First class was posted in this question, third class will be posted in a different question, please refer to it, thank you:...

Section 2: The Calculator class


Implement a class that calculates mathematic expressions. The input passed to this Calculator is in

infix notation, which gets converted to postfix internally, then the postfix expression is evaluated

(we evaluate in postfix instead of infix because of how much simpler it is). More details about

infix to postfix conversion can be found in the video lectures.



This calculator should support numeric values, five arithmetic operators (+, –, *, /, ^), and

parenthesis. Follow the PEMDAS order of operations (you can define precedence of operators

with a dictionary or a helper method). Note that exponentiation is ** in Python.



You can assume that expressions will have tokens (operators, operands) separated by a single space.

For the case of negative numbers, you can assume the negative sign will be prefixed to the number.

The str.split() method can be helpful to isolate tokens. Expressions are considered invalid if they

meet any of the following criteria:

• Contains unsupported operators 4 $ 5

• Contains consecutive operators 4 * + 5

• Has missing operands 4 +

• Has missing operators 4 5

• Has unbalanced parenthesis ) 4 + 5 ( or ( 4 + 5 ) )

• Tries to do implied multiplication 3(5) instead of 3 * ( 5 )

• Includes a space before a negative number 4 * - 5 instead of 4 * -5



Make sure to have proper encapsulation of your code by using proper variable scopes and writing

other helper methods to generalize your code for processes such as string processing and input

validation. Do not forget to document your code.

As a suggestion, start by implementing your methods assuming the expressions are always valid,

that way you have the logic implemented and you only need to work on validating the expressions.





Attributes

Type Name Description

str __expr The expression this calculator will evaluate



Methods

Type Name Description

None setExpr(self, new_expr) Sets the expression for the calculator to evaluate

str getExpr(self) Getter method for the private expression attribute

bool _isNumber(self, aSring) Returns True if aSring can be converted to a float

str _getPostfix(self, expr) Converts an expression from infix to postfix

float calculate(self) Calculates the expression stored in this calculator

setExpr(self, new_expr)

Sets the expression for the calculator to evaluate. This method is already implemented for you.

Input (excluding self)

str new_expr The new expression (in infix) for the calculator to evaluate





getExpr(self)

Property method for getting the expression in the calculator. This method is already implemented

for you.

Output

str The value stored in __expr





_isNumber(self, txt)
Returns True if txt is a string that can be converted to a float, False otherwise. Note that the type

conversion float('4.56') returns 4.56 but float('4 56') raises an exception. A try/except

block could be useful here.

Input (excluding self)

str txt The string to check if it represents a number



Output

bool True if txt can be successfully casted to a float, False otherwise





_getPostfix(self, expr)
Converts an expression from infix to postfix. All numbers in the output must be represented as a

float. You must use the Stack defined in section 1 in this method, otherwise your code will not

receive credit. (Stack applications video lecture can be helpful here).

Input (excluding self)

str expr The expression in infix form



Output

str The expression in postfix form

None None is returned if the input is an invalid expression

calculate(self)
A property method that evaluates the infix expression saved in self.__expr. First convert the

expression to postfix, then use a stack to evaluate the output postfix expression. You must use the

Stack defined in section 1 in this method, otherwise your code will not receive credit.

Input (excluding self)

str txt The string to check if it represents a number



Output

float The result of the expression

None None is returned if the input is an invalid expression or if expression can’t be

computed