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105 lines
4.3 KiB
Markdown
105 lines
4.3 KiB
Markdown
# Simple command line PMPV calculator in Python
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Author: Robert (Nayan) Sawyer\
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Date: Jan 24 2024
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### Description
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A simple command line calculator that supports addition,
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subtraction, variables, and parentheses. This program was
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written for a homework assignment for COS 301: Programming
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Languages at the University of Maine taught by Sudarshan S.
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Chawathe.
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### Language specification
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This assignment had intentionally vague instructions, however,
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I wrote my code to the following specifications:
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- The code accepts mathematical expressions that include plus,
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minus, variables, and parentheses (PMPV)
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- Variable names must meet python's criteria of letters,
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underscores, and numbers, so long as they don't start with a
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number.
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- Numbers must be positive or negative integers
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- Negative integers must be preceded by a space ' ' or an open
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paranthesis '('
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- Consecutive operators, dangling operators at the beginning or
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end of an expression, undefined variables, and mismatched
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parentheses are invalid syntax
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#### Terminals and Backus-Naur Form
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Terminals are numbers and identifiers (variable names), and
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['=', '+', '-', '(', ')']
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```
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BNF:
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<program> ::= <assignment> | <expression>
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<assignment> ::= identifier = <expression>
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<expression> ::= <expression> + <term> |
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<expression> - <term> |
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<term>
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<term> ::= (<expression>) | identifier | number
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```
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### Input examples
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```
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input output
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3 + 5 - -2 - 2 8
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x = 3 + 5 - -2 - 2
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y = x - (x - 2)
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y 2
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ans = (17 - (5 - 20)) - (1 - 11)
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ans 42
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ans = ans - (-42 - ans)
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ans 126
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(17-(5-20))-(1-11) 42
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```
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### How to run
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You can run the program interactively with the command \
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`python pmpv.py` \
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Alternatively, you can pipe files in and out.
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On linux that can be acheived with the arrow operator\
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`python pmpv.py < in.txt` or `python pmpv.py < in.txt > out.txt`\
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On windows output redirection works the same way, but you need to
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use piping for input redirection. It is the same in essence, but a
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little more clunky\
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`Get-Content in.txt | python pmpv.py > out.txt`
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### Dependencies
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This program uses python's built-in re module (regex), and there
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are no external dependencies. The test file uses unittest.
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### Diagnostics
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The program sends error messages to stderr, so you can redirect
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that away if you don't want those messages.
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There is a testing file inluded `test_pmpv.py` that can be run
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with the command `python test_pmpv.py` that will run a sereices
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of unit and integration tests. unittest was used as the testing
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library of choice. The selection of tests is not complete, but
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was enought to verify that the program met the requirements of the
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assignment.
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### Bugs and limitations
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Typical behaviour is to ignore invalid inputs and send a warning
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to stderr. It is possible for the program to crash or throw an
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exception, but I have tested it extensively, and crashes during
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normal use are *hopefully* rare.
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One issue with the program is that behaviour when given invalid
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input undefined. Some inputs will be ignored, but there may be
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some cases where the input is invalid, but the program interprets
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it anyway, and may give a mathematically incorrect result.
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### Sources used:
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- https://regexr.com/
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- https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions/Cheatsheet
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- https://www.delftstack.com/howto/python/python-print-to-stderr/
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- https://stlab.cc/legacy/how-to-write-a-simple-lexical-analyzer-or-parser.html
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- https://www.tutorialsteacher.com/python/public-private-protected-modifiers
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- https://stackoverflow.com/questions/1155617/count-the-number-of-occurrences-of-a-character-in-a-string
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- https://stackoverflow.com/questions/152580/whats-the-canonical-way-to-check-for-type-in-python
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- https://www.geeksforgeeks.org/python-method-overloading/
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- https://docs.python.org/3/library/functions.html#iter (for info on iter)
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- https://stackoverflow.com/questions/12179271/meaning-of-classmethod-and-staticmethod-for-beginner
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- https://stackoverflow.com/questions/11447598/redirecting-standard-input-output-in-windows-powershell
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- https://learn.microsoft.com/en-us/powershell/module/microsoft.powershell.core/about/about_pipelines?view=powershell-7.4
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