Practicing with Lex & Yacc

[cman]

I'm working on some practice programs with Lex/Yacc and having some problems ( I haven't received my copy of Lex & Yacc in the mail yet , damn eBay! ). I thought that it would be fun to write a parser for entering a system of linear equations from the console as practice ( and then send the results to some C code I wrote to do Guassian Elimination on the system and display the answers. Anyway , heres what I want to enter ( parse ):

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  3x + 3y = 3
  3x -  3y = 2
    x + 2y = 3



heres my lex and yacc scripts for a single equation ( just one  ( 3x - 2y = 3 ) or something :

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%{
#define end 260
#define equ 261
#define trm 262
#define num 263
#define pls 264
#define mun 265
#define otr 266
#define nl  267
#include <stdio.h>

#include <stdlib.h>
%}


end          "end"
term         [0-9]*[A-Za-z]
number       [0-9]+
plus "+"
minus        "-"
equal        "="
ws           [ \t]+
newline      [\n]
other   .
%%

{ws}      ;
{newline}   { return nl;  }
{term}      { return trm; }
{plus}     { return pls; }
{minus} { return mun; }
{equal} { return equ; }
{end} { return end; }
{number}    { return num; }
{other}    { return otr; }
%%
#ifdef debug
main ()
{

int ch;
do
{
ch = yylex();
if ( ch == trm )
printf ( "term\n" );
else if ( ch == pls )
printf ( "plus\n" );
else if ( ch == mun )
printf ( "minus\n" );
else if ( ch == equ )
printf ( "equal" );
else if ( ch == num )
printf ( "number\n" );
else if  ( ch == nl )
printf ( "newline\n" );

} while ( ch != end );


}
#endif
int yywrap ()
{
return 1;
}




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%{


#include "c:\programs\mat.cpp"
#include <stdio.h>
void yyerror (char const *);




%}

%token end trm num pls min equ otr nl

%start stmt

%%


stmt   : expr equ num nl      { printf ( "statement" );}
   ;

expr   : expr pls expr     { printf ( "expression" ); }
   | expr min expr
       | trm
       ;
%%


#include <stdio.h>

void main ()
{
do
{
yyparse();


}while ( !feof( stdin ) );

}
void
yyerror (char const *s)
{

  fprintf (stderr, "%s\n", s);
}



Any suggestions? Point me in the right direction! Thanks  :bg

[James Ladd]

I haven't received my copy of Lex & Yacc in the mail yet , damn eBay!

But lex and yacc are free !!

[Randall Hyde]

I haven't received my copy of Lex & Yacc in the mail yet , damn eBay!

But lex and yacc are free !!

Presumably, he means the book :-)
Cheers,
Randy Hyde

[James Ladd]

Doh !

[cman]

Yeah , I meant the book. Any ideas on the code? :bg

[cman]

If I define my lexical tokens like this:

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end          "end"
term         [0-9]*[A-Za-z] /* return trm */
number       [0-9]+         /* return num */
plus "+"           /* return pls */
minus        "-"           /* return min */
equal        "="           /* return equ */
ws           [ \t]+
newline      [\n]          /* return nl */
other   .



I should be able to define a equation in the system like this
( right? ):

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stmt   : expr equ num nl      { printf ( "statement" );}
   ;

expr   : expr pls expr        { printf ( "expression" ); }
       | expr min expr
       | trm     
   ;


This for an equation that looks like this:

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  3x + 4y - 2z = 3 <newline>



I'm I correct here? Thanks.....

[tenkey]

expr is ambiguous.

You haven't specified precedence or associativity. I don't know if Yacc has defaults for these things.

[cman]

Thanks tenkey. I defined the addition and subtraction operator:

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%left pls  /* plus operator */
%left min /* minus operator */


but I still get "syntax error" from the default error handler when I type in something like:

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  4c + 4d = 4



must be doing something wrong.....

[cman]

Hmmm..... VC++ 6 is giving me these warnings :

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--------------------Configuration: mat.tab - Win32 Debug--------------------
Compiling...
mat.tab.c
linsys.l(2) : warning C4005: 'end' : macro redefinition
        c:\programs\mat.tab.c(66) : see previous definition of 'end'
linsys.l(3) : warning C4005: 'equ' : macro redefinition
        c:\programs\mat.tab.c(73) : see previous definition of 'equ'
linsys.l(4) : warning C4005: 'trm' : macro redefinition
        c:\programs\mat.tab.c(67) : see previous definition of 'trm'
linsys.l(5) : warning C4005: 'num' : macro redefinition
        c:\programs\mat.tab.c(68) : see previous definition of 'num'
linsys.l(6) : warning C4005: 'pls' : macro redefinition
        c:\programs\mat.tab.c(71) : see previous definition of 'pls'
linsys.l(8) : warning C4005: 'otr' : macro redefinition
        c:\programs\mat.tab.c(69) : see previous definition of 'otr'
linsys.l(9) : warning C4005: 'nl' : macro redefinition
        c:\programs\mat.tab.c(70) : see previous definition of 'nl'
c:\program files\microsoft visual studio\vc98\include\stdlib.h(428) : warning C4005: 'min' : macro redefinition
        c:\programs\mat.tab.c(72) : see previous definition of 'min'
mat.tab.c(1115) : warning C4102: 'yyerrlab1' : unreferenced label
Linking...

mat.tab.exe - 0 error(s), 9 warning(s)



Seems all the tokens listed in my Bison script. Could this have anything to do with the problem? Maybe I didn't include something I should have :(.........

[cman]

I got it to work!  :bg I forgot to include the file "y.tab.h" in my flex script. I knew it was something like that! Thanks for the help!  :U

[cman]

I got my matrix parser to work using this grammer:

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%%


matrix        : equationList end { printf ( "matrix defined\n" ); }
 ;
equationList  : equation nl
                  | equation nl equationList
                  ;
equation      : expr equ num
                  ;
expr          : trm
                | expr min expr
                | expr pls expr
                 ;
%%





Pretty neat! Now I can enter a matrix like this:

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3c + 4d = 3
3c -  d    = 3

end



I have to yet get my copy of Lex & Yacc to see how to use the input to produce something I can pass to a set of C algorithms for matrixes. Let you know how it goes.... :bg

[cman]

Hmmm.... How do I "tell" Flex that :

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   (-5x)



and

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( - 5x )



are the same thing? I currently have a signed term defined as:

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   signedTerm        [(][+|-][0-9]*[A-Za-z][)]


Thanks for any help.  :toothy

[cman]

Opps! Just do this:

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sterm        [(][ ]*[+|-][ ]*[0-9]*[A-Za-z][ ]*[)]