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| From | Zoltan Kocsi <zoltan@bendor.com.au> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: In how many ways should this fail? |
| Date | 2012-02-01 00:46 +1100 |
| Organization | Bendor Research Pty. Ltd. |
| Message-ID | <20120201004609.389dffa4@tade.bendor.com.au> (permalink) |
| References | (4 earlier) <jg6mrf$6it$1@dont-email.me> <4F26E9F6.3020101@verizon.net> <jg6r4h$1fh$1@dont-email.me> <0.d2a24acbe4d003562f6e.20120130214854GMT.8739awsvy1.fsf@bsb.me.uk> <jg8no4$9de$1@dont-email.me> |
On Tue, 31 Jan 2012 12:43:46 -0000
"BartC" <bc@freeuk.com> wrote:
> > assignment-expression:
> > conditional-expression
> > unary-expression assignment-operator assignment-expression
> >
> > just says that an assignment expression can be a conditional expression
> > on its own.
>
> You have to look further into the Standard to discover that
> 'assignment-expression' just means 'any expression'. Whatever the merits of
> defining the grammar in this way, it's not very intuitive; the above
> suggests that any conditional expression is a form of assignment!
No, assignment expression is not the same as expression and no, the
conditional expression is not a form of assignment. The fact that the name of
the rule is assignment_expression does *not* mean that that rule specifies
the assignment expression. It specifies how to parse a subexpression at the
assignment operator's precedence level. It gives you two alternatives. If you
find a conditional expression, then that will do, you accept the parse. If
not, then you should look for a unary expression followed by an assignment
operator and then recursively look for an other expression but not any
expression, but one which has a precedence level of the assignment operator,
i.e. applying only the assignment_operator rules and rules depending on
those instead of starting again with the expressio rule. If what you have on
the input can not be parsed according to those rules, then it is syntactically
incorrect. If you can parse it, then it is correct and the parser will build
the subexpressions according to the precedence and associativity of the
operators described by the rules.
Defining the grammar that way has a few advantages:
- The operator precedence and associativity is implicitly defined by the
grammar.
- The grammar definition pretty much directly specifies a recursive descend
parser, if you want to parse that way.
- If you prefer a parser generator such as Yacc, you can use the definition
more or less as it is. Alternatively, you can specify operator precedence
for Yacc and the precedence table can be extracted from the grammar
mechanically.
Here is a very simple grammar:
expression:
addition_expression
addition_expression:
multiplication_expression
addition_expression '+' multiplication_expression
multiplication_expression:
tag
multiplication_expression '*' tag
tag:
number
'(' expression ')'
This will parse numeric expression using + and *, with * having precedence
over + and handling parenthesis properly; sequences of + -s or * -s will be
evaluated from left to right. The priority of * over + is expressed by
addition_expression refering to multiplication_expression and not the other
way around. The left to right evaluation is guaranteed by the second clauses
referencing their own rule on the left hand side of their operator. The
parenthesis overrules any precedence, because it recursively redirects from
the highest precedence level to the lowest and the very same thing
automatically guarantees proper handling of nested parens.
Writing a parser for this grammar from the grammar description is pretty much
a mechanical translation of the BNF to functions and writing a Yacc rule set
would be even easier.
Zoltan
--
Zoltán Kócsi
Bendor Research Pty. Ltd.
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In how many ways should this fail? Anders Wegge Keller <wegge@wegge.dk> - 2012-01-30 16:32 +0100
Re: In how many ways should this fail? James Kuyper <jameskuyper@verizon.net> - 2012-01-30 11:07 -0500
Re: In how many ways should this fail? James Kuyper <jameskuyper@verizon.net> - 2012-01-30 11:11 -0500
Re: In how many ways should this fail? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-01-30 17:42 +0000
Re: In how many ways should this fail? gwowen <gwowen@gmail.com> - 2012-01-30 08:16 -0800
Re: In how many ways should this fail? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-01-30 17:35 +0000
Re: In how many ways should this fail? gwowen <gwowen@gmail.com> - 2012-01-30 23:47 -0800
Re: In how many ways should this fail? Tim Rentsch <txr@alumni.caltech.edu> - 2012-02-01 00:00 -0800
Re: In how many ways should this fail? "BartC" <bc@freeuk.com> - 2012-02-01 12:14 +0000
Re: In how many ways should this fail? James Kuyper <jameskuyper@verizon.net> - 2012-02-01 08:04 -0500
Re: In how many ways should this fail? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-02-01 13:31 +0000
Re: In how many ways should this fail? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-02-01 12:59 +0000
Re: In how many ways should this fail? James Kuyper <jameskuyper@verizon.net> - 2012-02-01 08:37 -0500
Re: In how many ways should this fail? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-02-01 16:40 +0000
Re: In how many ways should this fail? James Kuyper <jameskuyper@verizon.net> - 2012-02-01 12:13 -0500
Re: In how many ways should this fail? Tim Rentsch <txr@alumni.caltech.edu> - 2012-03-08 12:49 -0800
Re: In how many ways should this fail? Anders Wegge Keller <wegge@wegge.dk> - 2012-01-30 19:20 +0100
Re: In how many ways should this fail? Devil with the China Blue Dress <chine.bleu@yahoo.com> - 2012-01-30 08:30 -0800
Re: In how many ways should this fail? "BartC" <bc@freeuk.com> - 2012-01-30 16:43 +0000
Re: In how many ways should this fail? Devil with the China Blue Dress <chine.bleu@yahoo.com> - 2012-01-30 09:17 -0800
Re: In how many ways should this fail? Kaz Kylheku <kaz@kylheku.com> - 2012-01-30 17:31 +0000
Re: In how many ways should this fail? James Kuyper <jameskuyper@verizon.net> - 2012-01-30 12:31 -0500
Re: In how many ways should this fail? "BartC" <bc@freeuk.com> - 2012-01-30 18:16 +0000
Re: In how many ways should this fail? James Kuyper <jameskuyper@verizon.net> - 2012-01-30 14:05 -0500
Re: In how many ways should this fail? "BartC" <bc@freeuk.com> - 2012-01-30 19:30 +0000
Re: In how many ways should this fail? Keith Thompson <kst-u@mib.org> - 2012-01-30 12:50 -0800
Re: In how many ways should this fail? James Kuyper <jameskuyper@verizon.net> - 2012-01-30 16:17 -0500
Re: In how many ways should this fail? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-01-30 21:48 +0000
Re: In how many ways should this fail? "BartC" <bc@freeuk.com> - 2012-01-31 12:43 +0000
Re: In how many ways should this fail? Zoltan Kocsi <zoltan@bendor.com.au> - 2012-02-01 00:46 +1100
Re: In how many ways should this fail? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-01-31 13:57 +0000
Re: In how many ways should this fail? "BartC" <bc@freeuk.com> - 2012-01-31 15:05 +0000
Re: In how many ways should this fail? Devil with the China Blue Dress <chine.bleu@yahoo.com> - 2012-01-31 07:16 -0800
Re: In how many ways should this fail? "BartC" <bc@freeuk.com> - 2012-01-31 17:37 +0000
Re: In how many ways should this fail? James Kuyper <jameskuyper@verizon.net> - 2012-01-31 11:34 -0500
Re: In how many ways should this fail? "BartC" <bc@freeuk.com> - 2012-01-31 17:39 +0000
Re: In how many ways should this fail? Keith Thompson <kst-u@mib.org> - 2012-01-31 19:42 -0800
Re: In how many ways should this fail? "BartC" <bc@freeuk.com> - 2012-02-01 11:57 +0000
Re: In how many ways should this fail? James Kuyper <jameskuyper@verizon.net> - 2012-02-01 08:14 -0500
Re: In how many ways should this fail? Philip Lantz <prl@canterey.us> - 2012-02-01 00:04 -0800
Re: In how many ways should this fail? James Kuyper <jameskuyper@verizon.net> - 2012-01-31 09:05 -0500
Re: In how many ways should this fail? Devil with the China Blue Dress <chine.bleu@yahoo.com> - 2012-01-31 06:45 -0800
Re: In how many ways should this fail? "BartC" <bc@freeuk.com> - 2012-01-31 17:34 +0000
Re: In how many ways should this fail? Keith Thompson <kst-u@mib.org> - 2012-01-31 19:26 -0800
Re: In how many ways should this fail? Keith Thompson <kst-u@mib.org> - 2012-01-31 19:29 -0800
Re: In how many ways should this fail? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-01-30 21:28 +0000
Re: In how many ways should this fail? timprince <tprince@computer.org> - 2012-01-30 11:44 -0500
Re: In how many ways should this fail? Kaz Kylheku <kaz@kylheku.com> - 2012-01-30 17:35 +0000
Re: In how many ways should this fail? Kaz Kylheku <kaz@kylheku.com> - 2012-01-30 17:21 +0000
Re: In how many ways should this fail? Anders Wegge Keller <wegge@wegge.dk> - 2012-01-30 19:22 +0100
Re: In how many ways should this fail? 88888 Dihedral <dihedral88888@googlemail.com> - 2012-01-30 11:09 -0800
Re: In how many ways should this fail? jacob navia <jacob@spamsink.net> - 2012-01-30 19:04 +0100
Re: In how many ways should this fail? "BartC" <bc@freeuk.com> - 2012-01-30 18:21 +0000
Re: In how many ways should this fail? Jens Gustedt <jens.gustedt@loria.fr> - 2012-01-30 20:15 +0100
Re: In how many ways should this fail? "BartC" <bc@freeuk.com> - 2012-01-30 19:32 +0000
Re: In how many ways should this fail? Jens Gustedt <jens.gustedt@loria.fr> - 2012-01-30 21:01 +0100
Re: In how many ways should this fail? "BartC" <bc@freeuk.com> - 2012-01-30 20:22 +0000
Re: In how many ways should this fail? Keith Thompson <kst-u@mib.org> - 2012-01-30 12:56 -0800
Re: In how many ways should this fail? blp@cs.stanford.edu (Ben Pfaff) - 2012-01-30 10:07 -0800
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