Skip to main content

Data Structures: Introduction to Queues

Queue is a linear data structure in which data can be added to one end and retrieved from the other. Just like the queue of the real world, the data that goes first into the queue is the first one to be retrieved. That is why queues are sometimes called as First-In-First-Out data structure.

In case of queues, we saw that data is inserted both from one end but in case of Queues; data is added to one end (known as REAR) and retrieved from the other end (known as FRONT).

The data first added is the first one to be retrieved while in case of queues the data last added is the first one to be retrieved.

A few points regarding Queues:

  1. Queues: It is a linear data structure; linked lists and arrays can represent it. Although representing queues with arrays have its shortcomings but due to simplicity, we will be representing queues with arrays in this article.

  2. Rear: A variable stores the index number in the array at which the new data will be added (in the queue).

  3. Front: It is a variable storing the index number in the array where the data will be retrieved.

Let us have look at the process of adding and retrieving data in the queue with the help of an example.

Suppose we have a queue represented by an array queue [10], which is empty to start with. The values of front and rear variable upon different actions are mentioned in {}.

queue [10]=EMPTY {front=-1, rear=0}

add (5)

Now, queue [10] = 5 {front=0, rear=1}

add (10)

Now, queue [10] = 5, 10 {front=0, rear=2}

retrieve () [It returns 5]

Now, queue [10] = 10 {front=1, rear=2}

retrieve () [now it returns 10]

Now, queue [10] is again empty {front=-1, rear=-1}

In this way, a queue like a stack, can grow and shrink over time.

Now have a look at the following example program that illustrates all this stuff:

  // -- A Queue Class in C++ --
  // example program in C++ to
  // illustrate queues represented
  // by arrays

  // macro to hold the max
  // number of elements
  // in the queue
  #define MAX 10

  // queue class
  class queue
   int arr[MAX];
   int front, rear;

   void add(int);
   int retrieve(void);
  // queue class ends

  // member functions
   // initialize index
   // variables

  void queue::add(int data)
    cout<<"QUEUE FULL!";

   // increase index
   // variable


  int queue::retrieve()
   int data;
    cout<<"QUEUE EMPTY!";
    return NULL;

   // if both index variables
   // point to the same location
   // then start afresh

   return data;
  // member functions ends

  void main(void)
   queue obj;
   int ch;
   int num;

    cout<<"1> ADD";
    cout<<"\n2> RETRIVE";
    cout<<"\n3> QUIT\n";


     case 1:
     cout<<"enter element:";


     case 2:
     cout<<"\n\nRetrieved: ";


Please do check back for updates!

Related Articles:

Popular posts from this blog

Fix For Toshiba Satellite "RTC Battery is Low" Error (with Pictures)

RTC Battery is Low Error on a Toshiba Satellite laptop "RTC Battery is Low..." An error message flashing while you try to boot your laptop is enough to panic many people. But worry not! "RTC Battery" stands for Real-Time Clock battery which almost all laptops and PCs have on their motherboard to power the clock and sometimes to also keep the CMOS settings from getting erased while the system is switched off.  It is not uncommon for these batteries to last for years before requiring a replacement as the clock consumes very less power. And contrary to what some people tell you - they are not rechargeable or getting charged while your computer or laptop is running. In this article, we'll learn everything about RTC batteries and how to fix the error on your Toshiba Satellite laptop. What is an RTC Battery? RTC or CMOS batteries are small coin-shaped lithium batteries with a 3-volts output. Most laptops use

The Best Way(s) to Comment out PHP/HTML Code

PHP supports various styles of comments. Please check the following example: <?php // Single line comment code (); # Single line Comment code2 (); /* Multi Line comment code(); The code inside doesn't run */ // /* This doesn NOT start a multi-line comment block /* Multi line comment block The following line still ends the multi-line comment block //*/ The " # " comment style, though, is rarely used. Do note, in the example, that anything (even a multi-block comment /* ) after a " // " or " # " is a comment, and /* */ around any single-line comment overrides it. This information will come in handy when we learn about some neat tricks next. Comment out PHP Code Blocks Check the following code <?php //* Toggle line if ( 1 ) {      // } else {      // } //*/ //* Toggle line if ( 2 ) {      // } else {      // } //*/ Now see how easy it is to toggle a part of PHP code by just removing or adding a single " / " from th

How to Create an HTML Form Linked with MySQL Database in PHP

If you're looking for example code and detailed discussion on how to create an HTML form that stores data in a MySQL database using PHP then this post might be what you're looking for. I assume that you're familiar with basic HTML, CSS, PHP coding, and  MySQL. I am going to divide this small project into two parts: The HTML form itself that takes input from the user and the PHP script that saves it into the database A table that displays the user-added data that has been saved in the database. We'll be dealing with the first part in this tutorial. Again I'd like to break this problem into a few parts so that it's easier for you to understand and probably gives you an insight into how breaking up a problem into smaller chunks can help make things clearer in your mind. Let's think about it, there is an HTML form (that is HTML code), then there is the PHP code that deals with the user-input data, and the MySQL database itself. For this tutorial, we'll b