Stack in python

A stack is a linear data structure that follows the principle of Last In First Out (LIFO) This means the last element inserted inside the stack is removed first.

LIFO Principle of Stack:

• putting an item on top of the stack is called push.
• removing an item is called pop.

Basic Operations of Stack:

• Push: Add an element to the top of a stack.
• Pop: Remove an element from the top of a stack.
• IsEmpty: Check if the stack is empty.
• IsFull: Check if the stack is full.
• Peek: Get the value of the top element without removing it.

Working of Stack Data Structure:

• A pointer called TOP is used to keep track of the top element in the stack.
• When initializing the stack, we set its value to -1 so that we can check if the stack is empty by comparing TOP == -1.
• On pushing an element, we increase the value of TOP and place the new element in the position pointed to by TOP.
• On popping an element, we return the element pointed to by TOP and reduce its value.
• Before pushing, we check if the stack is already full.
• Before popping, we check if the stack is already empty.

So,

• empyty stack => (empty bottle) => top = -1
• stack[1] => (push = 1) => top = 0
• stack[2] => (push = 2) => top = 1
• stack[3] => (push = 3) => top = 2
• return stack[2] => (pop = 3) => top = 1

Stack Implementations in Python

"""Stack code.""" def create_stack(): """Creating a stack.""" stack = [] return stack def check_empty(stack): """Creating an empty stack.""" return len(stack) == 0 def push(stack, item): """Adding items into the stack.""" stack.append(item) print("Pushing item " + item) def pop(stack): """Removing an element from the stack.""" if check_empty(stack): """Check for empty stack.""" return "stack is empty" return stack.pop() stack = create_stack() push(stack, str(1)) push(stack, str(2)) push(stack, str(3)) push(stack, str(4)) push(stack, str(5)) print("popped item: " + pop(stack)) print("stack after popping an elements " + str(stack)) 

the output

Pushing item 1 Pushing item 2 Pushing item 3 Pushing item 4 Pushing item 5 popped item: 5 stack after popping an elements ['1', '2', '3', '4']