C++ Queue Exercises: Remove all the elements from a queue

Last update on April 14 2025 12:46:35 (UTC/GMT +8 hours)

14. Remove All Elements from a Queue

Write a C++ program to remove all the elements from a queue.

Sample Solution:

C Code:

#include <iostream>
#include <map>
#include <cmath>

using namespace std;

const int MAX_SIZE = 100;

class Queue {
  private:
    int front; // Holds the front index of the queue
    int rear; // Holds the rear index of the queue
    int arr[MAX_SIZE]; // Array to hold queue elements

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

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

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

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

    void dequeue() {
      if (isEmpty()) {
        cout << "Error: Queue is empty" << endl; // Displays 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; // Displays error if the queue is empty
        return -1;
      }
      return arr[front]; // Returns the element at the front of the queue
    }

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

    void clear() {
      while (!isEmpty()) {
        dequeue(); // Removes all elements from the queue
      }
    }

};

int main() {
  cout << "Initialize a Queue." << endl;
  Queue q;
  cout << "\nInsert some elements into the queue:" << endl;
  q.enqueue(1);
  q.enqueue(2);
  q.enqueue(3);
  q.enqueue(4);
  q.enqueue(5);
  q.enqueue(6);
  q.display();
  cout << "\nRemove all the elements from the said queue:" << endl;
  q.clear();
  q.display(); // Display an empty queue
  cout << "\nInput two more elements into the queue:" << endl;
  q.enqueue(1);
  q.enqueue(2);
  q.display();
  cout << "\nRemove all the elements from the said queue:" << endl;
  q.clear();
  q.display(); // Display an empty queue again
  return 0;
}

Sample Output:

Initialize a Queue.

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

Remove all the elements from the said queue:
Error: Queue is empty

Input two more elements into the queue:
Queue elements are: 1 2 

Remove all the elements from the said queue:
Error: Queue is empty

Flowchart:

For more Practice: Solve these Related Problems:

Write a C++ program to completely clear a queue and verify that it is empty.
Write a C++ program to dequeue all elements from a queue one by one while printing each removed element.
Write a C++ program to empty a queue and then allow the user to refill it with new values.
Write a C++ program that clears a queue and then outputs a confirmation message along with the cleared count.

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Previous C++ Exercise: Remove a given element from a queue.
Next C++ Exercise: Remove all even elements from a queue.