Systems & CLI · Beginner
Build a Calculator
Build a command-line calculator that evaluates arithmetic expressions with correct operator precedence and parentheses using a tokenizer and a recursive-descent parser.
This project is a calculator that reads an expression like `2 + 3 * (4 - 1)` and prints the correct result by actually parsing the math rather than calling eval. You build a tokenizer that turns the raw string into numbers and operator tokens, then a recursive-descent parser that respects multiplication-before-addition precedence and nested parentheses. It is worth building because it is the gentlest real introduction to how interpreters and compilers work, and it teaches grammar, tokenization, and tree evaluation that show up in everything from query engines to programming languages. Implementing precedence correctly without a library is a concrete, demonstrable skill that distinguishes you from people who only ever call built-in parsers.
What you build
- Tokenizes an input string into numbers, operators, and parentheses
- Evaluates +, -, *, and / with mathematically correct precedence
- Handles parentheses and arbitrary nesting
- Supports unary minus for negative numbers
- Reports clear errors for malformed input like mismatched parentheses
- Reads expressions from arguments or an interactive prompt loop
- Supports floating-point as well as integer arithmetic
What it teaches
- Tokenizing raw text into a stream of typed tokens
- Grammar design and operator precedence
- Recursive-descent parsing
- Tree-walking evaluation of expressions
- Error reporting for invalid syntax
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Take it further
- Add support for exponentiation and a modulo operator with correct precedence.
- Add named variables and an assignment operator so users can store values.
- Build an abstract syntax tree explicitly and print it, separating parsing from evaluation.
