because valid indices are 0 to len(stack)-1.

2. Step 2: Confirm correct index for last element

   Last element index is len(stack)-1, so stack[len(stack)-1] is correct.

3. Final Answer:

   Index out of range when accessing stack[len(stack)] -> Option D

4. Quick Check:

   Index must be len(stack)-1, not len(stack) [OK]

Hint: Last element index = length minus one [OK]

Common Mistakes:

• Using len(stack) as index instead of len(stack)-1
• Ignoring slice update effect outside function
• Confusing slice syntax for popping

5. You want to implement a stack abstraction in Go that prevents direct access to the underlying array and only allows push and pop operations. Which approach best achieves this?

hard

A. Use a global slice variable and expose it directly for manipulation

B. Define a Stack struct with a private slice field and public Push/Pop methods

C. Use a map instead of a slice to store stack elements

D. Implement stack operations as standalone functions that take a slice parameter

Solution

1. Step 1: Understand encapsulation in Go

   Using a struct with private fields hides data and controls access via methods.

2. Step 2: Evaluate options for stack abstraction

   Define a Stack struct with a private slice field and public Push/Pop methods uses private slice and public methods, enforcing stack rules and hiding internals.

3. Final Answer:

   Define a Stack struct with a private slice field and public Push/Pop methods -> Option B

4. Quick Check:

   Encapsulation via struct + methods = safe stack [OK]

Hint: Use private fields and public methods for abstraction [OK]

Common Mistakes:

• Exposing slice directly allowing misuse
• Using map which is unordered, not stack-like
• Standalone functions can't hide data