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Introduction to Stack

As there are different types of books available on the rack or in the library, the moment they become handful, we start arranging them neatly. Stack of books or stack of dishes is the examples of stack in our daily life. Stacks are used in compilers in passing an expression by recursion; in memory management in operating system; etc.

Stack is a linear data structure and very much useful in various applications of computer science. Implementation of majority of the systems programs are simplified using this data structure.


DEFINITION

A stack is an ordered, collection of homogeneous data element where the insertion and deletion operations take place at one end only i.e., data is stored and retrieved in a Last In First Out (LIFO) order.




FIGURE 3.1 Schematic diagram of a stack

Like array and linked list, stack is also a linear data structure but the only difference is that in case of former two, insertion and deletion operations can take at place at any position. The insertion and deletion operations in case of stack are specially termed as PUSH and POP respectively and the position of the stack where these operations are performed is known as TOP of the stack. An element in a stack is termed as ITEM. The maximum number elements that a stack can accommodate are termed as SIZE.



OPERATIONS ON STACK

1. Algorithm for PUSH operation

Input: The new ITEM to be pushed onto it.
Output: A stack with newly published ITEM at the TOP position
Data structure: An array A with TOP as the pointer.

Steps:

1. [Check for stack overflow]
                     1. If top = size -1 then
                     2. Print “ Stack is full”
                     3. Exit
2. [Increment top pointer by one]
                     1. Top = Top + 1
3. [Perform Insertion]
                     1. Stack [Top] = value
4. Stop

2. Algorithm for POP operation

Input: A stack with elements.
Output: Removes an ITEM from the top of the stack if it is not empty.
Data structure: An array A with TOP as the pointer.

Steps:

1. [Check for stack underflow]
                     If top = -1
                     Print “Stack is empty”
                     Exit

2. [Remove the top information]
                     Value = stack[top]
                     Top = top – 1
3. Stop

3. Algorithm for STATUS operation

Input: A stack with elements.
Output: States whether it is empty or full, available free space and item at TOP.
Data structure: An array A with TOP as the pointer.

Steps:

1. if(top = -1) then
                     1. Print “ Stack is empty”
2. else if(top = size -1) then
                     1. Print “ stack is full”
3. else
                     1. Freespace = size – 1 – top
                     2. Print “ Available space “, freespace
4. endif
5. stop



Compiled By: Chaudhary Amit V.
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