Stack Concept and LIFO Principle in DSA Go - Build from Scratch

Stack Concept and LIFO Principle

📖 Scenario: Imagine you are managing a stack of books on a table. You can only add or remove the top book. This is called Last In, First Out (LIFO) because the last book you put on the stack is the first one you take off.

🎯 Goal: You will create a stack using a list, add books to it, remove the top book, and then show the final stack order.

📋 What You'll Learn

Create a stack as a slice of strings with exact book titles

Add a new book to the top of the stack

Remove the top book from the stack

Print the final stack showing the order of books from bottom to top

💡 Why This Matters

🌍 Real World

Stacks are used in undo features in apps, browser history, and managing tasks where the last added is handled first.

💼 Career

Understanding stacks helps in software development roles that involve managing data flow, parsing expressions, and designing algorithms.

1

Create the initial stack

Create a slice of strings called stack with these exact book titles in order: "Book A", "Book B", "Book C"

DSA Go

# Create the initial stack slice with the exact books
# Your code here

Hint

Use a slice literal with the exact book titles inside curly braces.

2

Add a new book to the stack

Add the book "Book D" to the top of the stack slice using the append function

DSA Go

package main

import "fmt"

func main() {
    stack := []string{"Book A", "Book B", "Book C"}
    // Add "Book D" to the top of the stack
    // Your code here
}

Hint

Use stack = append(stack, "Book D") to add the new book.

3

Remove the top book from the stack

Remove the top book from the stack slice by slicing off the last element

DSA Go

package main

import "fmt"

func main() {
    stack := []string{"Book A", "Book B", "Book C"}
    stack = append(stack, "Book D")
    // Remove the top book from the stack
    // Your code here
}

Hint

Use slicing to keep all elements except the last one: stack = stack[:len(stack)-1].

4

Print the final stack

Print the stack slice to show the final order of books from bottom to top

DSA Go

package main

import "fmt"

func main() {
    stack := []string{"Book A", "Book B", "Book C"}
    stack = append(stack, "Book D")
    stack = stack[:len(stack)-1]
    // Print the final stack
    // Your code here
}

Hint

Use fmt.Println(stack) to print the slice.

Practice

1. What does the LIFO principle mean in the context of a stack?

easy

A. Last In, First Out - the last element added is the first to be removed

B. Last In, First Out - the first element added is the first to be removed

C. First In, First Out - the first element added is the first to be removed

D. First In, Last Out - the first element added is the last to be removed

Solution

1. Step 1: Understand LIFO meaning

   LIFO stands for Last In, First Out, meaning the last item added to the stack is the first one removed.

2. Step 2: Match with stack behavior

   Stacks follow LIFO, so the last pushed element is popped first.

3. Final Answer:

   Last In, First Out - the last element added is the first to be removed -> Option A

4. Quick Check:

   LIFO = Last In, First Out [OK]

Hint: Remember stack removes newest item first [OK]

Common Mistakes:

• Confusing LIFO with FIFO
• Thinking first added is removed first
• Mixing up stack with queue behavior

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

easy

A. stack.push(5)

B. stack = append(stack, 5)

C. stack.add(5)

D. stack.insert(5)

Solution

1. Step 1: Recall Go slice push syntax

   In Go, stacks are often slices; to push, use append(slice, value).

2. Step 2: Check options for correct syntax

   Only stack = append(stack, 5) correctly adds 5 to the stack slice.

3. Final Answer:

   stack = append(stack, 5) -> Option B

4. Quick Check:

   Go stack push uses append [OK]

Hint: Use append to add elements to Go slices [OK]

Common Mistakes:

• Using non-existent push method on slice
• Using add or insert which are invalid for slices
• Forgetting to assign append result back

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

stack := []int{}
stack = append(stack, 10)
stack = append(stack, 20)
stack = append(stack, 30)
last := stack[len(stack)-1]
stack = stack[:len(stack)-1]
fmt.Println(last)

medium

A. 30

B. 20

C. 10

D. Error

Solution

1. Step 1: Trace stack push operations

   Elements 10, 20, 30 are pushed in order; stack is [10, 20, 30].

2. Step 2: Pop last element

   last = stack[len(stack)-1] gets 30; then stack is sliced to remove last element.

3. Final Answer:

   30 -> Option A

4. Quick Check:

   Last pushed element popped = 30 [OK]

Hint: Last appended element is popped first [OK]

Common Mistakes:

• Printing first element instead of last
• Forgetting to slice stack after pop
• Confusing index for pop operation

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

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

medium

A. Cannot print variable last

B. Syntax error in slice operation

C. Index out of range error due to accessing stack[len(stack)]

D. No error, code runs correctly

Solution

1. Step 1: Check index used for popping

   Indexing with stack[len(stack)] is invalid because slice indices go from 0 to len(stack)-1.

2. Step 2: Identify error type

   This causes an index out of range runtime error.

3. Final Answer:

   Index out of range error due to accessing stack[len(stack)] -> Option C

4. Quick Check:

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

Hint: Use len(stack)-1 for last index [OK]

Common Mistakes:

• Using len(stack) as index
• Ignoring zero-based indexing
• Assuming code runs without error

5. Given a stack implemented as a slice in Go, how would you reverse the order of elements using only stack operations (push and pop)?

hard

A. Pop elements and discard them to reverse the stack

B. Push elements onto the same stack again without popping

C. Use a queue instead of a stack to reverse elements

D. Pop all elements into a temporary slice, then push them back in order

Solution

1. Step 1: Pop all elements from original stack

   Pop elements one by one and store them in a temporary slice to hold reversed order.

2. Step 2: Push elements back to original stack

   Push elements from temporary slice back to stack; this reverses the order due to LIFO.

3. Final Answer:

   Pop all elements into a temporary slice, then push them back in order -> Option D

4. Quick Check:

   Reversing stack needs pop then push back [OK]

Hint: Pop all, then push back reverses stack [OK]

Common Mistakes:

• Pushing without popping does not reverse
• Discarding elements loses data
• Using queue is not stack operation