Path: utzoo!attcan!uunet!seismo!sundc!pitstop!sun!decwrl!purdue!haven!aplcen!mimsy!chris
From: chris@mimsy.UUCP (Chris Torek)
Newsgroups: comp.lang.c
Subject: `if (a = b)' (was Standard indentation?)
Message-ID: <14945@mimsy.UUCP>
Date: 10 Dec 88 20:22:18 GMT
References: <1988Dec8.173158.11839@utzoo.uucp> <846@starfish.Convergent.COM>
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742
Lines: 46

In article <846@starfish.Convergent.COM> jerry@starfish.Convergent.COM
(Gerald Hawkins) writes:
>Is it ever ok to use the form:
[slightly edited]
>	if (a = b * 2 + 39)	/* intentional assignment within condition */
>		...
>INSTEAD OF:
>	a = b * 2 + 39			/* more normal */
>	if (a)
>		...
>I say "no!"  The code is unsupportable.  EVERYONE who ever reads it will
>assume it is a trivial error (and perhaps try to correct it).

Not necessarily everyone, but it is certainly a dubious construct.
This is especially true since you can write

	if ((a = b * 2 + 39) != 0)
		...

and make it clear that you did not accidentally omit one `='.
I still prefer two separate statements, with one exception:

	/* typical sequence from Unibus drivers */
	if (unit >= NFOO)
		return (ENXIO);
	ui = fooinfo[unit];
	if (ui == NULL || !ui->ui_alive)
		return (ENXIO);

Here one would like to write:

	ui = fooinfo[unit];
	if (unit >= NFOO || ui == NULL || !ui->ui_alive)
		return (ENXIO);

but it is not safe to refer to (fooinfo[k] where k>=NFOO).
Hence:

	if (unit >= NFOO || (ui = fooinfo[unit]) == NULL || !ui->ui_alive)
		return (ENXIO);

This is rather borderline; if the contents of the `if' statement
were more complex I would be more solidly for it.
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163)
Domain:	chris@mimsy.umd.edu	Path:	uunet!mimsy!chris