G4G0-2/Data Structures/GPT Answers to Past Paper/Question 1.md

Write a Java method pop() for a stack implemented using an array.

public class Stack { 
	private int[] stackArray; // Array to hold stack elements 
	private int top; // Index of the top element 
	private int capacity; // Maximum capacity of the stack 
	// Constructor to initialize the stack 
	public Stack(int size) { 
		stackArray = new int[size]; 
		capacity = size; 
		top = -1; // Indicates the stack is empty 
	} 
	// Method to add an element to the stack 
	public void push(int item) { 
		if (isFull()) { 
			throw new RuntimeException("Stack overflow"); 
		} 
		stackArray[++top] = item; // Increment top and store the item 
	} 
	// Method to remove and return the top element of the stack 
	public int pop() { 
		if (isEmpty()) { 
			throw new RuntimeException("Stack underflow"); 
		} return stackArray[top--]; // Return the top element and decrement top 
	} 
	// Method to check if the stack is empty 
	public boolean isEmpty() {
		return top == -1; 
	} 
	// Method to check if the stack is full 
	public boolean isFull() { 
		return top == capacity - 1; 
	} 
	// Method to view the top element without removing it 
	public int peek() { 
		if (isEmpty()) { 
			throw new RuntimeException("Stack is empty"); 
		} return stackArray[top]; 
	} 
	public static void main(String[] args) { 
		Stack stack = new Stack(5); // Create a stack with capacity 5 
		// Example usage 
		stack.push(10); 
		stack.push(20); 
		stack.push(30); 
		System.out.println(stack.pop()); // Output: 30 
		System.out.println(stack.pop()); // Output: 20 
		System.out.println(stack.pop()); // Output: 10 
	} 
}

Explanation:

1. Array Storage: The stack elements are stored in an array (stackArray).
2. Pointer: The top variable keeps track of the current top of the stack.
3. Pop Operation: The pop() method checks if the stack is empty using the isEmpty() method. If not, it returns the element at the top index and decrements top.
4. Edge Cases:
   • Underflow: If the stack is empty, it throws a RuntimeException.
   • Overflow: The push() method checks for overflow before adding elements.

This ensures a robust implementation of the pop() operation.

• The expression stackArray[top--] first retrieves the value at stackArray[top] and then decrements top by 1.
• This uses the post-decrement operator (--), which means:
  1. The value of top is used to access the current top element.
  2. After accessing the value, top is decremented.

Why top Controls the Stack?

The value of top acts as a pointer to the last valid element in the stack. By decrementing it, you effectively "remove" the topmost element from the stack, even though the value remains in the array until overwritten by a push() operation.

Conclusion:

Yes, calling pop() does update top, which is crucial for maintaining the stack's state. This is a standard approach for implementing a stack using an array.