Path: utzoo!mnetor!uunet!sco!daniel
From: daniel@sco.COM (daniel edelson)
Newsgroups: comp.lang.c
Subject: Re: C sintax
Message-ID: <309@sysco>
Date: 8 Feb 88 22:32:17 GMT
References: <4083@june.cs.washington.edu<
Reply-To: daniel@sco.COM (daniel edelson)
Organization: The Santa Cruz Operation, Inc.
Lines: 38

In article <4083@june.cs.washington.edu< pardo@uw-june.UUCP (David Keppel) writes:
<
...
<<< 	;-D on  (My favorite sintax: switch(x+=*a+++*b){...})  Pardo
<<
<<What, precisely, does x+=*a+++*b mean?
<
<Well, actually it is ambiguous.  There are 2 (or more?) ways to parse
<it.  I'm not sure the behavior is defined from lexer to lexer as to
<which one is preferred.  Here are some possible interpretations:
<
<    tmp = *a++ + *b;	/* means add *a to *b and increment a */
<    x += tmp;
<    switch (x) {...}
<
<the (an) other is
<
<    tmp = *a + ++*b;	/* means increment *b and add that to *a */
<    x += tmp;
<    switch (x) {...}
<
...
<		    ;-D on  (What point?)  Pardo

It is ambiguous in that there are two valid expressions that
could look taht way. But correct compilers would only give
one of those twwo parsings. There is a little-used rule 
in K&R that says that when forming tokens, you ust the longest
string that could be a token. Thus, the expression a+++b
would be (correctly) parsed as a++ + b. Parsing it
as a + ++b would violate the longest-token rule.
Things becomes even more obfiscurated in ANSI C with the 
introduction of unary plus.
-- 
uucp:   {uunet|decvax!microsoft|ucbvax!ucscc|ihnp4|amd}!sco!daniel
ARPA:   daniel@sco.COM     Inter:  daniel@saturn.ucsc.edu--------------
pingpong: a dink to the right side with lots of top spin | Disclaimed |
fishing:  flies in morning and evening, day - spinners   | as usual   |