Question

1. (based on 8.38) A stack is a sequence container type that, like a queue, supports very restrictive access methods: all insertions and removals are from one end of the stack, typically referred to as the top of the stack. A stack is often referred to as a list-in first-out (LIFO) container because the last item inserted is the first removed. Implement a Stack class. It should support the following methods/functions:
   1. _init__ - Can construct either an empty stack, or initialized with a list of items, the first item is at the bottom, the last is at the top.
   2. push() – take an item as input and push it on the top of the stack
   3. pop() – remove and return the item at the top of the stack
   4. isEmpty() – returns True if the stack is empty, False otherwise
   5. [] – return the item at a given location, [0] is at the bottom of the stack
   6. len() – return length of the stack

The object is to make this client code work:

'''

>>> s = Stack()

>>> s.push('apple')

>>> s

Stack(['apple'])

>>> s.push('pear')

>>> s.push('kiwi')

>>> s

Stack(['apple', 'pear', 'kiwi'])

>>> top = s.pop()

>>> top

'kiwi'

>>> s

Stack(['apple', 'pear'])

>>> len(s)

2

>>> s.isEmpty()

False

>>> s.pop()

'pear'

>>> s.pop()

'apple'

>>> s.isEmpty()

True

>>> s = Stack(['apple', 'pear', 'kiwi'])

>>> s = Stack(['apple', 'pear', 'kiwi'])

>>> s[0]

'apple'

>>>

'''

use python3.7

>>> s = Stack()
>>> s.push('apple')
>>> s
Stack(['apple'])
>>> s.push('pear')
>>> s.push('kiwi')
>>> s
Stack(['apple', 'pear', 'kiwi'])
>>> top = s.pop()
>>> top
'kiwi'
>>> s
Stack(['apple', 'pear'])
>>> len(s)
2
>>> s.isEmpty()
False
>>> s.pop()
'pear'
>>> s.pop()
'apple'
>>> s.isEmpty()
True
>>> s = Stack(['apple', 'pear', 'kiwi'])
>>> s = Stack(['apple', 'pear', 'kiwi'])
>>> s[0]
'apple'
>>>

check that Stacks constructed without a list remain distinct
if you receive an error on s2 then you probably need to use a
default argument of None in __init__
(see the Queue class developed in class for an example)

>>> s = Stack()
>>> s.push('apple')
>>> s
Stack(['apple'])
>>> s2 = Stack()
>>> s2.push('pear')
>>> s2 # if this fails - see the TEST file for explanation
Stack(['pear'])

Answer 1

SOURCE CODE:

*Please follow the comments to better understand the code.

**Please look at the Screenshot below and use this code to copy-paste.

***The code in the below screenshot is neatly indented for better understanding.

class Stack:
    # Constructor
    # Can construct either an empty stack,
    # or initialized with a list of items,
    # the first item is at the bottom, the last is at the top
    def __init__(self, items=None):
        if items is None:
            self.data = []
        else:
            self.data = items

    # take an item as input and push it on the top of the stack
    def push(self,item):
        self.data.append(item)

    # remove and return the item at the top of the stack
    def pop(self):
        return self.data.pop(-1)

    #  returns True if the stack is empty, False otherwise
    def isEmpty(self):
        return len(self.data)==0

    # gives the length of the stack
    def __len__(self):
        return len(self.data)

    # gives the item at location
    def __getitem__(self, index):
        return self.data[index]

    # string representation
    def __str__(self):
        return 'Stack('+str(self.data)+')'

    def __repr__(self):
        return self.__str__()

==============================

SCREENSHOT:

OUTPUT: