Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!cs.utexas.edu!wuarchive!mit-eddie!mit-amt!mit-caf!vlcek
From: vlcek@mit-caf.MIT.EDU (Jim Vlcek)
Newsgroups: comp.lang.c
Subject: Re: A question of style
Message-ID: <3538@mit-caf.MIT.EDU>
Date: 1 Dec 89 23:22:12 GMT
References: <547@mars.Morgan.COM> <1989Nov30.001947.14883@aqdata.uucp> <427@jhereg.Minnetech.MN.ORG>
Reply-To: vlcek@mit-caf.UUCP (Jim Vlcek)
Organization: Microsystems Technology Laboratories, MIT
Lines: 62

In article <427@jhereg.Minnetech.MN.ORG> mark@jhereg.minnetech.mn.org (Mark H. Colburn) writes:
>In article <1989Nov30.001947.14883@aqdata.uucp> sullivan@aqdata.uucp (Michael T. Sullivan) writes:

Michael Sullivan (in the aqdata reference), proposes a 'proper' use of
the comma operator:

  while (c = getchar(), c != EOF)
  {
	...
  }

To which Mark Colburn replies:
  The code above could be written as 

	while (c = getchar && c  != EOF) { 
	    ... 
	}

  Which most programmers would find "more intuitive" than the comma
  separated one.

Sorry, but most programmers would find it "wrong".  In fact, I think
most programmers would find it difficult to get the thing _more_ wrong
than this.  First, you need the parenthesis after getchar (getchar())
- that omission, however, may be a lucky one in that it would keep
this mess from even compiling.  Second, the value assigned to ``c''
will not be the desired value (the value returned by getchar()), but
instead will be the value of the expression 

getchar() && c != EOF

This, of course, screws up the comparison with EOF, which ``c'' can
never equal (unless it was so initialized) as the && operator returns
only the values 0 and 1.  That comparison is also made with the value
of ``c'' from the last iteration of the loop, as the assignment
to ``c'' takes place _after_ the comparison to EOF.  Even if you
corrected all these problems by writing

while ((c = getchar()) && c != EOF) {
}

the semantics would still be changed in that the loop would terminate
on a return from getchar() of either 0 or EOF, whereas the previous
example terminates only on EOF (Barry Margolin already pointed this
out).

What you wanted, of course, was

while ((c = getchar()) != EOF) {
}

which, I am fairly confident, is given (probably verbatim) in K&R1.

Sorry if I seem pedantic, but I was amazed at how wrong this thing
was.  Even as I was writing this posting, I kept finding more things
wrong with it - I may have even missed a few.

Of course, if I'm wrong, this posting was done by an imposter, not Jim
Vlcek.

Fuck the moon; let's get     |  Jim Vlcek (vlcek@caf.mit.edu)
the Earth straightened out.  |            (vlcek@athena.mit.edu)