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
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