The old Google Groups will be going away soon, but your browser is incompatible with the new version.
Precedence problem
 There are currently too many topics in this group that display first. To make this topic appear first, remove this option from another topic. There was an error processing your request. Please try again. Standard view   View as tree
 4 messages

From:
To:
Cc:
Followup To:
Subject:
 Validation: For verification purposes please type the characters you see in the picture below or the numbers you hear by clicking the accessibility icon.

More options Oct 30 2008, 1:05 pm
From: "Todd O'Bryan" <toddobr...@gmail.com>
Date: Thu, 30 Oct 2008 13:05:44 -0400
Local: Thurs, Oct 30 2008 1:05 pm
Subject: [antlr-interest] Precedence problem
I've assigned my high school programming students a symbolic algebra
project and provided them an ANTLR parser so they could translate
Strings to values easily. I used an AST, so the translation is pretty
easy. The basic idea is a mapping like this:

"sin(x ^ 2)" --> new Sin(new Exp(new Var(), new Number(2.0))

Everything was working great, until... I wanted exponentiation to have
higher precedence than unary operators, so

"~x^3" ---> new Neg(new Exp(new Var(), new Number(2.0)))
"sin x ^ 2" ---> new Sin(new Exp(new Var(), new Number(2.0)))

But a student tried this:

"sin(x) ^ 2"

Clearly this SHOULD be new Exp(new Sin(new Var()), new Number(2.0)),
but since ^ has higher precedence than sin, it doesn't work.

I can't figure out how to fix it, however, because sin(...) should
have the same precedence as a parenthesized expression (higher than
^), but sin ... should have lower precedence. I tried a syntactic
predicate, but since the sin rule is in two rules, I can't get rid of
the ambiguity. Here's my grammar that doesn't work. Can anybody help?

grammar Expression;
options {
output = AST;
ASTLabelType=CommonTree;

}

;

;

multExpr : unaryExpr (('*'^|'/'^) unaryExpr)*
;

unaryExpr : ('sin'^|'cos'^|'ln'^|'~'^) expExpr
| expExpr
;

expExpr : atom ('^'^ atom)*
;

parenExpr : ('sin'^|'cos'^|'ln'^|'~'^)? '('! addExpr ')'!
;

atom : parenExpr
| NUMBER
| VAR
;

NUMBER : '-'? '0'..'9'+ ('.' '0'..'9'*)? ;
VAR : 'x' ;
WHITESPACE : (' '|'\t'|'\n'|'\r')+ { skip(); } ;

To post a message you must first join this group.
You do not have the permission required to post.
More options Oct 30 2008, 1:44 pm
From: "Tim Halloran" <hallor...@gmail.com>
Date: Thu, 30 Oct 2008 13:44:35 -0400
Local: Thurs, Oct 30 2008 1:44 pm
Subject: Re: [antlr-interest] Precedence problem
Well, one fix is to separate, taking "sin" as an example, sin SPACE from sin(

I just made the SPACE significant -- It seems to work on all your
examples.  Good luck

grammar Expression;
options {
output = AST;
ASTLabelType=CommonTree;

}

expr
;

: multExpr (('+'^|'-'^) multExpr)*
;

multExpr
: unaryExpr (('*'^|'/'^) unaryExpr)*
;

unaryExpr
: '~'^ expExpr
| sinExpr
| cosExpr
| lnExpr
| expExpr
;

sinExpr
: 'sin ' expExpr
;

cosExpr
: 'cos ' WHITESPACE expExpr
;

lnExpr
: 'ln ' WHITESPACE expExpr
;

expExpr
: atom ('^'^ atom)*
;

atom
: parenExpr
| NUMBER
| VAR
;

parenExpr
;

NUMBER : '-'? '0'..'9'+ ('.' '0'..'9'*)? ;
VAR : 'x' ;
WHITESPACE : (' '|'\t'|'\n'|'\r')+ { skip(); } ;

List: http://www.antlr.org:8080/mailman/listinfo/antlr-interest
Unsubscribe: http://www.antlr.org:8080/mailman/options/antlr-interest/your-email-a...

To post a message you must first join this group.
You do not have the permission required to post.
More options Oct 30 2008, 1:45 pm
From: "Tim Halloran" <hallor...@gmail.com>
Date: Thu, 30 Oct 2008 13:45:34 -0400
Local: Thurs, Oct 30 2008 1:45 pm
Subject: Re: [antlr-interest] Precedence problem

To post a message you must first join this group.
You do not have the permission required to post.
More options Oct 30 2008, 7:30 pm
From: "Todd O'Bryan" <toddobr...@gmail.com>
Date: Thu, 30 Oct 2008 19:30:57 -0400
Local: Thurs, Oct 30 2008 7:30 pm
Subject: Re: [antlr-interest] Precedence problem
That makes perfect sense! I was too busy thinking that spaces were
insignificant to realize that they're significant. Thanks!

List: http://www.antlr.org:8080/mailman/listinfo/antlr-interest
Unsubscribe: http://www.antlr.org:8080/mailman/options/antlr-interest/your-email-a...