Summary Stack Data Structure Notes

Stack Data Structure
A stack is an Abstract Data Type (ADT), commonly used in most programming
languages. It is named stack as it behaves like a real-world stack, for example – a
deck of cards or a pile of plates, etc.
A real-world stack allows operations at one end only. For example, we can place or
remove a card or plate from the top of the stack only. Likewise, Stack ADT allows all
data operations at one end only. At any given time, we can only access the top
element of a stack.
This feature makes it LIFO data structure. LIFO stands for Last-in-first-out. Here, the
element which is placed (inserted or added) last, is accessed first. In stack
terminology, insertion operation is called PUSH operation and removal operation is
called POP operation.

Stack Representation
The following diagram depicts a stack and its operations −




A stack can be implemented by means of Array, Structure, Pointer, and Linked List.
Stack can either be a fixed size one or it may have a sense of dynamic resizing.
Here, we are going to implement stack using arrays, which makes it a fixed size
stack implementation.

Basic Operations
Stack operations may involve initializing the stack, using it and then de-initializing it.
Apart from these basic stuffs, a stack is used for the following two primary operations
−

,  push() − Pushing (storing) an element on the stack.
 pop() − Removing (accessing) an element from the stack.
When data is PUSHed onto stack.
To use a stack efficiently, we need to check the status of stack as well. For the same
purpose, the following functionality is added to stacks −
 peek() − get the top data element of the stack, without removing it.
 isFull() − check if stack is full.
 isEmpty() − check if stack is empty.
At all times, we maintain a pointer to the last PUSHed data on the stack. As this
pointer always represents the top of the stack, hence named top. The top pointer
provides top value of the stack without actually removing it.
First we should learn about procedures to support stack functions −
peek()
Algorithm of peek() function −
begin procedure peek
return stack[top]
end procedure
Implementation of peek() function in C programming language −
Example
int peek() {
return stack[top];
}
isfull()
Algorithm of isfull() function −
begin procedure isfull

if top equals to MAXSIZE
return true
else
return false
endif

end procedure
Implementation of isfull() function in C programming language −
Example
bool isfull() {
if(top == MAXSIZE)
return true;
else
return false;
}