Question on examples/Lookahead

[Riadh Elloumi-4]

Hi all,

I am newbie to JavaCC and I started by the tutorials. I tried the
examples/Lookahead/Example1.jj and I am surprised that the grammar does
not accept "abc" as it is said on the tutorial. I am using JavaCC 5.0
and I compile and run the example with these commands:

riadh@giscard:Lookahead$ ../../bin/javacc Example1.jj
Java Compiler Compiler Version 5.0 (Parser Generator)
(type "javacc" with no arguments for help)
Reading from file Example1.jj . . .
Warning: Choice conflict in [...] construct at line 60, column 7.
         Expansion nested within construct and expansion following construct
         have common prefixes, one of which is: "c"
         Consider using a lookahead of 2 or more for nested expansion.
File "TokenMgrError.java" is being rebuilt.
File "ParseException.java" is being rebuilt.
File "Token.java" is being rebuilt.
File "SimpleCharStream.java" is being rebuilt.
Parser generated with 0 errors and 1 warnings.
riadh@giscard:Lookahead$ javac *.java
riadh@giscard:Lookahead$ java Example
abc
Exception in thread "main" ParseException: Encountered "<EOF>" at line
1, column 4.
Was expecting:
    "c" ...

        at Example.generateParseException(Example.java:206)
        at Example.jj_consume_token(Example.java:144)
        at Example.Input(Example.java:12)
        at Example.main(Example.java:6)
riadh@giscard:Lookahead$

Can you help me to understand why "abc" is not accepted?

Best regards,

Riadh.


---------------------------------------------------------------------
To unsubscribe, e-mail: users-unsubscribe@...
For additional commands, e-mail: users-help@...

[Tom Copeland]

On Oct 20, 2009, at 5:41 PM, Riadh Elloumi wrote:

> Hi all,
>
> I am newbie to JavaCC and I started by the tutorials. I tried the  
> examples/Lookahead/Example1.jj and I am surprised that the grammar  
> does not accept "abc" as it is said on the tutorial. I am using  
> JavaCC 5.0 and I compile and run the example with these commands:
>
> riadh@giscard:Lookahead$ ../../bin/javacc Example1.jj
> Java Compiler Compiler Version 5.0 (Parser Generator)
> (type "javacc" with no arguments for help)
> Reading from file Example1.jj . . .
> Warning: Choice conflict in [...] construct at line 60, column 7.
>        Expansion nested within construct and expansion following  
> construct
>        have common prefixes, one of which is: "c"
>        Consider using a lookahead of 2 or more for nested expansion.
> File "TokenMgrError.java" is being rebuilt.
> File "ParseException.java" is being rebuilt.
> File "Token.java" is being rebuilt.
> File "SimpleCharStream.java" is being rebuilt.
> Parser generated with 0 errors and 1 warnings.
> riadh@giscard:Lookahead$ javac *.java
> riadh@giscard:Lookahead$ java Example
> abc
> Exception in thread "main" ParseException: Encountered "<EOF>" at  
> line 1, column 4.
> Was expecting:
>   "c" ...
>
>       at Example.generateParseException(Example.java:206)
>       at Example.jj_consume_token(Example.java:144)
>       at Example.Input(Example.java:12)
>       at Example.main(Example.java:6)
> riadh@giscard:Lookahead$
>
> Can you help me to understand why "abc" is not accepted?

Hello Riadh -

I think the idea with examples/Lookahead/Example1.jj is to illustrate  
how backtracking affects a grammar.  In terms of BNF, Example1.jj can  
handle "abc", but since JavaCC parsers don't backtrack, the parser  
that JavaCC generates can't.  If you look at section 6 of the example/
Lookahead/README file you'll see a discussion of how to solve the  
problem in Example1.jj with semantic lookahead.

Hope this helps,

Yours,

Tom
http://generatingparserswithjavacc.com/


---------------------------------------------------------------------
To unsubscribe, e-mail: users-unsubscribe@...
For additional commands, e-mail: users-help@...