Queue Data Structure

What is a Queue?

A queue is a linear data structure where elements are stored in the FIFO (First In First Out) principle where the first element inserted would be the first element to be accessed. A queue is an Abstract Data Type (ADT) similar to stack, the thing that makes queue different from stack is that a queue is open at both its ends. The data is inserted into the queue through one end and deleted from it using the other end. Queue is very frequently used in most programming languages.

A real-world example of queue can be a single-lane one-way road, where the vehicle enters first, exits first. More real-world examples can be seen as queues at the ticket windows and bus-stops.

Representation of Queues

Similar to the stack ADT, a queue ADT can also be implemented using arrays, linked lists, or pointers. As a small example in this tutorial, we implement queues using a one-dimensional array.

Basic Operations in Queue

Queue operations also include initialization of a queue, usage and permanently deleting the data from the memory.

The most fundamental operations in the queue ADT include: enqueue(), dequeue(), peek(), isFull(), isEmpty(). These are all built-in operations to carry out data manipulation and to check the status of the queue.

Queue uses two pointers − front and rear. The front pointer accesses the data from the front end (helping in enqueueing) while the rear pointer accesses data from the rear end (helping in dequeuing).

Queue Insertion Operation: Enqueue()

The enqueue() is a data manipulation operation that is used to insert elements into the stack. The following algorithm describes the enqueue() operation in a simpler way.

Algorithm

1. START
2. Check if the queue is full.
3. If the queue is full, produce overflow error and exit.
4. If the queue is not full, increment rear pointer to point 
   the next empty space.
5. Add data element to the queue location, where the rear 
   is pointing.
6. return success.
7. END

Example

Following are the implementations of this operation in various programming languages −

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <stdbool.h>
#define MAX 6
int intArray[MAX];
int front = 0;
int rear = -1;
int itemCount = 0;
bool isFull(){
   return itemCount == MAX;
}
bool isEmpty(){
   return itemCount == 0;
}
int removeData(){
   int data = intArray[front++];
   if(front == MAX) {
      front = 0;
   }
   itemCount--;
   return data;
}
void insert(int data){
   if(!isFull()) {
      if(rear == MAX-1) {
         rear = -1;
      }
      intArray[++rear] = data;
      itemCount++;
   }
}
int main(){
   insert(3);
   insert(5);
   insert(9);
   insert(1);
   insert(12);
   insert(15);
   printf("Queue: ");
   while(!isEmpty()) {
      int n = removeData();
      printf("%d ",n);
   }
}

Queue: 3 5 9 1 12 15

#include <iostream>
#include <string>
#define MAX 6
int intArray[MAX];
int front = 0;
int rear = -1;
int itemCount = 0;
bool isFull(){
   return itemCount == MAX;
}
bool isEmpty(){
   return itemCount == 0;
}
int removeData(){
   int data = intArray[front++];
   if(front == MAX) {
      front = 0;
   }
   itemCount--;
   return data;
}
void insert(int data){
   if(!isFull()) {
      if(rear == MAX-1) {
         rear = -1;
      }
      intArray[++rear] = data;
      itemCount++;
   }
}
int main(){
   insert(3);
   insert(5);
   insert(9);
   insert(1);
   insert(12);
   insert(15);
   printf("Queue: ");
   while(!isEmpty()) {
      int n = removeData();
      printf("%d ",n);
   }
}

Queue: 3 5 9 1 12 15

import java.util.*;
public class Demo{
static final int MAX = 6;
static int intArray[] = new int[MAX];
static int front = 0;
static int rear = -1;
static int itemCount = 0;
public static boolean isFull(){
   return itemCount == MAX;
}
public static boolean isEmpty(){
   return itemCount == 0;
}
public static int removeData(){
   int data = intArray[front++];
   if(front == MAX) {
      front = 0;
   }
   itemCount--;
   return data;
}
public static void insert(int data){
   if(!isFull()) {
      if(rear == MAX-1) {
         rear = -1;
      }
      intArray[++rear] = data;
      itemCount++;
   }
}
public static void main(String[] args){
   insert(3);
   insert(5);
   insert(9);
   insert(1);
   insert(12);
   insert(15);
   System.out.print("Queue: ");
   while(!isEmpty()) {
      int n = removeData();
      System.out.print(n + " ");
   }
  }
}

Queue: 3 5 9 1 12 15 

MAX = 6;
intArray = [0] * MAX
front = 0;
rear = -1;
itemCount = 0;
def isFull():
    return itemCount == MAX
def isEmpty():
    return itemCount == 0
def removeData():
    data = intArray[front+1]
    if(front == MAX):
        front = 0
    itemCount-1
    return data
def insert(data):
    global rear, itemCount
    if(not isFull()):
        if(rear == MAX-1):
            rear = -1
        rear = rear + 1
        intArray[rear] = data
        itemCount+1
insert(3);
insert(5);
insert(9);
insert(1);
insert(12);
insert(15);
print("Queue: ")
for i in range(MAX):
    print(intArray[i], end = " ")
while(not isEmpty()):
    n = removeData()
    print(n, end = " ")

Queue: 
3 5 9 1 12 15

Queue Deletion Operation: dequeue()

The dequeue() is a data manipulation operation that is used to remove elements from the stack. The following algorithm describes the dequeue() operation in a simpler way.

Algorithm

1. START
2. Check if the queue is empty.
3. If the queue is empty, produce underflow error and exit.
4. If the queue is not empty, access the data where front 
   is pointing.
5. Increment front pointer to point to the next available 
   data element.
6. Return success.
7. END

Example

Following are the implementations of this operation in various programming languages −

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <stdbool.h>
#define MAX 6
int intArray[MAX];
int front = 0;
int rear = -1;
int itemCount = 0;
bool isFull(){
   return itemCount == MAX;
}
bool isEmpty(){
   return itemCount == 0;
}
void insert(int data){
   if(!isFull()) {
      if(rear == MAX-1) {
         rear = -1;
      }
      intArray[++rear] = data;
      itemCount++;
   }
}
int removeData(){
   int data = intArray[front++];
   if(front == MAX) {
      front = 0;
   }
   itemCount--;
   return data;
}
int main(){
   int i;
   
   /* insert 5 items */
   insert(3);
   insert(5);
   insert(9);
   insert(1);
   insert(12);
   insert(15);
   printf("Queue: ");
   for(i = 0; i < MAX; i++)
      printf("%d ", intArray[i]);

   // remove one item
   int num = removeData();
   printf("\nElement removed: %d\n",num);
   printf("Updated Queue: ");
   while(!isEmpty()) {
      int n = removeData();
      printf("%d ",n);
   }
}

Queue: 3 5 9 1 12 15 
Element removed: 3
Updated Queue: 5 9 1 12 15 

#include <iostream>
#include <string>
#define MAX 6
int intArray[MAX];
int front = 0;
int rear = -1;
int itemCount = 0;
bool isFull(){
   return itemCount == MAX;
}
bool isEmpty(){
   return itemCount == 0;
}
void insert(int data){
   if(!isFull()) {
      if(rear == MAX-1) {
         rear = -1;
      }
      intArray[++rear] = data;
      itemCount++;
   }
}
int removeData(){
   int data = intArray[front++];
   if(front == MAX) {
      front = 0;
   }
   itemCount--;
   return data;
}
int main(){
   int i;
   
   /* insert 5 items */
   insert(3);
   insert(5);
   insert(9);
   insert(1);
   insert(12);
   insert(15);
   printf("Queue: ");
   for(i = 0; i < MAX; i++)
      printf("%d ", intArray[i]);
   
   // remove one item
   int num = removeData();
   printf("\nElement removed: %d\n",num);
   printf("Updated Queue: ");
   while(!isEmpty()) {
      int n = removeData();
      printf("%d ",n);
   }
}

Queue: 3 5 9 1 12 15 
Element removed: 3
Updated Queue: 5 9 1 12 15 

public class Demo{
static final int MAX = 6;
static int intArray[] = new int[MAX];
static int front = 0;
static int rear = -1;
static int itemCount = 0;
public static boolean isFull(){
   return itemCount == MAX;
}
public static boolean isEmpty(){
   return itemCount == 0;
}
public static void insert(int data){
   if(!isFull()) {
      if(rear == MAX-1) {
         rear = -1;
      }
      intArray[++rear] = data;
      itemCount++;
   }
}
public static int removeData(){
   int data = intArray[front++];
   if(front == MAX) {
      front = 0;
   }
   itemCount--;
   return data;
}
public static void main(String[] args){
   int i;
   /* insert 5 items */
   insert(3);
   insert(5);
   insert(9);
   insert(1);
   insert(12);
   insert(15);
   System.out.print("Queue: ");
   for(i = 0; i < MAX; i++)
      System.out.print(intArray[i] + " ");
   
   // remove one item
   int num = removeData();
   System.out.print("\nElement removed: " + num);
   System.out.print("\nUpdated Queue: ");
   while(!isEmpty()) {
      int n = removeData();
      System.out.print(n + " ");
   }
  }
}

Queue: 3 5 9 1 12 15 
Element removed: 3
Updated Queue: 5 9 1 12 15 

MAX = 6
intArray = [0] * MAX
front = 0
rear = -1
itemCount = 0
def isFull():
    return itemCount == MAX
def isEmpty():
    return itemCount == 0
def insert(data):
    global rear, itemCount
    if not isFull():
        if rear == MAX-1:
            rear = -1
        rear += 1
        intArray[rear] = data
        itemCount += 1
def removeData():
    global front, itemCount
    data = intArray[front]
    if front == MAX-1:
        front = 0
    else:
        front += 1
    itemCount -= 1
    return data
insert(3);
insert(5);
insert(9);
insert(1);
insert(12);
insert(15);
print("Queue: ")
for i in range(MAX):
    print(intArray[i], end = " ")
num = removeData()
print("\nElement removed: ", num)
print("Updated Queue: ")
while(not isEmpty()):
    n = removeData()
    print(n, end = " ")

Queue: 
3 5 9 1 12 15 
Element removed:  3
Updated Queue: 
5 9 1 12 15

Queue - The peek() Operation

The peek() is an operation which is used to retrieve the frontmost element in the queue, without deleting it. This operation is used to check the status of the queue with the help of the pointer.

Algorithm

1. START
2. Return the element at the front of the queue
3. END

Example

Following are the implementations of this operation in various programming languages −

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <stdbool.h>
#define MAX 6
int intArray[MAX];
int front = 0;
int rear = -1;
int itemCount = 0;
int peek(){
   return intArray[front];
}
bool isFull(){
   return itemCount == MAX;
}
void insert(int data){
   if(!isFull()) {
      if(rear == MAX-1) {
         rear = -1;
      }
      intArray[++rear] = data;
      itemCount++;
   }
}
int main(){
   int i;
   
   /* insert 5 items */
   insert(3);
   insert(5);
   insert(9);
   insert(1);
   insert(12);
   insert(15);
   printf("Queue: ");
   for(i = 0; i < MAX; i++)
      printf("%d ", intArray[i]);
   printf("\nElement at front: %d\n",peek());
}

Queue: 3 5 9 1 12 15 
Element at front: 3

#include <iostream>
#include <string>
#define MAX 6
int intArray[MAX];
int front = 0;
int rear = -1;
int itemCount = 0;
int peek(){
   return intArray[front];
}
bool isFull(){
   return itemCount == MAX;
}
void insert(int data){
   if(!isFull()) {
      if(rear == MAX-1) {
         rear = -1;
      }
      intArray[++rear] = data;
      itemCount++;
   }
}
int main(){
   int i;
   /* insert 5 items */
   insert(3);
   insert(5);
   insert(9);
   insert(1);
   insert(12);
   insert(15);
   printf("Queue: ");
   for(i = 0; i < MAX; i++)
      printf("%d ", intArray[i]);
   printf("\nElement at front: %d\n",peek());
}

Queue: 3 5 9 1 12 15 
Element at front: 3

public class Demo{
final static int MAX = 6;
static int intArray[] = new int[MAX];
static int front = 0;
static int rear = -1;
static int itemCount = 0;
public static int peek(){
   return intArray[front];
}
public static boolean isFull(){
   return itemCount == MAX;
}
public static void insert(int data){
   if(!isFull()) {
      if(rear == MAX-1) {
         rear = -1;
      }
      intArray[++rear] = data;
      itemCount++;
   }
}
public static void main(String[] args){
   int i;
   /* insert 5 items */
   insert(3);
   insert(5);
   insert(9);
   insert(1);
   insert(12);
   insert(15);
   System.out.print("Queue: ");
   for(i = 0; i < MAX; i++)
      System.out.print(intArray[i] + " ");
   System.out.print("\nElement at front: " + peek());
  }
}

Queue: 3 5 9 1 12 15 
Element at front: 3

MAX = 6
intArray = [0] * MAX
front = 0
rear = -1
itemCount = 0
def peek():
    return intArray[front]
def isFull():
    return itemCount == MAX
def insert(data):
    global rear, itemCount
    if(not isFull()):
        if(rear == MAX-1):
            rear = -1
        rear  = rear + 1
        intArray[rear] = data
        itemCount+1
insert(3);
insert(5);
insert(9);
insert(1);
insert(12);
insert(15);
print("Queue: ")
for i in range(MAX):
    print(intArray[i], end = " ")
print("\nElement at front: ", peek())

Queue: 
3 5 9 1 12 15 
Element at front:  3

Queue - The isFull() Operation

The isFull() operation verifies whether the stack is full.

Algorithm

1. START
2. If the count of queue elements equals the queue size,
   return true
3. Otherwise, return false
4. END

Example

Following are the implementations of this operation in various programming languages −

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <stdbool.h>
#define MAX 6
int intArray[MAX];
int front = 0;
int rear = -1;
int itemCount = 0;
bool isFull(){
   return itemCount == MAX;
}
void insert(int data){
   if(!isFull()) {
      if(rear == MAX-1) {
         rear = -1;
      }
      intArray[++rear] = data;
      itemCount++;
   }
}
int main(){
   int i;
   /* insert 5 items */
   insert(3);
   insert(5);
   insert(9);
   insert(1);
   insert(12);
   insert(15);
   printf("Queue: ");
   for(i = 0; i < MAX; i++)
      printf("%d ", intArray[i]);
   printf("\n");
   if(isFull()) {
      printf("Queue is full!\n");
   }
}

Queue: 3 5 9 1 12 15 
Queue is full!

#include <iostream>
#include <string>
#define MAX 6
int intArray[MAX];
int front = 0;
int rear = -1;
int itemCount = 0;
bool isFull(){
   return itemCount == MAX;
}
void insert(int data){
   if(!isFull()) {
      if(rear == MAX-1) {
         rear = -1;
      }
      intArray[++rear] = data;
      itemCount++;
   }
}
int main(){
   int i;
   
   /* insert 5 items */
   insert(3);
   insert(5);
   insert(9);
   insert(1);
   insert(12);
   insert(15);
   printf("Queue: ");
   for(i = 0; i < MAX; i++)
      printf("%d ", intArray[i]);
   printf("\n");
   if(isFull()) {
      printf("Queue is full!\n");
   }
}

Queue: 3 5 9 1 12 15 
Queue is full!

import java.io.*;
public class QueueExample {
   static private int intArray[];
   private int front;
   private int rear;
   private int itemCount;
   static private int MAX;
   QueueExample(int size) {
      intArray = new int[size];
      front = 0;
      rear = -1;
      MAX = size;
      itemCount = 0;
   }
   public boolean isFull() {
      return itemCount == MAX;
   }
   public void insert(int key) {
      if(!isFull()) {
         if(rear == MAX-1) {
            rear = -1;
         }
         intArray[++rear] = key;
         itemCount++;
      }
   }
   public static void main (String[] args) {
      QueueExample q = new QueueExample(5);
      q.insert(1); // inserting 1 in the stack
      q.insert(2);
      q.insert(3);
      q.insert(4);
      q.insert(5);
      System.out.print("Queue: ");
      for(int i = 0; i<MAX; i++){
          System.out.print(intArray[i] + " ");
      }
      if(q.isFull()){
	  System.out.print("\nQueue is full!");
   }
  }
}

Queue: 1 2 3 4 5 
Queue is full!

#python code for isFull in Queue
MAX = 6
intArray = [None] * MAX
front = 0
rear = -1
itemCount = 0

def isFull():
    return itemCount == MAX

def insert(data):
    global rear, itemCount
    if not isFull():
        if rear == MAX-1:
            rear = -1
        rear += 1
        intArray[rear] = data
        itemCount += 1
#inserting 5 items into the Queue
insert(3)
insert(5)
insert(9)
insert(1)
insert(12)
insert(15)
print("Queue: ", end="")
for i in range(MAX):
    print(intArray[i], end=" ")
print()
if isFull():
    print("Queue is full!")

Queue: 3 5 9 1 12 15 
Queue is full!

Queue - The isEmpty() operation

The isEmpty() operation verifies whether the stack is empty. This operation is used to check the status of the stack with the help of top pointer.

Algorithm

1. START
2. If the count of queue elements equals zero, return true
3. Otherwise, return false
4. END

Example

Following are the implementations of this operation in various programming languages −

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <stdbool.h>
#define MAX 6
int intArray[MAX];
int front = 0;
int rear = -1;
int itemCount = 0;
bool isEmpty(){
   return itemCount == 0;
}
int main(){
   int i;
   printf("Queue: ");
   for(i = 0; i < MAX; i++)
      printf("%d ", intArray[i]);
   printf("\n");
   if(isEmpty()) {
      printf("Queue is Empty!\n");
   }
}

Queue: 0 0 0 0 0 0 
Queue is Empty!

#include <iostream>
#include <string>
#define MAX 6
int intArray[MAX];
int front = 0;
int rear = -1;
int itemCount = 0;
bool isEmpty(){
   return itemCount == 0;
}
int main(){
   int i;
   printf("Queue: ");
   for(i = 0; i < MAX; i++)
      printf("%d ", intArray[i]);
   printf("\n");
   if(isEmpty()) {
      printf("Queue is Empty!\n");
   }
}

Queue: 0 0 0 0 0 0 
Queue is Empty!

public class Demo{
final static int MAX = 6;
static int intArray[] = new int[MAX];
static int front = 0;
static int rear = -1;
static int itemCount = 0;
public static boolean isEmpty(){
   return itemCount == 0;
}
public static void main(String[] args){
   int i;
   System.out.print("Queue: ");
   for(i = 0; i < MAX; i++)
      System.out.print(intArray[i] + " ");
   if(isEmpty()) {
      System.out.print("\nQueue is Empty!");
   }
 }
}

Queue: 0 0 0 0 0 0 
Queue is Empty!

#python code for isFull in Queue
MAX = 6
intArray = [None] * MAX
front = 0
rear = -1
itemCount = 0

def isEmpty():
    return itemCount == 0

print("Queue: ", end="")
for i in range(MAX):
    print(intArray[i], end=" ")
print()
if isEmpty():
    print("Queue is empty!")

Queue: None None None None None None 
Queue is empty!

Queue Complete Implementation

Following are the complete implementations of Queue in various programming languages −

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <stdbool.h>
#define MAX 6
int intArray[MAX];
int front = 0;
int rear = -1;
int itemCount = 0;
int peek(){
   return intArray[front];
}
bool isEmpty(){
   return itemCount == 0;
}
bool isFull(){
   return itemCount == MAX;
}
int size(){
   return itemCount;
}
void insert(int data){
   if(!isFull()) {
      if(rear == MAX-1) {
         rear = -1;
      }
      intArray[++rear] = data;
      itemCount++;
   }
}
int removeData(){
   int data = intArray[front++];
   if(front == MAX) {
      front = 0;
   }
   itemCount--;
   return data;
}
int main(){
   
   /* insert 5 items */
   insert(3);
   insert(5);
   insert(9);
   insert(1);
   insert(12);
   insert(15);
   printf("Queue size: %d", size());
   printf("\nQueue: ");
   for(int i = 0; i < MAX; i++){
       printf("%d ", intArray[i]);
   }
   if(isFull()) {
      printf("\nQueue is full!");
   }

   // remove one item
   int num = removeData();
   printf("\nElement removed: %d", num);
   printf("\nSize of Queue after deletion: %d", size());
   printf("\nElement at front: %d", peek());
}

Queue size: 6
Queue: 3 5 9 1 12 15 
Queue is full!
Element removed: 3
Size of Queue after deletion: 5
Element at front: 5

#include <iostream>
using namespace std;
#include <string>
#define MAX 6
int intArray[MAX];
int front = 0;
int rear = -1;
int itemCount = 0;
int peek(){
   return intArray[front];
}
bool isEmpty(){
   return itemCount == 0;
}
bool isFull(){
   return itemCount == MAX;
}
int size(){
   return itemCount;
}
void insert(int data){
   if(!isFull()) {
      if(rear == MAX-1) {
         rear = -1;
      }
      intArray[++rear] = data;
      itemCount++;
   }
}
int removeData(){
   int data = intArray[front++];
   if(front == MAX) {
      front = 0;
   }
   itemCount--;
   return data;
}
int main(){
   
   /* insert 5 items */
   insert(3);
   insert(5);
   insert(9);
   insert(1);
   insert(12);
   insert(15);
   cout<<"Queue size: "<<size();
   cout<<"\nQueue: ";
   for(int i = 0; i < MAX; i++){
       cout<<intArray[i]<<" ";
   }
   if(isFull()) {
      cout<<"\nQueue is full!";
   }

   // remove one item
   int num = removeData();
   cout<<"\nElement removed: "<<num;
   cout<<"\nQueue size after deletion: "<<size();
   cout<<"\nElement at front: " <<peek();
}

Queue size: 6
Queue: 3 5 9 1 12 15 
Queue is full!
Element removed: 3
Queue size after deletion: 5
Element at front: 5

public class Demo{
final static int MAX = 6;
static int intArray[] = new int[MAX];
static int front = 0;
static int rear = -1;
static int itemCount = 0;
public static int peek(){
   return intArray[front];
}
public static boolean isEmpty(){
   return itemCount == 0;
}
public static boolean isFull(){
   return itemCount == MAX;
}
public static int size(){
   return itemCount;
}
public static void insert(int data){
   if(!isFull()) {
      if(rear == MAX-1) {
         rear = -1;
      }
      intArray[++rear] = data;
      itemCount++;
   }
}
public static int removeData(){
   int data = intArray[front++];
   if(front == MAX) {
      front = 0;
   }
   itemCount--;
   return data;
}
public static void main(String[] args){
   
   /* insert 5 items */
   insert(3);
   insert(5);
   insert(9);
   insert(1);
   insert(12);
   insert(15);
   System.out.print("Queue size: " + size());
   System.out.print("\nQueue: ");
   for(int i = 0; i < MAX; i++){
       System.out.print(intArray[i] + " ");
   }
   if(isFull()) {
      System.out.print("\nQueue is full!");
   }

   // remove one item
   int num = removeData();
   System.out.print("\nElement removed: " + num);
   System.out.print("\nQueue size after deletion: " + size());
   System.out.print("\nElement at front: " + peek());
 }
}

Queue size: 6
Queue: 3 5 9 1 12 15 
Queue is full!
Element removed: 3
Queue size after deletion: 5
Element at front: 5

MAX = 6
intArray = [0] * MAX
front = 0
rear = -1
itemCount = 0
def peek():
    return intArray[front]

def isEmpty():
    return itemCount == 0

def isFull():
    return itemCount == MAX

def size():
    return itemCount

def insert(data):
    global rear, itemCount
    if not isFull():
        if rear == MAX-1:
            rear = -1
        rear += 1
        intArray[rear] = data
        itemCount += 1

def removeData():
    global front, itemCount
    data = intArray[front]
    if front == MAX-1:
        front = 0
    else:
        front += 1
    itemCount -= 1
    return data

def main():
    insert(3)
    insert(5)
    insert(9)
    insert(1)
    insert(12)
    insert(15)
    print("Queue size: ", size())
    print("Queue: ")
    for i in range(MAX):
        print(intArray[i], end = " ")
    if isFull():
        print("\nQueue is full!")
    num = removeData()
    print("Element removed: ", num)
    print("Queue size after deletion: ", size())
    print("Element at front: ", peek())

main()

Queue size:  6
Queue: 
3 5 9 1 12 15 
Queue is full!
Element removed:  3
Queue size after deletion:  5
Element at front:  5

Queue Implementation in C

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