On Thu, 18 Nov 2004 20:21:41 UTC, "Bill Cunningham" <no****@nspam.n et>
wrote:
Would getc and ungetc be the best and most simple what to parse
expressions for a parser?
Yes. But be aware of that ungetc can only unget ONE char at a time.
On other hand you can simply by macro or function extend getc and
ungetc to unget more than one char. Your UNGETC() would accept a
number of chars to give back in your GETC() in reverse order. Your
GETC() will give back the chars UNGETC had received before it gets new
chars from the stream itself.
A bit tricky is to ungent a char that you have never gotten - legally
as there is nothing that forbids it and ungenc requires the char that
is to unget. Be sure that you does NOT tries to unget EOF, this won't
work. This can be very useful when you has a long list of keyword
delemiters with same meaning to a long list of similar keywords.
Your parser may convert keywords or keychars into tokens and save the
tokens until all or a part of the input stream is readed and then work
on the generated token, it may work in other ways you thinks it
matches your requirements.
getc() (in conjunktion with ungetc() ) gives you the strongest
possible control over the stream you can ever need. When needed you
can siply count the number of chars, words, lines... readed in as side
effect, reset these counters as needed..... You avoids supervising of
buffers - you does need one.
Build your parser as state mashine and you can reuse the same code
again and again beside the little number of statements you needs to
handle a specific (sub)state. You gets high flexibility as you would
easy extend the functionality of the parser by create a new
(sub)state. Makes maintenance an easy work.
--
Tschau/Bye
Herbert
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