Stack
A stack is an Abstract Data Type (ADT), commonly used in most programming languages. It is named stack as it behaves like a realworld stack, for example – a deck of cards or a pile of plates, etc.
A realworld 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 Lastinfirstout. 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 deinitializing 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;
}
isempty()
Algorithm of isempty() function −
begin procedure isempty
if top less than 1
return true
else
return false
endif
end procedure
Implementation of isempty() function in C programming language is slightly different. We initialize top at 1, as the index in array starts from 0. So we check if the top is below zero or 1 to determine if the stack is empty. Here's the code −
Example
bool isempty() {
if(top == 1)
return true;
else
return false;
}
Push Operation
The process of putting a new data element onto stack is known as a Push Operation. Push operation involves a series of steps −

Step 1 − Checks if the stack is full.

Step 2 − If the stack is full, produces an error and exit.

Step 3 − If the stack is not full, increments top to point next empty space.

Step 4 − Adds data element to the stack location, where top is pointing.

Step 5 − Returns success.
If the linked list is used to implement the stack, then in step 3, we need to allocate space dynamically.
Algorithm for PUSH Operation
A simple algorithm for Push operation can be derived as follows −
begin procedure push: stack, data
if stack is full
return null
endif
top ← top + 1
stack[top] ← data
end procedure
Implementation of this algorithm in C, is very easy. See the following code −
Example
void push(int data) {
if(!isFull()) {
top = top + 1;
stack[top] = data;
} else {
printf("Could not insert data, Stack is full.\n");
}
}
Pop Operation
Accessing the content while removing it from the stack, is known as a Pop Operation. In an array implementation of pop() operation, the data element is not actually removed, instead top is decremented to a lower position in the stack to point to the next value. But in linkedlist implementation, pop() actually removes data element and deallocates memory space.
A Pop operation may involve the following steps −

Step 1 − Checks if the stack is empty.

Step 2 − If the stack is empty, produces an error and exit.

Step 3 − If the stack is not empty, accesses the data element at which top is pointing.

Step 4 − Decreases the value of top by 1.

Step 5 − Returns success.
Algorithm for Pop Operation
A simple algorithm for Pop operation can be derived as follows −
begin procedure pop: stack
if stack is empty
return null
endif
data ← stack[top]
top ← top  1
return data
end procedure
Implementation of this algorithm in C, is as follows −
Example
int pop(int data) {
if(!isempty()) {
data = stack[top];
top = top  1;
return data;
} else {
printf("Could not retrieve data, Stack is empty.\n");
}
}