C++ Stack Exercises: Find the middle element of a stack (using an array)

Last update on April 14 2025 12:44:45 (UTC/GMT +8 hours)

8. Find the Middle Element of an Array-Based Stack

Write a C++ program to find the middle element of a stack (using an array).

If the number of elements (size) in the stack is odd, return the element at position (size/2) + 1.
If the number of elements is even, return the element at position size/2.

Test Data:
Input some elements onto the stack:
Stack elements are: 5 2 4 7
Middle element of the stack: 2

Sample Solution:

C++ Code:

#include <iostream>

using namespace std;

#define MAX_SIZE 15 // Maximum size of stack

class Stack {
private:
    int top; // Index of top element
    int arr[MAX_SIZE]; // Array to store elements

public:
    Stack() {
        top = -1; // Initialize top index to -1 (empty stack)
    }

    bool push(int x) {
        if (isFull()) {
            cout << "Stack overflow" << endl; // Display message if stack is full
            return false; // Return false to indicate failure in pushing element
        }
        // Increment top index and add element to array
        arr[++top] = x; 
        return true; // Return true to indicate successful element addition
    }

    int pop() {
        if (isEmpty()) {
            cout << "Stack underflow" << endl; // Display message if stack is empty
            return 0; // Return 0 to indicate failure in popping element
        }
        // Return top element and decrement top index
        return arr[top--]; 
    }

    int peek() {
        if (isEmpty()) {
            cout << "Stack is empty" << endl; // Display message if stack is empty
            return 0; // Return 0 to indicate failure in peeking element
        }
        // Return top element without modifying top index
        return arr[top]; 
    }

    bool isEmpty() {
        // Stack is empty if top index is -1
        return (top < 0); 
    }

    bool isFull() {
        // Stack is full if top index is equal to MAX_SIZE - 1
        return (top >= MAX_SIZE - 1); 
    }

    void display() {
        if (top < 0) {
            cout << "Stack is empty" << endl; // Display message if stack is empty
            return;
        }
        cout << "\nStack elements are: ";
        for (int i = top; i >= 0; i--)
            cout << arr[i] << " "; // Display elements of the stack
        cout << endl;
    }

    int findMiddleElement() {
        if (isEmpty()) {
            cout << "Stack is empty" << endl; // Display message if stack is empty
            return -1; // Return -1 to indicate empty stack
        }

        int size = top + 1;
        int mid = (size % 2 == 0) ? (size/2) : ((size/2) + 1);

        Stack temp;
        int middleElement;
        for (int i = 0; i < mid; i++) {
            int element = pop();
            if (i == mid - 1) {
                // Store the middle element
                middleElement = element; 
            }
            // Push other elements onto the temporary stack
            temp.push(element); 
        }
        while (!temp.isEmpty()) {
            // Return elements from the temporary stack to the original stack
            push(temp.pop()); 
        }
        return middleElement;
    }
};

int main() {
    //Initialize the stack stk
    Stack stk; 
    cout << "Input some elements onto the stack:";
    stk.push(7);
    stk.push(4);
    stk.push(2);
    stk.push(5);
    stk.display(); 
    // Find the maximum value 
    int middleElement = stk.findMiddleElement(); // Find the middle element of the stack
    cout << "Middle element of the stack: " << middleElement << endl;
    cout << "\nInput three more elements";
    stk.push(-1);
    stk.push(-2);
    stk.push(-3);
    stk.display();
    middleElement = stk.findMiddleElement(); // Find the middle element of the stack
    cout << "Middle element of the stack: " << middleElement << endl;
    return 0;
}

Sample Output:

Input some elements onto the stack:
Stack elements are: 5 2 4 7 
Middle element of the stack: 2

Input three more elements
Stack elements are: -3 -2 -1 5 2 4 7 
Middle element of the stack: 5

Flowchart:

For more Practice: Solve these Related Problems:

• Write a C++ program to determine the middle element of a stack (using an array) without altering its order.
• Develop a C++ program to compute the index of the middle element in an array-based stack and display its value.
• Design a C++ program that finds the middle element in an array-based stack and handles both even and odd sizes.
• Implement a C++ program to retrieve the middle element of a stack (using an array) by calculating its offset from the top.

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Previous C++ Exercise: Find the minimum element in a stack (using an array).
Next C++ Exercise: Delete the middle element of a stack (using an array).