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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>

struct Node {
   int data;
   struct Node *next;
};

struct Node* front = NULL;
struct Node* rear = NULL;

void enqueue(int x) {
   struct Node* temp =
      (struct Node*)malloc(sizeof(struct Node));
   temp->data = x;
   temp->next = NULL;
   if (front == NULL && rear == NULL) {
      front = rear = temp;
      return;
   }
   rear->next = temp;
   rear = temp;
}

void dequeue() {
   struct Node* temp = front;
   if (front == NULL) {
      printf("Queue is empty\n");
      return;
   }
   if (front == rear) {
      front = rear = NULL;
   } else {
      front = front->next;
   }
   free(temp);
}

void display() {
   struct Node* temp = front;
   if (front == NULL) {
      printf("Queue is empty\n");
      return;
   }
   while (temp != NULL) {
      printf("%d ", temp->data);
      temp = temp->next;
   }
   printf("\n");
}

int is_empty() {
   return front == NULL;
}

int main() {
	printf("Initialize a queue!");
    // Insert some elements into the queue.
    printf("\nCheck the queue is empty or not? %s\n", is_empty() ? "Yes" : "No");
	printf("\nInsert some elements into the queue:\n");
    enqueue(1);
    enqueue(2);
    enqueue(3);
    display();
	printf("\nInsert another element into the queue:\n");
    enqueue(4);
    display();
    printf("\nCheck the queue is empty or not? %s\n", is_empty() ? "Yes" : "No");
    return 0;
}

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:

Flowchart: Implement a queue using a linked list with insert, display.
Flowchart: Implement a queue using a linked list with insert, display.

C Programming Code Editor:

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Previous: Remove an element from a queue.
Next: Remove an element from a queue (using an linked list).

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C Programming: Tips of the Day

C Programming - How do you pass a function as a parameter in C?

Declaration

A prototype for a function which takes a function parameter looks like the following:

void func ( void (*f)(int) );

This states that the parameter f will be a pointer to a function which has a void return type and which takes a single int parameter. The following function (print) is an example of a function which could be passed to func as a parameter because it is the proper type:

void print ( int x ) {
  printf("%d\n", x);
}

Function Call

When calling a function with a function parameter, the value passed must be a pointer to a function. Use the function's name (without parentheses) for this:

func(print);

would call func, passing the print function to it.

Function Body

As with any parameter, func can now use the parameter's name in the function body to access the value of the parameter. Let's say that func will apply the function it is passed to the numbers 0-4. Consider, first, what the loop would look like to call print directly:

for ( int ctr = 0 ; ctr < 5 ; ctr++ ) {
  print(ctr);
}

Since func's parameter declaration says that f is the name for a pointer to the desired function, we recall first that if f is a pointer then *f is the thing that f points to (i.e. the function print in this case). As a result, just replace every occurrence of print in the loop above with *f:

void func ( void (*f)(int) ) {
  for ( int ctr = 0 ; ctr < 5 ; ctr++ ) {
    (*f)(ctr);
  }
}

Ref : https://bit.ly/3skw9Um