Re: BNF DEFINITION - help!

mab@dst17.wdl.loral.com (Mark A Biggar)
Tue, 20 Sep 1994 20:32:36 GMT

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BNF DEFINITION - help! banks@iowasp.physics.uiowa.edu (1994-09-11)
Re: BNF DEFINITION - help! eanders+@CMU.EDU (Eric A. Anderson) (1994-09-12)
Re: BNF DEFINITION - help! ellard@bbn.com (1994-09-17)
Re: BNF DEFINITION - help! godau@dec2.wi-inf.uni-essen.de (H.-Juergen Godau) (1994-09-18)
Re: BNF DEFINITION - help! mab@dst17.wdl.loral.com (1994-09-20)
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Newsgroups: comp.compilers
From: mab@dst17.wdl.loral.com (Mark A Biggar)
Keywords: parse, yacc, comment
Organization: Loral Western Development Labs
References: 94-09-024 94-09-067
Date: Tue, 20 Sep 1994 20:32:36 GMT

<banks@iowasp.physics.uiowa.edu> wrote:
> Using character set C={a,b} write a BNF definition for the language
>consisting of all strings with odd length whose first and middle characters
>are the same.


ellard@bbn.com (Dan Ellard) writes:
>In a yacc-like syntax:
>S : a middleA a // an "a" followed by something
> | a middleA b // with an "a" in the middle.
> // followed by any single char.
> | b middleB a // a "b" followed by something
> | b middleB b // with a "b" in the middle,
> // followed by any single char.
> | a
> | b
> ;
>middleA : a middleA a
> | a middleA b
> | b middleA a
> | b middleA b
> | a
> ;
>middleB : a middleB a
> | a middleB b
> | b middleB a
> | b middleB b
> | b
> ;


This can be made simpler if you are willing to use one more non-terminal:


S : a A X
| b B X
| X
;


A : X A X
| a
;


B : X B X
| b
;


X : a
| b
;


--
Mark Biggar
mab@wdl.loral.com
[I think we can now consider this homework problem solved. -John]
--


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