Re: [help-bison]How to stock information?

[Hans Aberg]

On 20 Jul 2005, at 07:49, soledady wrote:
> can i construct a top-down grammar?

It isn't the grammar that is top-down or not -- it is the parse tree  
that one build. The Bison generated parser builds it bottom-up,  
because Bison uses LALR(1). The other method is LL(k). In Bison, one  
sometimes has to introduce a global variable, which carries some  
semantic information. That is essentially a top-down construct.

> > For example, if you have a rule
> > term:
> >    term "+" term {...}
> > then $1, $2, and $3 will be used to assign values to the RHS
> > occurrences of "term", which can be forwarded to the LHS by the use
> > of $$. If you let $$ be the root of a tree, and $1, $2 and $3 be
> > branches attached to $$, you get a syntax tree:
> >          $$
> >       /  |  \
> >      $1  $2  $3
>
>  the $$ could be use again in other rule couldn't it?

Every rule and token has its own semantic value, represented locally  
in the rule actions by the symbol $$.

> just a precision what the meaning of RHS and LHS(Right...? and Left
> something?)

See
  http://en.wikipedia.org/wiki/LHS
  http://en.wikipedia.org/wiki/RHS

> > > so for this part of an example (without all the rest of the example)
> > > U{N}=U{N-1}+3*U{N-2}; U{0}=1; U{1}=2;
> > > it's give me:
> > > *3+    ;  1  2   =U{1}=U{0} U{n-2} U{n-1}    =U{N}
> > >
> > >
> > > how can i do to recover correctly which number for which part of
> > > code ?
> >
> > My guess is that you ask for the semantic value. It will be held in
> > the variables $$ and $k.
>
> ok but how can i do to know that *3+ means +3*
> and to know which kind of term (u(n) or u(n+1) and so on...) are in  
> use

Yo define a grammar, just as you already have started, and then in  
each action you have the input semantic values $1, $2, ..., $k, where  
k is the rule length, and then you use them to define a value of $$.  
Bison generates a parser that makes sure that his hangs together.


> > statement:
> >    "{" statement_body "}"
> >
> > statement_body: /* Builds "u(n)=u(n-1)+3*u(n-2);u{0}=1;u{1}=2;" */
> yes but how can i built this expression

You have to build a grammar. This is difficult at firts. The Bison  
manual has a calculator example. The book by Aho, Sethi, Ullman,  
"Compilers" (the "dragon book") has an example of using Lex and Yacc.  
Look in grammar-like BNF notation of already existing computer  
languages, as that of the C/C++ standards. The newsgroup  
comp.compilers has a FAQ, published there monthly, which you can look  
into.

  Hans Aberg