C++ Calculator

A command-line calculator written in C++17 featuring a hand-written recursive-descent parser and operator precedence handling. Parses and evaluates multi-value arithmetic expressions with correct operator precedence using a hand-written recursive-descent parser.

> 3 + 9 * 99
= 894

> (3 + 9) * 99
= 1188

> 100 / 5 * 2 + 3 - 1
= 42

---

Features

• Supports chained arithmetic expressions — chain as many numbers and operators as you like
• Correct operator precedence — * and / evaluate before + and -
• Parentheses — override precedence with ( and )
• Unary minus — negative numbers and negated sub-expressions (-5, -(3+4))
• Floating-point support — 1.5 * 2.0, .5 + .25
• Clear error messages — division by zero, unknown characters, mismatched parentheses
• Unit tests — full test suite via doctest

---

Project Structure

calculator/
├── include/
│   └── calculator.h        # Public API — tokenizer + evaluator declarations
├── src/
│   ├── calculator.cpp      # Lexer and recursive-descent parser implementation
│   └── main.cpp            # Interactive CLI entry point
├── tests/
│   └── test_calculator.cpp # Unit tests (doctest)
├── CMakeLists.txt
├── .gitignore
└── README.md

---

How It Works

Expression evaluation happens in two stages:

1. Lexer (tokenize) — scans the input string character by character and produces a flat list of typed tokens: NUMBER, PLUS, MINUS, MULTIPLY, DIVIDE, LPAREN, RPAREN.

2. Recursive-descent parser (evaluate) — walks the token list using a grammar that encodes precedence directly in the call hierarchy:

expr   → term   (('+' | '-') term)*       ← lowest precedence
term   → factor (('*' | '/') factor)*
factor → NUMBER | '(' expr ')' | '-' factor  ← highest precedence

Because term is called inside expr, multiplication and division always bind tighter than addition and subtraction — no precedence tables or shunting-yard bookkeeping needed.

---

Build

Requirements

• CMake ≥ 3.15
• A C++17-compatible compiler (GCC ≥ 7, Clang ≥ 5, MSVC 2017+)
• Internet connection on first build (CMake fetches the doctest header automatically)

Steps

# Clone
git clone https://github.com/<your-username>/calculator.git
cd calculator

# Configure
cmake -S . -B build

# Build
cmake --build build

# Run
./build/calculator

Run tests

ctest --test-dir build --output-on-failure

Or run the test binary directly for verbose output:

./build/tests

---

Usage

=== C++ Calculator ===
Type an expression and press Enter. Type 'help' or 'quit'.

> 3 + 9 * 99
= 894

> (2 + 3) * (4 - 1)
= 15

> 1 / 0
Error: Division by zero

> help

Calculator — supported operations
  +   addition          e.g.  3 + 5
  -   subtraction       e.g.  10 - 4
  *   multiplication    e.g.  6 * 7
  /   division          e.g.  9 / 3
  ()  parentheses       e.g.  (2 + 3) * 4
  Operator precedence is respected: * and / before + and -

Commands
  help   show this message
  quit   exit the program

> quit
Goodbye!