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# Assignment 2: Evaluating Expressions using stacks

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Assignment 2: Evaluating Expressions using stacks
(http://www.openbookproject.net/books/pythonds/BasicDS/InfixPrefixandPostfixExpressions.
html#postfix-evaluation )
For this project, you will implement a program to evaluate an infix expression, the kind of expression used in
standard arithmetic. The program will do this is two steps, each step implemented as a separate Python
function:
1. Convert the infix expression to a postfix expression.
2. Evaluate the postfix expression
Both these steps make use of a stack in an interesting way. Many language translators (e.g. compiler) do
something similar to convert expressions into code that is easy to execute on a computer.
link above has a detailed discussion of this material that you may find helpful before building your
implementation. You program must use your own implementations of the Abstract Data Type Stack. Your
programs should work with either of the implementations you did for the Lab.
Note:
• For this assignment, in addition to the operators (+,-,*, /), your programs should handle the
exponentiation operator. In this assignment, the exponential operator will be denoted by ^. For
example, 2^3→8 and 3^2→9.
• The exponentiation operator has higher precedence than the * or /. For example, 2*3^2 = 2*9 = 18 not
6^2=36
• Also, the exponentiation operator associates from right to left. The other operators (+,-,*,/) associate
left to right. Think carefully about what this means. For example: 2^3^2 = 2^(3^2) = 2^9 = 512 not
(2^3)^2= 8^2=64
• Every class and function must come with a brief purpose statement in its docstring. In separate
comments you should explain the arguments and what is returned by the function or method.
• You must provide test cases all functions that do not involve I/O (Input/Output).
• However, you do not need to provide test cases for functions that only produce output or consume input,
but design (or refactor) your code so that the functions that involve I/O are as small as possible and only
handle I/O. Write data definitions, function signatures and purpose statements in order to get full credit.
• Use descriptive names for data structures and helper functions. You must name your files, classes, and
functions (methods) as specified below.
• You will not get full credit if you use built-in functions unless they are explicitly stated as being
allowed.
2 Functions
The following bullet points provide a guide to implement some of the data structures and individual functions of
• exp_eval.py (contains infix_to_postfix(infix_expr)and postfix_eval (postfix_expr)
• exp_eval_testcases.py

def infixToPostfix(infixexpr):
“””Converts an infix expression to an equivalent postfix expression”””
“””Input argument: a string containing an infix expression where tokens are
space separated. Tokens are either operators {+ – * / ^} or numbers
Returns a string containing a postfix expression “””
Use the split function to convert the input to a list of tokens
PostfixEval.py
def postfixEval(postfixExpr):
“””Evaluates a postfix expression”””
“””Input argument: a string containing a postfix expression where tokens
are space separated. Tokens are either operators {+ – * / ^} or numbers”””
3 Tests
• Write sufficient tests using unittest to ensure full functionality and correctness of your program. You do
not need to provide test cases for you stack since you did that for Lab 2.
• Make sure that your own tests test each branch of your program and any edge conditions. You do not
need to test for correct input in the assignment. You may assume that when
infixToPostfix(infixexpr)is called that infixexp is a well formatted, correct infix expression
containing only numbers and the specified operators and the tokens are space separated.
4. Submission
Submit two files to PolyLearn: exp_eval.py and exp_eval_testcases.py

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