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Path: utzoo!mnetor!seismo!husc6!necntc!ima!haddock!karl
From: karl@haddock.UUCP (Karl Heuer)
Newsgroups: comp.lang.c
Subject: Re: Style question re common C expression.
Message-ID: <588@haddock.UUCP>
Date: Fri, 19-Jun-87 15:24:19 EDT
Article-I.D.: haddock.588
Posted: Fri Jun 19 15:24:19 1987
Date-Received: Mon, 22-Jun-87 03:51:43 EDT
References: <4934@utcsri.UUCP>
Reply-To: karl@haddock.ISC.COM.UUCP (Karl Heuer)
Distribution: world
Organization: Interactive Systems, Boston
Lines: 57

In article <4934@utcsri.UUCP> flaps@utcsri.UUCP (Alan J Rosenthal) writes:
>Frequently I write expressions of the form "x ? x : y".  For example,
>in unix where "getenv" returns an environment variable, I might have
>something like: ... (t = getenv("EDITOR")) ? t : "/bin/ed" ...
>My complaint about this syntax is mostly about having to use the
>temporary variable.

I partially agree; one measure of the power of an operator (or language) is
its ability to save you a temporary variable.

I once proposed extending the "||" operator to do what you want -- this would
break only those rare programs that pass non-booleans to "||" *and* use the
result in a non-boolean context -- but decided it was a bad idea because

(a) it blurs the distinction between boolean and integer, which is already
fuzzy in C; and

(b) (really a consequence of (a), I guess) it's not quite as powerful as such
a construct could be, because the only comparison allowed is equality with 0.
(Consider the similar example "(c=getchar()) != EOF ? c : defchar".)

So, with tongue in cheek, I proposed the new syntax:
	getenv() unless ==0 inwhichcase "/bin/ed"
(which immediately inspired my colleague to propose the "ifontheotherhand ...
wemaysafelyassume" construct).

Actually, I think what you really want is a pronoun.  I first encountered this
in PDL, in the construct
   if token_scanner_backed_up_flag is set
      unset it
      ...
   endif
; given this, your expression is just (getenv() ? it : "/bin/ed") and mine is
(getchar() == EOF ? defchar : it).  Of course, the problem now becomes one of
defining the antecedent of the pronoun in a consistent way.

>So the main question is, is there some nice way I missed to specify this?

No, you need an explicit temporary (or a function call, in which the formal
argument is the temporary).

>And the secondary question is, if not then is there a standard macro
>name and syntax I should use, like the standard [STREQ macro]?

How will putting it into a macro solve anything?  You still need the temp, and
for a general macro you don't even have a consistent type for it.  (Integer or
pointer?  Pointer to what?)  I recommend you leave it as inline code.

(Or else make a smarter function -- I don't bother for string getenv, but for
reading numbers from the environment I've found
   int env(char *name, int defval) {
      char *ep;
      return ((ep = getenv(name)) == NULL || *ep == '\0' ? defval : atoi(ep));
   }
useful at times.)

Karl W. Z. Heuer (ima!haddock!karl or karl@haddock.isc.com), The Walking Lint