Expression interpreter written in Python

Last update: Oct 17, 2021

Overview

Calc Interpreter

An interpreter modeled after a calculator implemented in Python 3. The program currently only supports basic mathematical expressions. The package uses Reverse Polish Notation also known as postfix expression internally to easily represent mathematical expressions. This is powerful as it allows operators to be organised in such a way that precedence is absolute meaning an operator that is encountered first will always be executed first.

However, it is obvious that postfix is not normal or rather not usually taught in schools and thus infix expression is still the way for the user to write expressions. The interpreter uses a modified Shunting-Yard algorithm to produce Abstract Syntax Trees. Evaluation of ASTs is trivial, and they are flexible making it easy to extend the grammar and functionality later on.

Installation

The package is available in PyPI and can be installed via pip.

pip install expr-calc
#or
python3 -m pip install expr-calc

Usage

The best way to run the program currently is to execute the REPL and can be done in a python file or through your terminal.

Assuming your present working directory is inside the cloned repo, you can run the following command without the comment.

# inside /clone_path/expr_calc/
python -m expr_calc

The test suite can also be ran with pytest when inside the cloned repo

pytest  # or python -m pytest

Example

Once inside the REPL, you can start evaluating expressions. Currently, only operators listed in Features are supported.

calc> 1 + 1
2

calc> 345--500
845

calc> -2
-2

calc> 123 ^ 4
228886641

calc> 32 / 1.5
21.33333333333333333333333333

calc> 123 * 456
56088

calc> 34 % 5
4

calc> 4 ^ 1/2
0.5

calc> 4 ^ (1/2)
2.0

Features

Infix expressions
Basic operators such as +, -, *, /, %, ^
Tokens created from an expression can also be fetched to be manipulated if one wanted to do so
Expressions are transformed into m-ary Tree objects connected to each other

Features I want to add later

variable support
custom functions
more mathematical functions such as sin, cos, tan, etc
and possibly a simple symbolic computation support

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Comments

Better lexing(and scanning) and parsing
There is currently an issue with the lexing strategy for unary operators. Because i have only partially implemented it, constructs that uses unary operators are note exactly recognised. For example

> -(1) 1 # should be -1

This is problematic because it affects expressions such as to give wrong the answer

> -1^2 1 # should be -1 > -(1^2) 1 # should be -1

The main reason for this is that unary operators are not actually lexed themselves, when an expression such as -1 is passed, the program checks if the immediately succeeding character is a number and immediately binds the sign to the number after it, a bracket/parentheses is not a number and thus the operator doesn't get lexed independently at all disabling the parser to not recognise have any knowledge of it.

Also note that + and - are always initially tokenised as Token.BINARY_OPs and I believe this should change.

see this line
bug enhancement
opened by lestherll 2
Support for high precision floating-points
Currently, the interpreter uses raw Python as the engine for evaluating mathematical expressions. This is good in normal cases but as you may already know, computers aren't so good at dealing with high precision floating-points. It would be good to be able to utilise perhaps some module to deal with this.

> 9.999999800000001 * 10^15 9999999800000002.0 # should be 9999999800000001
bug enhancement
opened by lestherll 0
Create custom error codes
Currently, there are no custom error codes. Adding one would make the project more concrete and robust.

Example

# inside REPL > 1.1.1 SyntaxError: Numbers can only have one decimal point
enhancement
opened by lestherll 0

Releases(v0.1.2)

v0.1.2(Sep 7, 2021)

Source code(tar.gz)
Source code(zip)

Owner

BSc Comp Sci at Newcastle University

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Expression interpreter written in Python

Related tags

Overview

Calc Interpreter

Installation

Usage

Example

Features

Features I want to add later

Resources

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