This document discusses the POP operation in stacks. POP removes elements from the stack, deleting them in reverse order of how they were added. It works by removing the topmost element, making the next element the new top, and decrementing the top pointer. An example shows five elements A-E being pushed onto a stack and then popped off in reverse order E-A. The POP algorithm checks for underflow and removes the top element, decrementing the top pointer.

1. Lecture Note-6: Stack Operations 2015
By Rajesh K Shukla, HOD, Department of CSE, SIRT Bhopal
Downloaded from www.RajeshkShukla.com
Stack Operations
There are two basic operations that are usually performed with stacks.
1. PUSH
2. POP
Let us discuss POP Operation in detail.
POP Operation
POP Operation is basically used to remove the elements from the stack. This is the process used to delete
the elements from the stack. So when we say pop an element we mean delete an element from the stack.
Suppose we want to remove all the five elements from the stack shown in figure 3.6, the graphical
representation of such removal from the Stack is shown here in figure 3.7
The most recently inserted element i.e. E is at the Top so it is deleted first from the stack. The second to top
element i.e. D is deleted next because the previous top element E is removed so the next older item D in the
stack becomes the new Top element. The value of Top is changed after the deletion in Stack takes place. In
this way the element inserted first i.e. A is deleted last from the stack. When the last item in the stack is
deleted the stack is set to empty state. The elements from the stack are always retrieved in the reverser order
as we can see from figure 3.7 the elements are removed in the reverse order (E, D, C, B, A) in which they
were pushed (A, B, C, D, E) into the stack as shown in figure 3.6. Removal of an element depends upon its
position in the stack; you have to first remove all the elements above it. Therefore you cannot delete B unless
all the elements above it i.e. C, D and E are removed. We can see in the figure 3.7 that after every pop
operation the stack Top is decremented by one. The most frequently accessible element in the stack is the
topmost element whereas the least accessible element is the bottom of the stack. When all the elements from
the stack are removed mean stack is empty then we cannot delete anything from the stack. This situation is
called Underflow. If the pop operation is performed in the empty stack then the stack underflow condition
occurs so one must test whether there is an element in the stack to be deleted. We can perform the pop
operation only when the stack is not empty. The algorithm for POP operation is given below as
ALGORITHM-2. Let STACK is an array of MAX size and we want to delete the ELEMENT from the
STACK
ALGORITHM-2: POP (STACK, MAX, TOP, ITEM)
Step1. Check for Underflow
If Top=-1 then Print “Underflow” and Return
Else
a. ELEMENT =STACK[Top] // Delete an ELEMENT from the STACK
b. Top=Top-1 // Decrement the Top by One
End if
Step2. Return
Fig 3.7: Graphical representation of Stack POP Operation
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Top=4(stack is given) Top=3(E is Removed) Top=2(D is Removed) Top=1(C is Removed) Top=0(B is Removed) Top=-1(A is Removed)