Stack

# Global
sample = [None for i in range(6)]
null_pointer = -1
head_pointer = null_pointer


# Initialising stack
def init_stack(Array):
    global head_pointer
    for i in range(len(Array)):
        Array[i] = None
    head_pointer = null_pointer
    print("Initialised Successfully")


# Push
def push(Array, Value):
    global head_pointer
    head_pointer += 1
    if head_pointer >= len(Array):
        print("ERROR! Stack is full")
    else:
        Array[head_pointer] = Value
        print("Pushed Successfully")


# Pop
def pop(Array):
    global head_pointer, null_pointer
    if head_pointer == null_pointer:
        print("ERROR! Stack is empty")
        return -1
    else:
        head_pointer -= 1
        print("Popped Successfully")
        return Array[head_pointer + 1]


# Display
def display(array):
    if head_pointer == null_pointer:
        print("Stack is empty")
    else:
        for i in range(len(array)-1, null_pointer, -1):
            if i == head_pointer:
                print(array[i], "\t <-- Head")
            else:
                print(array[i])


# Menu
status = False
while not status:
    print("Choice 1: Initialise Stack")
    print("Choice 2: Push an item")
    print("Choice 3: Pop an item")
    print("Choice 4: Display the stack")
    print("Choice 5: Terminate")
    print("")
    choice = input("Enter choice number: ")
    if choice == '1':
        init_stack(sample)
    elif choice == '2':
        print("")
        i = input("Enter data to be pushed: ")
        push(sample, i)
    elif choice == '3':
        print(pop(sample))
        input("")
    elif choice == '4':
        display(sample)
        input()
    elif choice == '5':
        break
    else:
        print("_______________")
        continue
    print("_______________")