vault backup: 2024-12-01 23:50:59

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

@@ -0,0 +1,76 @@
+# Write a Java method pop() for a stack implemented using an array.
+
+```java
+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.