Data Structures - Stacks and Queues in Python

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Implementing Stack in Python using a List

def push(stack, element):
    stack.append(element)
    top = len(stack) - 1

def pop(stack):
    if stack == []:
        return 'UNDERFLOW'
    element = stack.pop()
    if len(stack) == 0:
        top = None
    else:
        top = len(stack) - 1
    return element

def peek(stack):
    if stack == []:
        return 'UNDERFLOW'
    top = len(stack) - 1
    return stack[top]

def display(stack):
    if stack == []:
        print('STACK EMPTY')
    else:
        top = len(stack) - 1
        print(str(stack[top]) + "<--top")
        for i in range(top - 1, -1, -1):
            print(stack[i])

stack = []
top = None
while True:
    print('1. PUSH ELEMENT')
    print('2. POP ELEMENT')
    print('3. PEEK STACK')
    print('4. DISPLAY ELEMENTS')
    choice = int(input('ENTER YOUR CHOICE: '))
    if choice == 1:
        element = int(input('Element to be pushed: '))
        push(stack, element)
    elif choice == 2:
        element = pop(stack)
        if element == 'UNDERFLOW':
            print('STACK UNDERFLOW!')
        else:
            print(str(element) + ' POPPED')
    elif choice == 3:
        element = peek(stack)
        if element == 'UNDERFLOW':
            print('STACK EMPTY')
        else:
            print('TOPMOST ELEMENT: ' + str(element))
    elif choice == 4:
        display(stack)
    else:
        print('Bye!')
        break

Implementing Queue in Python using a List

def enqueue(queue, element):
    queue.append(element)
    if len(queue) == 1:
        front = 0
        rear = 0
    else:
        rear = len(queue) - 1

def dequeue(queue):
    if queue == []:
        return 'UNDERFLOW'
    element = queue.pop(0)
    if len(queue) == 0:
        front = None
        rear = None
    return element

def peek(queue):
    if queue == []:
        return 'UNDERFLOW'
    return queue[0]

def display(queue):
    if queue == []:
        print('QUEUE EMPTY')
    elif len(queue) == 1:
        print(str(queue[0]) + "<---front, rear")
    else:
        front = 0
        rear = len(queue) - 1
        print(str(queue[front]) + "<--front")
        for i in range(1, rear):
            print(queue[i])
        print(str(queue[rear]) + "<--rear")

queue = []
front = None
rear = None
while(True):
    print('1. ENQUEUE')
    print('2. DEQUEUE')
    print('3. PEEK QUEUE')
    print('4. DISPLAY ELEMENTS')
    choice = int(input('ENTER YOUR CHOICE: '))
    if choice == 1:
        element = int(input('Element to enqueue: '))
        enqueue(queue, element)
    elif choice == 2:
        element = dequeue(queue)
        if element == 'UNDERFLOW':
            print('QUEUE UNDERFLOW!')
        else:
            print(str(element) + ' REMOVED')
    elif choice == 3:
        element = peek(queue)
        if element == 'UNDERFLOW':
            print('QUEUE EMPTY')
        else:
            print('FIRST ELEMENT: ' + str(element))
    elif choice == 4:
        display(queue)
    else:
        print('Bye!')
        break

Implementing Stack using Class and Object

class stack:
    def __init__(self):
        self.s = []
        self.top = -1
    def push(self, data):
        self.s.append(data)
        self.top = self.top + 1
    def pop(self):
        if self.top == -1:
            return -9999
        self.top = self.top - 1
        return self.s.pop()
    def peek(self):
        if self.top == -1:
            print('Stack empty!')
        else:
            data = self.s[self.top]
            print('Topmost element: ' + str(data))
    def display(self):
        if self.top == -1:
            print('Stack empty!')
        else:
            print(str(self.s[self.top]) + '<--top')
            for i in range(self.top - 1, -1, -1):
                print(self.s[i])
my_stack = stack()
while True:
    print('1. Push element')
    print('2. Pop element')
    print('3. Peek stack')
    print('4. Display elements')
    choice = int(input('Enter your choice: '))
    if choice == 1:
        data = int(input('Element to be pushed: '))
        my_stack.push(data)
    elif choice == 2:
        d = my_stack.pop()
        if d == -9999:
            print('Stack underflow!')
        else:
            print(str(d) + ' popped!')
    elif choice == 3:
        my_stack.peek()
    elif choice == 4:
        my_stack.display()
    else:
        print('Bye!')
        break

Implementing Queue using Class and Object

class queue:
    def __init__(self):
        self.q = []
        self.front = -1
        self.rear = -1
    def enqueue(self, data):
        self.q.append(data)
        if self.rear == -1:
            self.front = 0
            self.rear = 0
        else:
            self.rear = self.rear + 1
    def dequeue(self):
        if self.q == []:
            return -9999
        return self.q.pop(self.front)
    def display(self):
        if self.q == []:
            print('Queue empty!')
        elif self.front == self.rear:
            print(str(self.q[self.front]) + '<--front, rear')
        else:
            print(str(self.q[self.front]) + '<--front')
            for i in range(1, self.rear):
                print(str(self.q[i]))
            print(str(self.q[self.rear]) + '<--rear')

my_queue = queue()
while True:
    print('1. Enqueue')
    print('2. Dequeue')
    print('3. Display')
    choice = int(input('Enter your choice: '))
    if choice == 1:
        data = int(input('Data to be enqueued: '))
        my_queue.enqueue(data)
    elif choice == 2:
        data = my_queue.dequeue()
        if data == -9999:
            print('Queue empty!')
        else:
            print(str(data) + ' deleted')
    elif choice == 3:
        my_queue.display()
    else:
        print('Bye')
        break
Location: Asansol, West Bengal, India

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