Difference Between Stack and Queue in Data Structure

Stack and Queue are the very important data structure in programming. Whether you are writing complex program or preparing for placement or getting into the career, you will come across questions related to the basic difference between stack and queue.

With stack and queue data structures, it is very easy to solve even complex programming questions.

For the sake of the simplicity, we can define stack and queue as the collections of the objects, just like an array data structure.

These are all the similarities between stack and queue. But they are not mean to be similar in any way.

Then, what is the difference between stack and queue?

Let’s start with definitions of Stack and Queue.

What is Stack?

The stack is a data structure where the user can add a data object any time. But the user can only remove the data which is added last.

The stack follows LIFO (Last In First out ) Mechanism. It means the object which is added last only can be removed.

The reverse is for the queue.

What is Queue?

Again it is data structure where the user can add any data object any time. But the user can only remove the data which is added first.

The queue follows FIFO (First In First Out) mechanism. It means an object which is added first only can be removed.

The removal of data order defines the use of these two data structures. Let’s see their uses with applications.

Applications of stack and queue:

These data structure would not be in our context if we don’t know where to use them.

Where is stack useful?

I use Stack when I need recently added object to be treated/processed first.

For example, if you are sitting in the restaurant, new patron get served first.

In the searching algorithm, one of the primary application of the stack is DFS (Depth-first search).

Where should I use the queue?

The queue is useful when I want to write a program where elements which I add first get first preference to treat/process.

The person enters restaurant first gets service first.

BFS is breadth-first search algorithm. It uses Queue as its prominent data structure.

Comparing Array with Stack and Queue:

Let’s talk about Array…

I know it’s not the part of this article. But it’s good for understanding, how stack and queue are different from the array.

I am not going to compare Array here. But it is a data structure where you can add or remove any object you need. It is possible by assigning an index to each element of the array and call that object by its index. It’s pretty simple, though.

Understanding Stack and Queue for Career point of view:

If you are preparing for GATE or any other competitive CS exam, you may be asked to find the valid sequence for the elements stored in stack or queue.

So understanding the difference between the operation of stack and queue is crucial.

Let’s see how to find the valid pattern for both stack and queue and the way stack and queue operates.

Difference Between Stack and Queue by its Operation:

What are the operations are involved in Stack?

Stack uses ‘top’ pointer which is always pointed to the most recent added element in the stack.

PUSH Operation on Stack:

The operation of adding an element to the stack is PUSH operation.

Steps:

• Find the top pointer of the stack.
• Add object to the top of the stack
• Increment the top pointer and point it to the newly added object.

POP Operation on Stack:

The operation of removing top elements from the stack is PUSH operation.

Steps:

• Check if the stack is not empty.
• Access the topmost object using the top pointer. (Save the object in a local variable to use in the application.)
• Remove the object from the top.
• Point the ‘top’ pointer to next object in the stack.

Even it is possible to implement stack operation using array.

What are the operations are involved in Queue?

Unlike stack data structure, queue requires two pointers so-called FRONT and REAR.

Here in above image, ‘Back’ pointer is same as ‘Rear’ pointer. So don’t confuse yourself.

Enqueue operation on Queue:

The operation of adding an element in the queue is Enqueue operation.

Steps:

• Find the REAR pointer of the queue.
• Increment the REAR pointer.
• Add new object at the REAR pointer.

Dequeue operation on Queue:

The operation of removing an element from the queue is Dequeue operation.

Steps:

• Ensure if the queue is not empty.
• Get access to the object pointed by the FRONT pointer.
• Remove the object. (Store in a local variable to use in the application.)
• Increment the FRONT pointer to point it to next front object.

Operation Bound:

The stack is bounded from one end. There is only one end to carry both push and pop operation.

The queue is free from both sides. One can dequeue element from one end and enqueue element from another end.

These operations are crucial to finding the difference between stack and queue.

Stack Vs Queue for Memory Usage:

Both data structure are useful to store the data objects. So it will take almost same size memory to store the same amount of data objects.

If you go down the further…

The stack needs only one pointer so-called top pointer. (It points to the topmost object in the stack.)

The queue requires two pointers. (One is FRONT and other is REAR pointer.)

An example of Stack:

When a program calls a subroutine, which calls another subroutine and so on… You have a stack to store these recursive subroutine calls. So the subroutine execution occurs in reverse order (LIFO).

An example of Queue:

Whenever the system receives interrupt call, the system adds it to the event queue. System services them in the same order as they appear.

Summarizing Comparison Table for Stack and Queue:

Related Read: difference between Min and Max Heap

Final Thought over Stack Vs Queue:

Needless to say, stack and queue have similarity as they stores the collections of data objects. But they are entirely different for their mechanism to access and remove data.

The stack is LIFO and Queue is FIFO data structure. Both are very useful in the context of writing a complex program. You have to be clever to understand the difference between stack and queue, whether you need stack or queue in your program.