Check if Stack is Empty or Full in DSA Java - Execution Trace

Concept Flow - Check if Stack is Empty or Full

Start

↓

Check if stack pointer == -1?

Yes→Stack is Empty

|No↓

Check if stack pointer == max size - 1?

Yes→Stack is Full

|No↓

Stack has space and elements

↓

End

We first check if the stack pointer is -1 to see if the stack is empty. If not empty, we check if the pointer reached max size - 1 to see if it is full.

Execution Sample

DSA Java

int top = -1;
int maxSize = 3;

// Check empty
boolean isEmpty = (top == -1);
// Check full
boolean isFull = (top == maxSize - 1);

This code checks if the stack is empty or full by comparing the top pointer with -1 and maxSize - 1.

Execution Table

Step	Operation	top Pointer	maxSize	Is Empty?	Is Full?	Visual Stack State
1	Initialize top to -1	-1	3	true	false	[] (empty)
2	Push element 10 (top++)	0	3	false	false	[10]
3	Push element 20 (top++)	1	3	false	false	[10, 20]
4	Push element 30 (top++)	2	3	false	true	[10, 20, 30]
5	Check if empty (top == -1)	2	3	false	true	[10, 20, 30]
6	Check if full (top == maxSize - 1)	2	3	false	true	[10, 20, 30]
7	Pop element (top--)	1	3	false	false	[10, 20]
8	Check if empty (top == -1)	1	3	false	false	[10, 20]
9	Check if full (top == maxSize - 1)	1	3	false	false	[10, 20]
10	Pop element (top--)	0	3	false	false	[10]
11	Pop element (top--)	-1	3	true	false	[] (empty)
12	Check if empty (top == -1)	-1	3	true	false	[] (empty)
13	Check if full (top == maxSize - 1)	-1	3	true	false	[] (empty)
14	End	N/A	N/A	N/A	N/A	Final empty stack
15	Exit	N/A	N/A	Stack empty or full checked	N/A	N/A

💡 Execution stops after checking empty/full states and performing push/pop operations within stack size limits.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	After Step 7	After Step 10	After Step 11	Final
top	-1	0	1	2	1	0	-1	-1
maxSize	3	3	3	3	3	3	3	3
Is Empty?	true	false	false	false	false	false	true	true
Is Full?	false	false	false	true	false	false	false	false
Visual Stack State	[]	[10]	[10, 20]	[10, 20, 30]	[10, 20]	[10]	[]	[]

Key Moments - 3 Insights

Why does the stack pointer start at -1 instead of 0?

The stack pointer starts at -1 to indicate the stack is empty before any push. This is shown in execution_table step 1 where top = -1 means empty stack.

How do we know the stack is full?

When the top pointer equals maxSize - 1, the stack is full. This is shown in step 4 where top = 2 and maxSize = 3, so stack is full.

What happens to the top pointer after popping all elements?

After popping all elements, the top pointer returns to -1, indicating the stack is empty again, as shown in step 11.

Visual Quiz - 3 Questions

Look at the execution_table at step 4. What is the value of 'Is Full?' and why?

A true, because top equals maxSize - 1

B false, because top is less than maxSize - 1

C false, because stack is empty

D true, because top is -1

💡 Hint

Check the 'Is Full?' column and the 'top Pointer' value at step 4 in execution_table.

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

A Step 7

B Step 10

C Step 11

D Step 4

💡 Hint

Look at the 'top Pointer' and 'Is Empty?' columns in execution_table after pop operations.

If maxSize was increased to 4, how would the 'Is Full?' value change at step 4?

A It would be true because top is still 2

B It would be false because top is less than maxSize - 1

C It would be true because stack is empty

D It would be false because top is -1

💡 Hint

Compare 'top Pointer' and 'maxSize - 1' values in execution_table step 4.

Concept Snapshot

Stack uses a pointer 'top' starting at -1.
Empty stack: top == -1.
Full stack: top == maxSize - 1.
Push increments top; pop decrements top.
Check empty/full by comparing top with -1 and maxSize - 1.
Visualize stack as array with top indicating last element.

Full Transcript

This visualization shows how to check if a stack is empty or full using the top pointer. The top pointer starts at -1 indicating an empty stack. When pushing elements, top increments. When popping, top decrements. The stack is empty if top equals -1 and full if top equals maxSize - 1. The execution table traces these changes step-by-step with the stack's visual state. Key moments clarify why top starts at -1, how full is detected, and what happens after popping all elements. The quiz tests understanding of these states and pointer values.

Practice

1. What does the isEmpty() method in a stack typically check?

easy

A. If the stack has exactly one element

B. If the stack is full

C. If the stack has no elements

D. If the stack is initialized

Solution

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

   The isEmpty() method checks whether the stack contains any elements or not.

2. Step 2: Identify the condition checked

   If the stack has no elements, it is empty, so isEmpty() returns true.

3. Final Answer:

   If the stack has no elements -> Option C

4. Quick Check:

   Empty stack means no elements = true [OK]

Hint: Empty means zero elements in stack [OK]

Common Mistakes:

• Confusing isEmpty() with isFull()
• Thinking isEmpty() checks for one element
• Assuming isEmpty() checks initialization

2. Which of the following is the correct syntax to check if a stack named stack is full in Java?

easy

A. stack.isEmpty();

B. stack.isFull();

C. stack.full();

D. stack.isFilled();

Solution

1. Step 1: Identify the standard method name for full check

   In stack implementations, the method to check if the stack is full is usually named isFull().

2. Step 2: Match the correct syntax

   The correct syntax to call this method on a stack object named stack is stack.isFull();.

3. Final Answer:

   stack.isFull(); -> Option B

4. Quick Check:

   Method to check full = isFull() [OK]

Hint: Full check method is isFull() [OK]

Common Mistakes:

• Using isEmpty() to check full
• Using non-existent methods like full()
• Misspelling method names

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

class Stack {
  int maxSize = 3;
  int top = -1;
  int[] arr = new int[maxSize];

  boolean isFull() {
    return top == maxSize - 1;
  }

  boolean isEmpty() {
    return top == -1;
  }

  void push(int val) {
    if (!isFull()) {
      arr[++top] = val;
      System.out.println("Pushed " + val);
    } else {
      System.out.println("Stack is full");
    }
  }
}

public class Main {
  public static void main(String[] args) {
    Stack stack = new Stack();
    stack.push(10);
    stack.push(20);
    stack.push(30);
    stack.push(40);
  }
}

medium

A. Pushed 10
Pushed 20
Pushed 30
Stack is full

B. Pushed 10
Pushed 20
Pushed 30
Pushed 40

C. Stack is full
Stack is full
Stack is full
Stack is full

D. Pushed 10
Pushed 20
Stack is full
Stack is full

Solution

1. Step 1: Trace push operations and stack state

   Initially, top = -1. Each push increments top and adds value if stack not full. Max size is 3, so max top index is 2.

2. Step 2: Check each push call

   Push 10: top becomes 0, prints "Pushed 10".
   Push 20: top becomes 1, prints "Pushed 20".
   Push 30: top becomes 2, prints "Pushed 30".
   Push 40: top is 2 (full), so prints "Stack is full".

3. Final Answer:

   Pushed 10 Pushed 20 Pushed 30 Stack is full -> Option A

4. Quick Check:

   Stack max size 3, 4th push fails [OK]

Hint: Stack full when top == maxSize-1 [OK]

Common Mistakes:

• Assuming push always succeeds
• Ignoring top index limits
• Confusing isEmpty() with isFull()

4. Find the error in this stack implementation snippet that checks if the stack is empty:

public boolean isEmpty() {
  if (top == 0) {
    return true;
  } else {
    return false;
  }
}

medium

A. No error, the code is correct

B. The method should return false when top == 0

C. The method should check if top == maxSize

D. The condition should be top == -1 to check empty

Solution

1. Step 1: Understand stack empty condition

   In stack, empty means no elements pushed, so top is usually -1.

2. Step 2: Identify incorrect condition

   The code checks top == 0, which means one element is present, so it is wrong for empty check.

3. Final Answer:

   The condition should be top == -1 to check empty -> Option D

4. Quick Check:

   Empty stack top = -1 [OK]

Hint: Empty stack top is -1, not 0 [OK]

Common Mistakes:

• Checking top == 0 for empty
• Confusing full and empty conditions
• Returning wrong boolean values

5. You have a stack with max size 5 and current top = 3. Which of the following statements is true about the stack's state?

hard

A. The stack is neither full nor empty

B. The stack is empty

C. The stack is full

D. The stack has exactly 5 elements

Solution

1. Step 1: Understand stack indexing and size

   Stack max size is 5, so valid top indices are 0 to 4. top = 3 means 4 elements are present (indices 0,1,2,3).

2. Step 2: Determine if stack is full or empty

   Full means top == maxSize - 1 which is 4 here, so not full.
   Empty means top == -1, so not empty.
   Therefore, stack is neither full nor empty.

3. Final Answer:

   The stack is neither full nor empty -> Option A

4. Quick Check:

   top=3, maxSize=5 means partial fill [OK]

Hint: Full if top = maxSize-1, empty if top = -1 [OK]

Common Mistakes:

• Assuming top=3 means full
• Confusing top index with element count
• Thinking top=3 means empty