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From: bobg+@andrew.cmu.edu (Robert Steven Glickstein)
Newsgroups: comp.unix.questions
Subject: YACC question
Message-ID: <8ZgGBgG00VsnQDgUNO@andrew.cmu.edu>
Date: 15 Jan 90 01:12:12 GMT
Organization: Information Technology Center, Carnegie Mellon, Pittsburgh, PA
Lines: 48


Here is a fragment of Yacc code:

    %token PLUS MINUS

    %%

    expr:       mulexpr PLUS mulexpr
        | mulexpr MINUS mulexpr

It's very straightforward; the yylex() routine must be written to return
the constant PLUS when it encounters a '+' in the input, and the
constant MINUS when it encounters a '-' in the input.  However, Yacc
allows you to rewrite the above fragment as

    %%

    expr:       mulexpr '+' mulexpr
        | mulexpr '-' mulexpr

My question is, where does Yacc find the '+' and the '-' characters? 
Apparently they're not gotten via a call to yylex().  Does Yacc simply
do a getchar()?

I ask because I have written a parser which can be configured to read
from various input sources (standard input, file, string).  There are
many places in the parser where token-name constants are used when a
single character will do; however, if the single character is retrieved
by getchar() or some other hardwired mechanism, I'll have to stick to
the yylex() approach (since my yylex() knows where to read characters
from).

Please e-mail your replies.  Thanks in advance.

_______________________________
Bob Glickstein, System Designer
Information Technology Center  room 220
Carnegie Mellon University
Pittsburgh, PA  15213-3890
(412) 268-6743

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