Re: How to get the text corresponding to a bison rule

[sgaurelius]

It is easy to recover the number of the dimensions. But I want to find what
are the dimensions as well. If I count the brackets, that won't give much of
information will it ?

Stratis


Laurence Finston wrote:
> 
>> Basically, I want to derive information from it. For example, if I got a
>> matrix like [[1,2],[3,4]], I want to find the dimensions of it. 
> 
> It's not that difficult to do this sort of thing.  
> 
> I almost always call the object I pass to Bison along with the parameter 
> for `yylex' `scanner_node' and it is of type `Scanner_Node', whereby
> `Scanner_Node' is a typedef for  
> `struct Scanner_Type *'.  When I've written my own version of `yylex' 
> (which isn't hard to do, if you don't need or want low-level 
> scanning using regular expressions), one can just pass a `Scanner_Node' 
> (cast to `void*') to `yyparse' and `yylex' without the extra rigamarole.  
> However, it's not that difficult to attach it to the `yylex' parameter 
> when using Flex;  I just would have to look up how I did this, because I 
> don't know off the top of my head.
> 
> I believe there's something fairly new in Bison where one can declare
> the type one passes as a parameter to `yyparse', so one doesn't need
> to cast to and from `void*' all the time.  However, I couldn't get
> this to work when using `yylex' as well.  Perhaps it is possible, but
> casting to and from `void*' is not a big problem.
> 
> The point of all this is that one can declare `Scanner_Type' (or whatever
> you 
> choose to call it) something like this:
> 
> struct Scanner_Type
> {
> 
>    int bracket_counter;
> 
>    <other data members>
> 
>    <member function declarations>
> 
> };
> 
> Then, in the constructor or constructors for `Scanner_Type', set 
> `bracket_counter' to 0, e.g.,
> 
> Scanner_Type::Scanner_Type(void)
> {
>    bracket_counter = 0;
> 
>    return;
> }
> 
> Of course, if you're using C, you'll have to write an `init' function to 
> initialize your `Scanner_Type' and ensure that it's called when you 
> allocate memory for each `Scanner_Type'.  If you have multiple calls
> to `yyparse' with a thread for each one, you will have multiple
> `Scanner_Types' (or `Scanner_Nodes'): one for each thread and instance
> of `yyparse'.  Then, all you have to do is increment
> `scanner_node->bracket_ctr' when `yylex' produces a `[' (a `LEFT_BRACKET'
> token) and decrement it when it produces a `]' (a `RIGHT_BRACKET'
> token).  For what it's worth, I always use names for tokens and never
> use characters or strings, but that's purely a matter of taste.
> 
> You'll have to keep track of when you get left and right brackets, but 
> I'm as sure as I can be that it can be done in the same way, without 
> actually having ever implemented this myself.  I've got variables with
> multiple dimensions, where the brackets don't have to all be together, 
> in the GNU 3DLDF language (based on Knuth's METAFONT language),
> e.g.,  `a[]bc[]def[][]g' would be a valid declaration,
> but I don't actually specifically keep track of how many
> dimensions such a variable has.  I could get this information at a
> later time if I needed it, though.
> 
> I hope this is of some help.
> 
> Laurence 
> 
>> 
>> Basically, I want to derive information from it. For example, if I got a
>> matrix like [[1,2],[3,4]], I want to find the dimensions of it. The thing
>> is, taht I had already a pretty good function for doing this, when I had
>> manipulating the expression without bison. now with bison, if I want to
>> the
>> same thing, i have to make it from the start. I will try to do it just
>> with
>> bison, but if it gets too complicated, I might use my old function to
>> find
>> dimensions and then do whatever I want with bison.
>> 
>> Stratis
>> 
>> 
>> Laurence Finston wrote:
>> > 
>> > 
>> > On Fri, 28 Nov 2008, sgaurelius wrote:
>> > 
>> >> Flex scanner has previously analyzed the string and extracted the
>> tokens.
>> >> However, if I want the text that corresponds to this expression, for
>> >> example
>> >> the "(1+2)" and not just the tokens, what should I do ? Is there some
>> >> solution ?
>> > 
>> > Save `yytext' somewhere "persistent".  `yytext' is available in the 
>> > actions of your scanner.  You can save it in a data member of an object 
>> > that you pass as a parameter to `yylex'.  If you use Flex, you can pass
>> a 
>> > parameter of a certain type (I'd have to look up what it's called) and 
>> > declare an object of a type of your choice to be a data member of this 
>> > object.  I use this to carry around information that needs to be
>> available 
>> > within my scanner and parser rules, and outside them.  This is a 
>> > thread-safe approach.
>> > 
>> > Alternatively, you could pass the strings back to `yyparse' as the 
>> > semantic values of the tokens, _unless_ you need the semantic values to
>> be 
>> > numeric values or something else.  This might be easier, because you 
>> > wouldn't have to keep track of which string is associated with a given 
>> > token.  If you chose this approach, you might have to convert strings
>> to 
>> > numbers in your parser rules, which isn't very difficult.
>> > 
>> > There are probably other approaches, but these are the ones that occur
>> to 
>> > me off the top of my head.  However, I can't think of any reason why I 
>> > would want to keep the strings.
>> > 
>> > Laurence Finston
>> > 
>> > 
>> > _______________________________________________
>> > address@hidden http://lists.gnu.org/mailman/listinfo/help-bison
>> > 
>> > 
>> 
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>> 
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>> 
> 
> 
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> 

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