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

[soledady]

thanks fo your help...


Le mercredi 20 juillet 2005 à 10:34 +0200, Hans Aberg a écrit :
> 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
> 
> 
>