FunctionSolver

A desktop app for plotting and comparing two mathematical functions, built around a hand-written lexer and parser (using PLY — Python's Lex/Yacc) that validates expressions before they're ever evaluated.

Originally built for Master Micro's 2025 Winter SW internship.

What it does

• Scans and parses mathematical expressions from scratch. Input isn't just passed to eval — it goes through a proper tokenizer (ExpressionLexer) and a grammar-based parser (ExpressionParser) built with PLY, enforcing operator precedence (+, -, *, /, ^, unary minus) and validating supported functions (log, sqrt) and variable names before anything is plotted.
• Catches invalid input with specific, readable errors — invalid characters, unsupported variable names, incomplete expressions, and syntax errors are all detected at the lexer/parser stage and surfaced to the user with a clear message rather than a stack trace.
• Plots two functions simultaneously and finds their intersection points (via SymPy) once both expressions pass validation.
• Interactive plot with hover information for exploring function values.

Architecture

src/function_solver/
├── core/
│   ├── expression_lexer.py    # Tokenizer (PLY lex) — numbers, operators, log/sqrt, variables
│   ├── expression_parser.py   # Grammar + precedence rules (PLY yacc)
│   └── solver.py              # SymPy-based equation solving and evaluation
├── gui/
│   ├── app.py                 # Main window
│   └── components/            # Input widget (validation + error display), plotter widget
└── main.py                    # Entry point

The GUI only calls into Solver once both input expressions pass ExpressionParser.validate(), keeping expression validation fully decoupled from the UI and testable on its own (see tests/core/).

Installation

# Clone the repository
git clone https://github.com/Zaituny/FunctionSolver.git
cd FunctionSolver

# Install dependencies
python -m pip install -r requirements.txt

Running

python -m src.function_solver.main

Running tests

pytest

Covers the lexer, parser, solver, and GUI (tests/core/, tests/gui/).

Screenshots

Main window

Plotting functions

Example 1: f(x) = x^2 + 1 and g(x) = -x^2 + 2

Example 2: f(x) = log(x) and g(x) = -x^2 + 2

Example 3: f(x) = log(x^2+2) and g(x) = -x^2+2

Validation errors

Both input fields empty

One input field empty

Invalid variable name in both fields

Invalid variable name in one field

Invalid log expression

Incomplete expression

Tech stack

Python, PLY (Lex/Yacc), SymPy, PySide2 (Qt for Python), Matplotlib, pytest / pytest-qt

License

MIT License — see LICENSE file for details.