*at: https://github.com/jamesgregson/expression_parser*

**An updated version of this library supporting custom variables/functions as well as boolean expressions is available from Github**I recent wrote a recursive-descent mathematical expression parser. I started mainly to get a handle on the expression aspects of a mostly compliant NIST RS274NGC GCode interpreter that I would like to write. It is written in ANSI C with the hopes of being able to cross-compile for AVR, ARM, PIC and Propeller devices, while still supporting standard PCs as well. This pretty much rules out anything but C. Note that

*this parser does not support the expression syntax for GCode*, but was written as a warmup exercise.

It's a pretty fun and surprisingly easy programming exercise to write one of these parsers; I recommend it for everyone. But for those who don't want to, I'm releasing the source code free for non-commercial use. For those who don't want to write it themselves but do want to to use it commercially (probably no-one), please contact me and generous terms can be arranged.

Anyway, the parser supports the standard infix mathematical notation, with operator precendence, unary +/-, an exponentiation operator (^) and the most useful of the math.h functions, like sin(), cos(), etc. The code is well commented for those that want to dive in and modify it.

Shown below is a sample driver script illustrating use of the parser:

#include<math.h> #include<stdio.h> #include"expression_parser.h" /** @brief macro to compare values as computed by C to those computed by the parser. creates a scope, initializes the parser and parses the string, then prints the expression, C result and parsed result. Does not handle the exponent operator '^', since it is not equivalent in C. */ #define parser_check( expr ) printf( "Parsing: '%s'\n", #expr ); \ printf( " C: %f\n", expr ); \ printf( " parser: %f\n\n", parse_expression( #expr ) ); void parser_test( const char *expr ){ double val = parse_expression( expr ); printf( "%s=%f\n", expr, val ); } int main( int argc, char **argv ){ parser_check( 1.0 + 2.0 ); parser_check( -1.e-03 + 2E+1 ); parser_check( asin( sin( 1.0 ) ) ); parser_check( pow( sin(1.0), 2.0 ) + pow( cos(1.0), 2.0 ) ); parser_check( (0.1 + 4.9)*(2.5*2)*(-3.0-2.0) ); parser_check( log( exp( 25.0 ) ) ); parser_test( "2^2^3" ); parser_test( "sin(1.0)^2 + cos(1.0)^2" ); parser_test( "2^-2" ); parser_test( "2^-(2.0*fabs(-sqrt(sin(0.5)^2 + cos(0.5)^2)))" ); return 0; }

The above code produces the output below:

Parsing: '1.0 + 2.0' C: 3.000000 parser: 3.000000 Parsing: '-1.e-03 + 2E+1' C: 19.999000 parser: 19.999000 Parsing: 'asin( sin( 1.0 ) )' C: 1.000000 parser: 1.000000 Parsing: 'pow( sin(1.0), 2.0 ) + pow( cos(1.0), 2.0 )' C: 1.000000 parser: 1.000000 Parsing: '(0.1 + 4.9)*(2.5*2)*(-3.0-2.0)' C: -125.000000 parser: -125.000000 Parsing: 'log( exp( 25.0 ) )' C: 25.000000 parser: 25.000000 2^2^3=256.000000 sin(1.0)^2 + cos(1.0)^2=1.000000 2^-2=0.250000 2^-(2.0*fabs(-sqrt(sin(0.5)^2 + cos(0.5)^2)))=0.250000

You can the source code from Github: https://github.com/jamesgregson/expression_parser