Re: [Boost-users] Parsing math expression

30 Nov 2010

      Wow ! Excellent :)

I thought it would be more verbose than that.

Thank you very much,
Eloi Du Bois.

2010/11/30 Hartmut Kaiser 
...
...
...
...
Is there a simple math expression parser in boost ?
I only need to parse those kind of expression: "1+2/3".
I believe Spirit is exactly what you're looking for. It has almost a
dozen
different calculator examples to choose from as a starting point for
your
needs.
...
I have been thinking on boost::spirit, but it is quite hard for what
I
need.
Why?
I very enjoy playing with spirit, but I was looking for a 5-line code
that
only boost have the secret...
Well, not exactly five lines, but here is a simple calculator grammar
written in Spirit:
namespace qi = boost::spirit::qi;
template <typename Iterator>
   struct calculator : qi::grammar
   {
       calculator() : calculator::base_type(expression)
       {
           expression = term >> *( ('+' >> term) | ('-' >> term) ) ;
           term = factor >> *( ('*' >> factor) | ('/' >> factor) ) ;
           factor = qi::uint_ | '(' >> expression >> ')' |
                            ('-' >> factor) | ('+' >> factor) ;
       }
       qi::rule expression, term, factor;
   };
And here is how to use it:
std::string input("1+2/3");
   calculatorstd::string::const_iterator calc;
   if (phrase_parse(input.begin(), input.end(), calc, qi::space))
       std::cout << "successfully parsed: " << input << std::endl;
Regards Hartmut
---------------
http://boost-spirit.com
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