C++ Queue Exercises: N^th element from the top of a queue

C++ Queue: Exercise-23 with Solution

Write a C++ program to get the N^th element from the top of a queue.

Sample Solution:

C Code:

#include <iostream> // Including necessary library for input and output operations

using namespace std;

const int MAX_SIZE = 100; // Maximum size for the queue

class Queue {
private:
    int front; // Front of the queue
    int rear; // Rear of the queue
    int arr[MAX_SIZE]; // Array to store queue elements

public:
    Queue() {
        front = -1; // Initializing front index to -1
        rear = -1; // Initializing rear index to -1
    }

    bool isFull() {
        return (rear == MAX_SIZE - 1); // Check if the queue is full
    }

    bool isEmpty() {
        return (front == -1 && rear == -1); // Check if the queue is empty
    }

    void enqueue(int x) {
        if (isFull()) {
            cout << "Error: Queue is full" << endl; // Display error if the queue is full
            return;
        }
        if (isEmpty()) {
            front = 0;
            rear = 0;
        } else {
            rear++;
        }
        arr[rear] = x; // Add an element to the rear of the queue
    }

    void dequeue() {
        if (isEmpty()) {
            cout << "Error: Queue is empty" << endl; // Display error if the queue is empty
            return;
        }
        if (front == rear) {
            front = -1;
            rear = -1;
        } else {
            front++;
        }
    }

    int peek() {
        if (isEmpty()) {
            cout << "Error: Queue is empty" << endl; // Display error if the queue is empty
            return -1;
        }
        return arr[front]; // Return the element at the front of the queue
    }

    int find_Nth_From_Top(Queue q, int n) {
        int size = 0;
        Queue temp;

        // Find the size of the queue
        while (!q.isEmpty()) {
            temp.enqueue(q.peek());
            q.dequeue();
            size++;
        }

        // If the given index is greater than the size of the queue
        if (n > size) {
            cout << "Error: Queue does not have " << n << " elements." << endl;
            return -1;
        }

        // Copy the elements back to the original queue
        for (int i = 0; i < size; i++) {
            q.enqueue(temp.peek());
            temp.dequeue();
        }

        // Dequeue the first (n-1) elements
        for (int i = 0; i < n - 1; i++) {
            q.enqueue(q.peek());
            q.dequeue();
        }

        // Return the nth element from the top
        return q.peek();
    }

    void display() {
        if (isEmpty()) {
            cout << "Error: Queue is empty" << endl; // Display error if the queue is empty
            return;
        }
        cout << "Queue elements are: ";
        for (int i = front; i <= rear; i++) {
            cout << arr[i] << " "; // Display all elements in the queue
        }
        cout << endl;
    }
};

int main() {
    cout << "Initialize a Queue." << endl;
    Queue q1; // Creating an instance of Queue
    cout << "\nInsert some elements into the queue:" << endl;
    q1.enqueue(0);
    q1.enqueue(1);
    q1.enqueue(2);
    q1.enqueue(3);
    q1.enqueue(4);
    q1.enqueue(5);
    q1.enqueue(6);
    q1.display(); // Display elements of the queue
    int n = 2;
    cout << "\nNth element from top is " << q1.find_Nth_From_Top(q1, n) << " Where n = " << n << endl;
    n = 3;
    cout << "\nNth element from top is " << q1.find_Nth_From_Top(q1, n) << " Where n = " << n << endl;
    n = 4;
    cout << "\nNth element from top is " << q1.find_Nth_From_Top(q1, n) << " Where n = " << n << endl;
    return 0;
}

Sample Output:

Initialize a Queue.

Insert some elements into the queue:
Queue elements are: 0 1 2 3 4 5 6 

Nth element from top is 1 Where n = 2

Nth element from top is 2 Where n = 3

Nth element from top is 3 Where n = 4

Flowchart:

CPP Code Editor:

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