Math parser and calculator
Switch branches/tags
Nothing to show
Clone or download


leo is an expression parser and calculator.

From string inputs, leo can handle basic math operations, variables, and calling external functions.


leo is actually a library and a repl. To build just the library (ie, for use in other programs), just run make. To build the repl, run make repl.

Try things out to see what you can do. If you experience unexpected results, please let me know!

Variables and functions

Variables are rather lax; they can be multiple characters long and contain any character that is not ()+-/*^ ,.

A "variable" followed by a ( is considered to be a function. External function call support relies on the system having x86_64 system-V ABI compliance. Functions are whitelisted by passing an array to rpn_calc: char *whitelist[] = {"allowed func 1", "allowed func 2", NULL}. Optionally, the whitelist can be NULL to disallow any functions, or {"*", NULL} to allow all.

Development notes

I wrote leo mostly from scratch, but following this page for guidelines on how to design the algorithms.

leo implements a tokenizer (to convert the user input string into tokens), a shunting-yard algorithm (to convert the infix tokens into reverse polish notation), and a reverse polish notation calculator (to compute the value of the expression). To support these parts, it also implements a simple stack and queue.

As always, most things are written in ANSI C, with the exception of run_function.c, since it relies on the asm() "function" that wasn't part of the original C standard.