Stack Implementation Using Array in DSA Go - Execution Trace

Concept Flow - Stack Implementation Using Array

Start

↓

Check if stack is full?

Yes→Cannot push, overflow

No↓

Add element at top index

↓

Increment top pointer

↓

Done Push

↓

Start

↓

Check if stack is empty?

Yes→Cannot pop, underflow

No↓

Return element at top

↓

Decrement top pointer

↓

Done Pop

This flow shows how elements are added (push) and removed (pop) from the stack using an array and a top pointer.

Execution Sample

DSA Go

stack := []int{}
stack = append(stack, 10) // push 10
stack = append(stack, 20) // push 20
popVal := stack[len(stack)-1] // pop
stack = stack[:len(stack)-1]

This code pushes two elements onto the stack and then pops one element from it.

Execution Table

Step	Operation	Top Index	Stack Elements	Pointer Changes	Visual State
1	Initialize empty stack	-1	[]	top = -1	Stack: [] (empty)
2	Push 10	0	[10]	top: -1 -> 0	Stack: [10] (top at index 0)
3	Push 20	1	[10, 20]	top: 0 -> 1	Stack: [10, 20] (top at index 1)
4	Pop element	0	[10]	top: 1 -> 0	Stack: [10] (top at index 0)
5	Pop element	-1	[]	top: 0 -> -1	Stack: [] (empty)
6	Pop on empty stack	-1	[]	No change	Underflow error - cannot pop from empty stack

💡 Execution stops after attempting to pop from empty stack (underflow).

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 5	After Step 6
top	-1	0	1	0	-1	-1
stack	[]	[10]	[10, 20]	[10]	[]	[]

Key Moments - 3 Insights

Why does the top index start at -1?

The top index is -1 when the stack is empty, meaning no elements are present. This is shown in execution_table step 1 where top = -1 and stack is empty.

What happens when we push an element?

When pushing, the top index increases by 1 and the new element is added at that position. See steps 2 and 3 in execution_table where top moves from -1 to 0 and then 1.

Why can't we pop when top is -1?

Top = -1 means stack is empty, so popping is not possible. This causes underflow as shown in step 6 of execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what is the top index after pushing 20?

A 1

B 0

C -1

D 2

💡 Hint

Check step 3 in the execution_table where push 20 updates top index.

At which step does the stack become empty again after popping?

A Step 4

B Step 5

C Step 3

D Step 6

💡 Hint

Look at the stack elements column in execution_table to see when stack is [].

If we try to pop when top is 0, what will be the new top index?

A 1

B 0

C -1

D 2

💡 Hint

Refer to step 5 in execution_table where popping from top=0 results in top=-1.

Concept Snapshot

Stack using array:
- Use an array and a top index.
- top = -1 means empty stack.
- Push: increment top, add element.
- Pop: remove element at top, decrement top.
- Overflow if top reaches max size.
- Underflow if pop when empty.

Full Transcript

This visualization shows how a stack is implemented using an array and a top pointer. Initially, the stack is empty with top = -1. When pushing, the top index increases and the new element is added at that position. When popping, the element at the top is removed and the top index decreases. Trying to pop when the stack is empty causes underflow. The execution table tracks each step, showing stack elements and top pointer changes. Key moments clarify why top starts at -1, how push and pop affect top, and what underflow means. The visual quiz tests understanding of top index changes and stack emptiness during operations.

Practice

1. What is the main purpose of using an array to implement a stack?

easy

A. To sort elements automatically

B. To store elements in a first-in, first-out (FIFO) order

C. To store elements in a last-in, first-out (LIFO) order

D. To allow random access to elements

Solution

1. Step 1: Understand stack behavior

   A stack stores elements so the last added is the first removed, called LIFO.

2. Step 2: Role of array in stack

   An array holds elements in order, supporting push/pop at one end to maintain LIFO.

3. Final Answer:

   To store elements in a last-in, first-out (LIFO) order -> Option C

4. Quick Check:

   Stack = LIFO [OK]

Hint: Stack means last-in, first-out order [OK]

Common Mistakes:

• Confusing stack with queue (FIFO)
• Thinking array sorts elements automatically
• Assuming random access is main stack feature

2. Which of the following is the correct way to declare a stack using an array in Go?

easy

A. stack := []int{}

B. stack := array int

C. var stack [10]int

D. var stack int[]

Solution

1. Step 1: Understand Go slice declaration

   In Go, slices are dynamic arrays declared as []type{}, e.g., []int{}.

2. Step 2: Check options for correct syntax

   stack := []int{} uses correct slice literal syntax to create an empty int slice.

3. Final Answer:

   stack := []int{} -> Option A

4. Quick Check:

   Go slice syntax = []type{} [OK]

Hint: Use slice literal []int{} to declare empty stack [OK]

Common Mistakes:

• Using array keyword incorrectly
• Wrong order of type and brackets
• Declaring fixed size array instead of slice

3. What will be the output of the following Go code snippet implementing a stack push and pop?

package main
import "fmt"
func main() {
  stack := []int{}
  stack = append(stack, 10)
  stack = append(stack, 20)
  top := stack[len(stack)-1]
  stack = stack[:len(stack)-1]
  fmt.Println(top)
  fmt.Println(stack)
}

medium

A. 10
[]

B. 20
[10]

C. 20
[10 20]

D. 10
[20]

Solution

1. Step 1: Trace push operations

   stack starts empty, append 10 -> [10], append 20 -> [10, 20]

2. Step 2: Trace pop operation

   top = last element = 20, then remove last element -> stack becomes [10]

3. Final Answer:

   20 [10] -> Option B

4. Quick Check:

   Pop returns 20, stack left with 10 [OK]

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

Common Mistakes:

• Printing stack before pop
• Confusing first element with last
• Not slicing stack correctly after pop

4. Identify the error in this Go code snippet for stack push operation:

package main
func main() {
  var stack []int
  stack[len(stack)] = 5
}

medium

A. Index out of range error

B. Syntax error: missing import

C. No error, code runs fine

D. Type mismatch error

Solution

1. Step 1: Understand slice indexing

   Accessing stack[len(stack)] tries to assign beyond current slice length, causing error.

2. Step 2: Correct push uses append

   To add element, use append(stack, 5) instead of direct index assignment.

3. Final Answer:

   Index out of range error -> Option A

4. Quick Check:

   Assigning at len(slice) causes index error [OK]

Hint: Use append to add elements, not direct index [OK]

Common Mistakes:

• Trying to assign at slice length index
• Forgetting to import fmt (not needed here)
• Assuming slice auto-expands on index assignment

5. Given a stack implemented using an array in Go, how would you modify the pop operation to safely handle popping from an empty stack without causing a runtime error?

hard

A. Replace the stack with a map to avoid empty pop errors

B. Always pop without checking; Go handles empty slices gracefully

C. Use a for loop to pop all elements at once

D. Check if stack length is zero before popping and return an error or zero value

Solution

1. Step 1: Understand pop risk on empty stack

   Popping from empty slice causes index out of range runtime error.

2. Step 2: Add length check before pop

   Check if len(stack) == 0; if yes, return error or zero value to avoid crash.

3. Final Answer:

   Check if stack length is zero before popping and return an error or zero value -> Option D

4. Quick Check:

   Empty stack pop needs length check [OK]

Hint: Always check stack length before pop to avoid errors [OK]

Common Mistakes:

• Ignoring empty stack check causing runtime panic
• Using map instead of array unnecessarily
• Trying to pop all elements in a loop without condition