Trying to understand parsers

Question

I'm trying to use JavaCC to build a simple command line calculator that can handle a variety of expressions. While there are plenty of tutorials out there on how to write grammars, none that I've seen so far explain what happens afterwards.

What I understand right now is that after a string is passed into the parser, it's split into a tokens and turned into a parse tree. What happens next? Do I traverse through the parse tree doing a bunch of if-else string comparisons on the contents of each node and then perform the appropriate function?

Answer 1

I highly suggest you watch Scott Stanchfield's ANTLR 3.x tutorials. Even if you don't end up using ANTLR, which may be overkill for your project but I doubt it, you will learn a lot by watching him go through the thought process.

In general the process is...

1. Build a lexer to understand your tokens
2. Build a parser that can validate and understand and organize the input into an abstract syntax tree (AST) which should represent a simplified/easy-to-work-with version of your syntax
3. Run any calculation based on the AST