Why Check if Stack is Empty or Full in DSA Go?

The Big Idea

What if your program crashes just because it tried to add to a full stack or remove from an empty one?

The Scenario

Imagine you have a stack of plates in your kitchen. You want to add a new plate or take one out, but you don't know if the stack is already full or empty. You try blindly, and plates might fall or you might waste time searching.

The Problem

Without knowing if the stack is empty or full, you risk errors like trying to remove a plate when none are there or adding a plate beyond the stack's limit. This causes confusion and mistakes, just like dropping plates or wasting time.

The Solution

By checking if the stack is empty or full before adding or removing plates, you avoid mistakes. This simple check keeps your stack safe and operations smooth, just like knowing when you can safely add or remove a plate.

Before vs After

✗ Before

if len(stack) == 0 {
    // no check, might cause error
}
stack = append(stack, newPlate)

✓ After

if len(stack) == 0 {
    fmt.Println("Stack is empty")
}
if len(stack) == capacity {
    fmt.Println("Stack is full")
}

What It Enables

This check lets you safely manage your stack, preventing errors and making your program reliable and easy to use.

Real Life Example

Think of a parking garage with limited spots. Before letting a car in or out, the system checks if the garage is full or empty to avoid accidents or confusion.

Key Takeaways

Checking if a stack is empty or full prevents errors.

It helps manage limited space safely and efficiently.

Simple checks make your code more reliable and user-friendly.

Practice

1. What does the IsEmpty() function check in a stack data structure?

easy

A. If the stack has no elements inside

B. If the stack is full and cannot add more elements

C. If the stack has exactly one element

D. If the stack has more than one element

Solution

1. Step 1: Understand the purpose of IsEmpty()

   The IsEmpty() function checks whether the stack contains any elements or not.

2. Step 2: Identify what empty means for a stack

   An empty stack means it has zero elements inside, so no items to pop or peek.

3. Final Answer:

   If the stack has no elements inside -> Option A

4. Quick Check:

   IsEmpty() means stack is empty = true [OK]

Hint: Empty means zero elements inside stack [OK]

Common Mistakes:

• Confusing empty with full
• Thinking empty means one element
• Mixing IsEmpty with IsFull

2. Which of the following is the correct Go syntax to check if a stack is full when using a fixed size array?

easy

A. if len(stack) == 0 { /* stack is full */ }

B. if top > len(stack) { /* stack is full */ }

C. if top == len(stack) - 1 { /* stack is full */ }

D. if top == 0 { /* stack is full */ }

Solution

1. Step 1: Understand stack indexing in Go

   For a fixed size array stack, top points to the last filled index. The last valid index is length - 1.

2. Step 2: Check condition for full stack

   When top equals len(stack) - 1, the stack is full because no more elements can be added.

3. Final Answer:

   if top == len(stack) - 1 { /* stack is full */ } -> Option C

4. Quick Check:

   top == max index means full [OK]

Hint: Full means top equals last index (length - 1) [OK]

Common Mistakes:

• Using > instead of == for full check
• Checking if top == 0 for full
• Confusing length with top index

3. Given the following Go code, what will be printed?

type Stack struct {
    items []int
    top   int
}

func (s *Stack) IsEmpty() bool {
    return s.top == -1
}

func main() {
    s := Stack{items: make([]int, 5), top: -1}
    fmt.Println(s.IsEmpty())
    s.top = 4
    fmt.Println(s.IsEmpty())
}

medium

A. true\nfalse

B. false\ntrue

C. false\nfalse

D. true\ntrue

Solution

1. Step 1: Check initial top value and IsEmpty() result

   Initially, top is -1, so IsEmpty() returns true because stack is empty.

2. Step 2: Update top to 4 and check IsEmpty() again

   After setting top to 4, IsEmpty() returns false because stack now has elements.

3. Final Answer:

   true\nfalse -> Option A

4. Quick Check:

   top -1 means empty true, top 4 means not empty false [OK]

Hint: top -1 means empty, any other top means not empty [OK]

Common Mistakes:

• Assuming top 4 means empty
• Confusing slice length with top
• Ignoring initial top value

4. Find the error in this Go code snippet that checks if a stack is full:

func (s *Stack) IsFull() bool {
    return s.top == len(s.items)
}

medium

A. The function should return 's.top > len(s.items)' instead

B. The condition should be 's.top == len(s.items) - 1' to check full correctly

C. The function should check if 's.top == 0' to be full

D. No error, the code is correct

Solution

1. Step 1: Understand stack indexing for full condition

   Stack top index starts at -1 and max valid index is length - 1.

2. Step 2: Identify correct full condition

   Full means top equals last valid index, which is len(s.items) - 1, not len(s.items).

3. Final Answer:

   The condition should be 's.top == len(s.items) - 1' to check full correctly -> Option B

4. Quick Check:

   Full means top == max index (length - 1) [OK]

Hint: Full means top equals last index, not length [OK]

Common Mistakes:

• Using length instead of length - 1
• Checking top == 0 for full
• Returning wrong boolean condition

5. You have a stack implemented with a fixed size array of length 3 and a top index starting at -1. After pushing two elements, you pop one element, then push two more elements. Which statement about the stack state is correct?

hard

A. The stack is empty and top index is -1

B. The stack is not full and top index is 1

C. The stack is full but top index is 3

D. The stack is full and top index is 2

Solution

1. Step 1: Track stack operations and top index changes

   Start: top = -1 (empty). Push two elements: top = 1. Pop one element: top = 0. Push two elements: top = 2 (max index for length 3).

2. Step 2: Determine if stack is full

   Since top = 2 and max index is 2 (length 3 - 1), stack is full.

3. Final Answer:

   The stack is full and top index is 2 -> Option D

4. Quick Check:

   Push/pop changes top; top 2 means full for size 3 [OK]

Hint: Track top index after each push/pop carefully [OK]

Common Mistakes:

• Assuming top can be 3 for full
• Forgetting pop decreases top
• Confusing empty with full states