Queue in data structures and algorithms :
Queue is useful data structure in programming. it is similar to the ticket Queue outside the cinema hall, where the first person entering the Queue is the first person who gets the ticket.
Queue follows FIRST IN FIRST OUT ( FIFO ) Rule :
In the above figure, since 1 was kept in the queue before 2, it is the first to be removed from the queue as well. its follows the FIFO Rule.
In programming terms :
enqueue : putting item into the queue is called enqueue.
dequeue : removing item from the queue is called dequeue.
we can implement the queue in any programming languages like c, java, python etc.
Operation of Queue :
- enqueue : Add an element to the end of the queue.
- dequeue : remove an element from the front of the Queue.
- isEmpty : check if the queue is Empty.
- isFull : check if the queue is Full.
- Peek : get the value of the front of the queue without removing it.
Working of Queue :
- Two pointers FRONT and REAR
- FRONT track the first element of the queue.
- REAR track the last element of the Queue.
- Initially, set value of FRONT and REAR is to -1.
enqueue and dequeue operations :
Queue Implementation C, C++, JAVA, PYTHON :
// Queue implementation in C
#include <stdio.h>
#define SIZE 5
void enQueue(int);
void deQueue();
void display();
int items[SIZE], front = -1, rear = -1;
int main() {
//deQueue is not possible on empty queue
deQueue();
//enQueue 5 elements
enQueue(1);
enQueue(2);
enQueue(3);
enQueue(4);
enQueue(5);
// 6th element can't be added to because the queue is full
enQueue(6);
display();
//deQueue removes element entered first i.e. 1
deQueue();
//Now we have just 4 elements
display();
return 0;
}
void enQueue(int value) {
if (rear == SIZE - 1)
printf("\nQueue is Full!!");
else {
if (front == -1)
front = 0;
rear++;
items[rear] = value;
printf("\nInserted -> %d", value);
}
}
void deQueue() {
if (front == -1)
printf("\nQueue is Empty!!");
else {
printf("\nDeleted : %d", items[front]);
front++;
if (front > rear)
front = rear = -1;
}
}
// Function to print the queue
void display() {
if (rear == -1)
printf("\nQueue is Empty!!!");
else {
int i;
printf("\nQueue elements are:\n");
for (i = front; i <= rear; i++)
printf("%d ", items[i]);
}
printf("\n");
}
Queue Implementation C++ :
// Queue implementation in C++
#include <iostream>
#define SIZE 5
using namespace std;
class Queue {
private:
int items[SIZE], front, rear;
public:
Queue() {
front = -1;
rear = -1;
}
bool isFull() {
if (front == 0 && rear == SIZE - 1) {
return true;
}
return false;
}
bool isEmpty() {
if (front == -1)
return true;
else
return false;
}
void enQueue(int element) {
if (isFull()) {
cout << "Queue is full";
} else {
if (front == -1) front = 0;
rear++;
items[rear] = element;
cout << endl
<< "Inserted " << element << endl;
}
}
int deQueue() {
int element;
if (isEmpty()) {
cout << "Queue is empty" << endl;
return (-1);
} else {
element = items[front];
if (front >= rear) {
front = -1;
rear = -1;
} /* Q has only one element, so we reset the queue after deleting it. */
else {
front++;
}
cout << endl
<< "Deleted -> " << element << endl;
return (element);
}
}
void display() {
/* Function to display elements of Queue */
int i;
if (isEmpty()) {
cout << endl
<< "Empty Queue" << endl;
} else {
cout << endl
<< "Front index-> " << front;
cout << endl
<< "Items -> ";
for (i = front; i <= rear; i++)
cout << items[i] << " ";
cout << endl
<< "Rear index-> " << rear << endl;
}
}
};
int main() {
Queue q;
//deQueue is not possible on empty queue
q.deQueue();
//enQueue 5 elements
q.enQueue(1);
q.enQueue(2);
q.enQueue(3);
q.enQueue(4);
q.enQueue(5);
// 6th element can't be added to because the queue is full
q.enQueue(6);
q.display();
//deQueue removes element entered first i.e. 1
q.deQueue();
//Now we have just 4 elements
q.display();
return 0;
}
Queue Implementation JAVA :
// Queue implementation in Java
public class Queue {
int SIZE = 5;
int items[] = new int[SIZE];
int front, rear;
Queue() {
front = -1;
rear = -1;
}
boolean isFull() {
if (front == 0 && rear == SIZE - 1) {
return true;
}
return false;
}
boolean isEmpty() {
if (front == -1)
return true;
else
return false;
}
void enQueue(int element) {
if (isFull()) {
System.out.println("Queue is full");
} else {
if (front == -1)
front = 0;
rear++;
items[rear] = element;
System.out.println("Inserted " + element);
}
}
int deQueue() {
int element;
if (isEmpty()) {
System.out.println("Queue is empty");
return (-1);
} else {
element = items[front];
if (front >= rear) {
front = -1;
rear = -1;
} /* Q has only one element, so we reset the queue after deleting it. */
else {
front++;
}
System.out.println("Deleted -> " + element);
return (element);
}
}
void display() {
/* Function to display elements of Queue */
int i;
if (isEmpty()) {
System.out.println("Empty Queue");
} else {
System.out.println("\nFront index-> " + front);
System.out.println("Items -> ");
for (i = front; i <= rear; i++)
System.out.print(items[i] + " ");
System.out.println("\nRear index-> " + rear);
}
}
public static void main(String[] args) {
Queue q = new Queue();
// deQueue is not possible on empty queue
q.deQueue();
// enQueue 5 elements
q.enQueue(1);
q.enQueue(2);
q.enQueue(3);
q.enQueue(4);
q.enQueue(5);
// 6th element can't be added to because the queue is full
q.enQueue(6);
q.display();
// deQueue removes element entered first i.e. 1
q.deQueue();
// Now we have just 4 elements
q.display();
}
}
Queue Implementation PYTHON :
# Queue implementation in Python
class Queue:
def __init__(self):
self.queue = []
# Add an element
def enqueue(self, item):
self.queue.append(item)
# Remove an element
def dequeue(self):
if len(self.queue) < 1:
return None
return self.queue.pop(0)
# Display the queue
def display(self):
print(self.queue)
def size(self):
return len(self.queue)
q = Queue()
q.enqueue(1)
q.enqueue(2)
q.enqueue(3)
q.enqueue(4)
q.enqueue(5)
q.display()
q.dequeue()
print("After removing an element")
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