Re: %left is not working

[Hans Aberg]

On 3 Aug 2010, at 18:54, Sasan Forghani wrote:

> I have been writing a simple compiler for a CS class and have run  
> into a few
> problems that I can't figure out how to solve.  %left does not seem  
> to be
> working as Bison is derivating immediately.
>
> For example:
>
> a := b + c + d
>
> expr : expr ADDOP expr
> expr: ref_name
> ref_name: IDENTIFIER
>
> ADDOP is specified with the %left
>
> The directed translation in parse tree representation is
>
> E
> E         +         E
> R         E         +         E
> b          c                    d
>
> Since ADDOP is specified with % left shouldn't the tree look like:
>
> E
> E        +         E
> E        +        E         R
> R                  R         d
> b                   c

Bison uses LALR(1), so it build it bottom-up and from the right, as in  
the first diagram.

> Another problem I have run into is sending data back when a  
> recursive call
> is finished. If the input is a := (b + c) + d, the code I have is as
> follows:
>
> stmt : ref_name BL_ASSIGNMENT expr {
>
>    fprintf(ptrToIrFile, "mov         %s          %s\n",  
> $<blIdentifier>3,
>                    $<blIdentifier>1);
> }
>
> expr : expr BL_ADDOP expr {
>
>    if(strcmp($<blOperator>2, "+"))
>        strcpy($<blOperator>2, "add");
>    else
>        strcpy($<blOperator>2, "sub");
>
>    char bufferT[6];
>    changeToString(++amtOfTemps, "T", bufferT);
>    fprintf(ptrToIrFile, "%s         %s          %s          %s\n",
>                    $<blOperator>2, $<blIdentifier>1, $<blIdentifier>3,
> bufferT);
>
>    strcpy($<blIdentifier>1, bufferT);
> }
>
> | ( expr)
>
> as long as there isn't ( expr ) in the input input move up without  
> problem.
> However once input does have an ( expr) part, $<blIdentifier>1 at  
> the ADDOP
> level has the value of $<blIdentifier>1 at the ASSIGNMENT level.   
> When I add
> the following action to the ( expr ) production-  
> {strcpy($<blIdentifier>1,
> $<blIdentifier>2);}, $<blIdentifier>1 at the ADDOP level has the  
> correct
> value but then the $<blIdentifier>1 at the ASSIGNMENT level value is  
> what I
> changed $<blIdentifier>1 to be at the ( expr ) level.  I'm very  
> confused.

Can't see the details, if you do strcpy() without properly allocating  
first, there will be problems. Make sure its OK in the lexer, too.