Question

Q: Assistance in understanding and solving this example from Data Structure & Algorithms (Computer Science) with the steps of the solution to better understand, thanks.

##Using R studio language and code to use provided below, thanks.

In this assignment, we will implement a function to evaluate an arithmetic expression. You will use two stacks to keep track of things. The function is named ‘evaluate’, it takes a string as input parameter, and returns an integer. The input string represents an arithmetic expression, with each character in the string being a token, while the returned value should be the value of the expression.

Examples:

• evaluate ('1+2*3') ➔ 7

• evaluate ('1*2+3') ➔ 5

##This is the code provided to use

"""Basic example of an adapter class to provide a stack interface to

Python's list."""

class ArrayStack:

"""LIFO Stack implementation using a Python list as underlying

storage."""

def __init__ (self):

"""Create an empty stack."""

self._data = [] # nonpublic list instance

def __len__ (self):

"""Return the number of elements in the stack."""

return len (self._data)

def is_empty (self):

"""Return True if the stack is empty."""

return len (self._data) == 0

def push (self, e):

"""Add element e to the top of the stack."""

self._data.append (e) # new item stored at end of

list

def top (self):

"""Return (but do not remove) the element at the top of the stack.

Raise Empty exception if the stack is empty.

"""

if self.is_empty ():

raise Empty ('Stack is empty')

return self._data [-1] # the last item in the list

def pop (self):

"""Remove and return the element from the top of the stack (i.e.,

LIFO).

Raise Empty exception if the stack is empty.

"""

if self.is_empty ():

raise Empty ('Stack is empty')

return self._data.pop () # remove last item from list

class Empty (Exception):

"""Error attempting to access an element from an empty container."""

pass

if __name__ == '__main__':

S = ArrayStack () # contents: [ ]

S.push (5) # contents: [5]

S.push (3) # contents: [5, 3]

print (len (S)) # contents: [5, 3]; outputs 2

print (S.pop ()) # contents: [5]; outputs 3

print (S.is_empty ()) # contents: [5]; outputs False

print (S.pop ()) # contents: [ ]; outputs 5

print (S.is_empty ()) # contents: [ ]; outputs True

S.push (7) # contents: [7]

S.push (9) # contents: [7, 9]

print (S.top ()) # contents: [7, 9]; outputs 9

S.push (4) # contents: [7, 9, 4]

print (len (S)) # contents: [7, 9, 4]; outputs 3

print (S.pop ()) # contents: [7, 9]; outputs 4

S.push (6) # contents: [7, 9, 6]

S.push (8) # contents: [7, 9, 6, 8]

print (S.pop ()) # contents: [7, 9, 6]; outputs 8

Answer 1