Introduction to Stacks

What are Stacks?

A Stack is a Linear Data Structure that follows a process of FILO or First In - Last Out.

To understand how a stack works, you can think of it as a pile of plates. If you were to add a plate, you would add it to the top of the pile, and if you were to remove a plate, you can only remove the top plate.

We will be using an array called stack for the demonstration and a variable top initialized as -1 to indicate an empty stack and let us assume that the stack has to be of size 5. The various operations that can be performed on a stack are:

Push

The push operation adds the value to the top of the stack. If the top value is the same as the size of the stack, if a push operation is attempted, it will lead to Stack Overflow indicating that you're trying to add a value past the allocated space. The function for the push operation:

def push():
    #let us assume the stack to be of size 5
    global top
    global stack 

# value from 0 to size-1, as top begins from -1
    if(top == 4):  
        print("Stack Overflow")
    else:
        a = int(input("Enter value"))
        stack.append(a)
        top+=1
        print(f"{a} pushed to stack")

Pop

The pop operation removes the value at the top of the stack. If the value of top is -1 (ie, the stack is empty) and the pop operation is attempted, it will lead to Stack Underflow indicating that you're trying to remove a value from an empty stack. The function for the pop operation:

def pop():
    global top
    global stack 

    if(top == -1):
        print("Stack Underflow")
    else:
        print(f"{stack[top]} has been popped")
        stack.pop(top)
        top -= 1

Peek

The Peek Operation displays the value at the top of the stack. The function for the peek operation is:

def peek():
    global top
    global stack 

    if(top == -1):
        print("Stack is empty")
    else:
        print(f"Top: {stack[top]}")

State of Stack

This function combines the three operations is_empty() which tells you whether or not the stack is empty, is_full() which tells you whether or not the stack is full. The function for the operation is:

def state_of_stack():
    if(top==4):
        print("Stack is full")
    elif(top == -1):
        print("Stack is empty")
    else:
        print("Stack is partially empty")

To simulate the functioning of the stack, the complete code is given below

import array as ar

## Global variables

stack = ar.array('i', [])
top = -1


def push():
    global top
    global stack 

    if(top == 4):
        print("Stack Overflow")
    else:
        a = int(input("Enter value"))
        stack.append(a)
        top+=1
        print(f"{a} pushed to stack")


def pop():
    global top
    global stack 

    if(top == -1):
        print("Stack Underflow")
    else:
        print(f"{stack[top]} has been popped")
        stack.pop(top)
        top -= 1


def peek():
    global top
    global stack 

    if(top == -1):
        print("Stack is empty")
    else:
        print(f"Top: {stack[top]}")


def state_of_stack():
    if(top==4):
        print("Stack is full")
    elif(top == -1):
        print("Stack is empty")
    else:
        print("Stack is partially empty")


## Driver Code
while(True):
    print("1.Push\n2.Pop\n3.Peek\n4.State of Stack\n5.Exit")

    ch = int(input())

    if(ch==1):
        push()
    elif(ch==2):
        pop()
    elif(ch==3):
        peek()
    elif(ch==4):
        state_of_stack()
    else:
        break

Note

Since we will be modifying the stack in the functions, we use the global value to modify the variables that are being used