C Exercises: Implement a queue using a linked list with insert, display

C Queue: Exercise-3 with Solution

Write a C program to implement a queue using a linked list. Programs should contain functions for inserting elements into the queue, displaying queue elements, and checking whether the queue is empty or not.

Sample Solution:

C Code:

#include <stdio.h>
#include <stdlib.h>
// Define a structure for a node in the queue
struct Node {
   int data;
   struct Node *next;
};

struct Node* front = NULL; // Declare a pointer to the front of the queue
struct Node* rear = NULL; // Declare a pointer to the rear of the queue

// Function to insert an element into the queue
void enqueue(int x) {
   // Allocate memory for a new node
   struct Node* temp = (struct Node*)malloc(sizeof(struct Node));
   temp->data = x; // Assign data to the new node
   temp->next = NULL; // Set the next pointer of the new node to NULL

   // Check if the queue is empty
   if (front == NULL && rear == NULL) {
      front = rear = temp; // If empty, set both front and rear to the new node
      return;
   }
   rear->next = temp; // Otherwise, link the new node to the rear of the queue
   rear = temp; // Update the rear pointer to the new node
}

// Function to remove an element from the queue
void dequeue() {
   struct Node* temp = front; // Store the front node in a temporary variable
   // Check if the queue is empty
   if (front == NULL) {
      printf("Queue is empty\n"); // Print error message if empty
      return;
   }
   // Check if there's only one node in the queue
   if (front == rear) {
      front = rear = NULL; // If yes, set both front and rear to NULL
   } else {
      front = front->next; // Otherwise, move front to the next node
   }
   free(temp); // Free memory allocated for the removed node
}

// Function to display the elements in the queue
void display() {
   struct Node* temp = front; // Initialize a temporary pointer to traverse the queue
   // Check if the queue is empty
   if (front == NULL) {
      printf("Queue is empty\n"); // Print message if empty
      return;
   }
   // Traverse the queue and print each element
   while (temp != NULL) {
      printf("%d ", temp->data); // Print the data of the current node
      temp = temp->next; // Move to the next node
   }
   printf("\n"); // Print a newline after displaying all elements
}

// Function to check whether the queue is empty
int is_empty() {
   return front == NULL; // Return 1 if front is NULL (indicating an empty queue), otherwise return 0
}

int main() {
    printf("Initialize a queue!"); // Print message to indicate initializing a queue
    // Insert some elements into the queue.
    printf("\nCheck the queue is empty or not? %s\n", is_empty() ? "Yes" : "No"); // Check if the queue is empty
	printf("\nInsert some elements into the queue:\n"); // Print message to indicate inserting elements into the queue
    enqueue(1); // Insert element 1 into the queue
    enqueue(2); // Insert element 2 into the queue
    enqueue(3); // Insert element 3 into the queue
    display(); // Display the elements of the queue
	printf("\nInsert another element into the queue:\n"); // Print message to indicate inserting another element
    enqueue(4); // Insert element 4 into the queue
    display(); // Display the elements of the queue
    printf("\nCheck the queue is empty or not? %s\n", is_empty() ? "Yes" : "No"); // Check if the queue is empty
    return 0; // Return from the main function
}

Sample Output:

Initialize a queue!
Check the queue is empty or not? Yes

Insert some elements into the queue:
1 2 3 

Insert another element into the queue:
1 2 3 4 

Check the queue is empty or not? No

Flowchart:

C Programming Code Editor:

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