Re: [Boost-users] Parsing math expression

Subject: Re: [Boost-users] Parsing math expression
From: Hartmut Kaiser (hartmut.kaiser_at_[hidden])
Date: 2010-11-30 09:22:08

> > > 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<Iterator, qi::space_type>
    {
        calculator() : calculator::base_type(expression)
        {
            expression = term >> *( ('+' >> term) | ('-' >> term) ) ;
            term = factor >> *( ('*' >> factor) | ('/' >> factor) ) ;
            factor = qi::uint_ | '(' >> expression >> ')' |
                             ('-' >> factor) | ('+' >> factor) ;
        }
        qi::rule<Iterator, qi::space_type> expression, term, factor;
    };

And here is how to use it:

    std::string input("1+2/3");
    calculator<std::string::const_iterator> calc;
    if (phrase_parse(input.begin(), input.end(), calc, qi::space))
        std::cout << "successfully parsed: " << input << std::endl;

Regards Hartmut
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