Simple command line PMPV calculator in Python

Author: Robert (Nayan) Sawyer
Date: Jan 24 2024

Description

A simple command line calculator that supports addition, subtraction, variables, and parentheses. This program was written for a homework assignment for COS 301: Programming Languages at the University of Maine taught by Sudarshan S. Chawathe.

Language specification

This assignment had intentionally vague instructions, however, I wrote my code to the following specifications:

The code accepts mathematical expressions that include plus, minus, variables, and parentheses (PMPV)
Variable names must meet python's criteria of letters, underscores, and numbers, so long as they don't start with a number.
Numbers must be positive or negative integers
Negative integers must be preceded by a space ' ' or an open paranthesis '('
Consecutive operators, dangling operators at the beginning or end of an expression, undefined variables, and mismatched parentheses are invalid syntax

Terminals and Backus-Naur Form

Terminals are numbers and identifiers (variable names), and ['=', '+', '-', '(', ')']

BNF:
<program> ::= <assignment> | <expression>
<assignment> ::= identifier = <expression>
<expression> ::= <expression> + <term> | 
                 <expression> - <term> | 
                 <term>
<term> ::= (<expression>) |  identifier  |  number

Input examples

input                                   output
3 + 5 - -2 - 2                          8
x = 3 + 5 - -2 - 2
y = x - (x - 2)
y                                       2
ans = (17 - (5 - 20)) - (1 - 11)
ans                                     42

ans = ans - (-42 - ans)
ans                                     126
(17-(5-20))-(1-11)                      42

How to run

You can run the program interactively with the command
python pmpv.py
Alternatively, you can pipe files in and out. On linux that can be acheived with the arrow operator
python pmpv.py < in.txt or python pmpv.py < in.txt > out.txt
On windows output redirection works the same way, but you need to use piping for input redirection. It is the same in essence, but a little more clunky
Get-Content in.txt | python pmpv.py > out.txt

Dependencies

This program uses python's built-in re module (regex), and there are no external dependencies

Diagnostics

The program sends error messages to stderr, so you can redirect that away if you don't want those messages.

Bugs and limitations

Typical behaviour is to ignore invalid inputs and send a warning to stderr. It is possible for the program to crash or throw an exception, but I have tested it extensively, and crashes during normal use are hopefully rare.

One issue with the program is that behaviour when given invalid input undefined. Some inputs will be ignored, but there may be some cases where the input is invalid, but the program interprets it anyway, and may give a mathematically incorrect result.

README.md