A Beginner’s Guide to Building a Simple Expression Evaluator

Parsing Math: Writing a Simple Expression Evaluator is a classic software engineering project that teaches you how programming languages understand and compute code. Instead of executing unsafe string evaluations (like JavaScript’s eval() or Python’s eval()), building a dedicated evaluator involves transforming a raw text string like “3 + 5(10 - 2)” into a precise, calculated number.

An expression evaluator is constructed using a standard three-stage pipeline:

“3 + 5 * 2” ──> [ Lexer / Tokenizer ] ──> [ Token Stream ] │ ▼ Result: 13 <── [ Evaluator ] <── [ Abstract Syntax Tree (AST) ] <── [ Parser ] Step 1: Lexical Analysis (The Lexer / Tokenizer)

The Lexer reads the input string character-by-character and groups them into meaningful units called Tokens. It strips out unneeded whitespace and categorizes characters into structured data types. For example, the string “12.5 + 4” is broken down into: Token(TYPE_NUMBER, “12.5”) Token(TYPE_PLUS, “+”) Token(TYPE_NUMBER, “4”) Step 2: Syntactic Analysis (The Parser) Writing a Math Expression Parser | by Ryan Dabler – ITNEXT