[antlr-interest] Re: modifying expression grammar ; unexpected token null

[Peggy Fieland]

You are missing the case for unary plus and minus:

    | (PLUS^ | MINUS^) expression

as one of the options at the lowest level should do
it.

Peggy
  
--- tjs <tjs_tjs4 at yahoo.com> wrote:

> I still get a null token error
> I tried to redefine the grammar like this, using
> only sumExpr as the
> root, but it still seems to try to read more input
> than it is given:
> 
> class ExpressionParser extends Parser;
> options { buildAST=true; }
> 
> //expr     : (LPAREN^ sumExpr RPAREN!) ;
> sumExpr  : prodExpr ((PLUS^|MINUS^) prodExpr)* ;
> prodExpr : powExpr ((MUL^|DIV^|MOD^) powExpr)* ;
> powExpr  : atom (POW^ atom)? ;
> atom     : (LPAREN^ sumExpr RPAREN!) | INT;
> 
> class ExpressionLexer extends Lexer;
> 
> LPAREN : '(';
> RPAREN : ')';
> PLUS  : '+' ;
> MINUS : '-' ;
> MUL   : '*' ;
> DIV   : '/' ;
> MOD   : '%' ;
> POW   : '^' ;
> SEMI  : ';' ;
> protected DIGIT : '0'..'9' ;
> INT   : (DIGIT)+ ;
> 
> 
> {import java.lang.Math;}
> class ExpressionTreeWalker extends TreeParser;
> 
> sumExpr returns [double r]
>   { double a,b; r=0; }
> 
>   : #(PLUS  a=sumExpr b=sumExpr)  { r=a+b; }
>   | #(MINUS a=sumExpr b=sumExpr)  { r=a-b; }
>   | #(MUL   a=sumExpr b=sumExpr)  { r=a*b; }
>   | #(DIV   a=sumExpr b=sumExpr)  { r=a/b; }
>   | #(MOD   a=sumExpr b=sumExpr)  { r=a%b; }
>   | #(POW   a=sumExpr b=sumExpr)  { r=Math.pow(a,b);
> }
>   | #(LPAREN a=sumExpr) { r=a;}
>   | i:INT { r=(double)Integer.parseInt(i.getText());
> }
>   ;
>     
> 
> 
> 
> 
> --- In antlr-interest at yahoogroups.com, "tjs"
> <tjs_tjs4 at y...> wrote:
> > I am trying to modify the Expression grammar given
> in the 
> > JS Mill's tutorial (as of this time down;
> > http://supportweb.cs.bham.ac.uk/documentation/tuto
> >
>
rials/docsystem/build/tutorials/antlr/antlrhome.html)
> > 
> > I have 1 main goal here:
> > To make it so you don't need parantheses around
> the top expression
> > I did this by recrusively defining an expression
> as the same as in the
> > tutorial (fairly straightforward), and instead of
> > 
> > | expr
> > 
> > on the atom rule, I used
> > 
> > | (LPAREN^ expr RPAREN!) 
> > 
> > Which is the way the Java grammars I've look at
> handle expressions.
> > All this grammar does is parse math expressions
> and enforce
> > parantheses (for example, 2+2 outputs 4.0, and
> (1-3)*4 outputs -8, or
> > it SHOULD anyway). Again, I modified the ORIGINAL
> grammar.
> > 
> > My PROBLEM is that when I feed it input (in the
> form of a text file
> > that is only 3 characters in length, it gives me
> the following error:
> > 
> > line 1:4 unexpected token: null
> >  ( + 2 )
> > (AST): unexpected end of subtree
> > 
> > My interpretation of this is that it is trying to
> read the 4th
> > character of the input (which doesn't exist, hence
> null) and then the
> > AST doesn't understand why it can't find the end
> of the subtree, it
> > apparently also thinks it needs another token when
> it shouldn't. I
> > can't figure out why it wants a 4th character or
> any other problem.
> > 
> > The following are 1. the input text 2. the grammar
> 3. the ORIGINAL
> > grammar ( which does work properly, but you need
> to do (2+2) instead
> > of 2+2, because it makes you use parantheses
> around everything ), 4.
> > the way in which I load the text file into the
> lexer
> > 
> > finally, before you think i am a "Newbie" who
> doesn't deserve any
> > help, i have read 50+ pages of ANTLR
> documentation, searched
> > everything in /docs and this forum, experimented
> with this grammar for
> > over 4 hours, looked at Java grammars to see how
> they handled
> > expressions, and wrote a very crude expression
> parser that was very
> > messy before to graph functions (its messiness is
> part of the reason i
> > am using ANTLR to redo it). i just can't figure
> out my dumb problem,
> > that's all. also, if you help me i'll give you
> credit in my program
> > for correcting my dumb mistakes!
> > 
> > ------- 1. input text
> > 2+2
> > 
> > (this is only 3 characters, no newlines, or
> anything else)
> > 
> > ------- 2. my expression grammar
> > 
> > class ExpressionParser extends Parser;
> > options { buildAST=true; }
> > 
> > expr     :  sumExpr ;
> > sumExpr  : prodExpr ((PLUS^|MINUS^) prodExpr)* ;
> > prodExpr : powExpr ((MUL^|DIV^|MOD^) powExpr)* ;
> > powExpr  : atom (POW^ atom)? ;
> > atom     : INT | (LPAREN^ expr RPAREN!) ;
> > 
> > class ExpressionLexer extends Lexer;
> > 
> > LPAREN : '(';
> > FUNCSIN : 's';
> > RPAREN : ')';
> > PLUS  : '+' ;
> > MINUS : '-' ;
> > MUL   : '*' ;
> > DIV   : '/' ;
> > MOD   : '%' ;
> > POW   : '^' ;
> > SEMI  : ';' ;
> > protected DIGIT : '0'..'9' ;
> > INT   : (DIGIT)+ ;
> > 
> > {import java.lang.Math;}
> > class ExpressionTreeWalker extends TreeParser;
> > 
> > expr returns [double r]
> >   { double a,b; r=0; }
> > 
> >   : #(PLUS  a=expr b=expr)  { r=a+b; }
> >   | #(MINUS a=expr b=expr)  { r=a-b; }
> >   | #(MUL   a=expr b=expr)  { r=a*b; }
> >   | #(DIV   a=expr b=expr)  { r=a/b; }
> >   | #(MOD   a=expr b=expr)  { r=a%b; }
> >   | #(POW   a=expr b=expr)  { r=Math.pow(a,b); }
> >   | #(LPAREN a=expr) { r=a;}
> >   | i:INT {
> r=(double)Integer.parseInt(i.getText()); }
> >   ;
> >     
> > ------ 3. the original grammar from Mill's
> tutorial (working, but noly
> > when the input is (2+2)  not 2+2 )
> > 
> > class ExpressionParser extends Parser;
> > options { buildAST=true; }
> > 
> > expr     : (LPAREN^ sumExpr RPAREN!) ;
> > sumExpr  : prodExpr ((PLUS^|MINUS^) prodExpr)* ;
> > prodExpr : powExpr ((MUL^|DIV^|MOD^) powExpr)* ;
> > powExpr  : atom (POW^ atom)? ;
> > atom     : INT | expr ;
> > 
> > class ExpressionLexer extends Lexer;
> > 
> > LPAREN : '(';
> > RPAREN : ')';
> > PLUS  : '+' ;
> > MINUS : '-' ;
> > MUL   : '*' ;
> > DIV   : '/' ;
> > MOD   : '%' ;
> 
=== message truncated ===



		
__________________________________
Do you Yahoo!?
Yahoo! Mail - 50x more storage than other providers!
http://promotions.yahoo.com/new_mail


 
Yahoo! Groups Links

<*> To visit your group on the web, go to:
    http://groups.yahoo.com/group/antlr-interest/

<*> To unsubscribe from this group, send an email to:
    antlr-interest-unsubscribe at yahoogroups.com

<*> Your use of Yahoo! Groups is subject to:
    http://docs.yahoo.com/info/terms/