Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!watmath!clyde!rutgers!lll-lcc!seismo!mimsy!chris
From: chris@mimsy.UUCP
Newsgroups: comp.lang.c
Subject: Re: short circuit evaluation
Message-ID: <5178@mimsy.UUCP>
Date: Sat, 24-Jan-87 12:45:20 EST
Article-I.D.: mimsy.5178
Posted: Sat Jan 24 12:45:20 1987
Date-Received: Sun, 25-Jan-87 05:44:12 EST
References: <425@bobkat.UUCP> <102600001@datacube> <34@umich.UUCP>
Organization: U of Maryland, Dept. of Computer Science, Coll. Pk., MD 20742
Lines: 66

>>>    x = (a | b | c);
>>>if the variable `a' contains zero, the compiler must still OR the
>>>contents of `b' and `c' to determine the result.

Almost true.

>>>These are bitwise logical operators.  Short-circuiting these makes
>>>no more sense than short-circuiting a sequence of multiplies as
>>>soon as one of the operands evaluates to `1'.
>>>Mike McNally                                    Digital Lynx Inc.

Not so!

>In article <102600001@datacube> stephen@datacube.UUCP writes:
>>This posting indicates a misunderstanding of how short-circuit evaluation
>>works. In the case of the '|' expression above, the decision to not evaluate
>>is would occur when a or b are all ones, NOT when a or b was zero.

Correct.

In article <34@umich.UUCP>jtr485@umich.UUCP (Johnathan Tainter) writes:
>You mean if the '|' had been a '||', of course.

No, he means in the case of the `|' expression above.

>And actually, when a or b is NONZERO not ALL ONES.

Yes for `||', no for `|'.

C guarantees short-circuit left-to-right evaluation for `||' and
`&&'.  It makes no guarantees for `|' and `&'.  Here is a table
enumerating all possibilities.

    [For these, read `all zeroes bit pattern' for `OFF', and `all ones
     bit pattern' for `ON'.  This is a text compression device.]

      Given:	Possible methods of evalution:
      ------	------------------------------
	&&	Left side evaluated.  If zero, result is zero,
		and evaluation stops.  If nonzero, right side evaluated;
		result is 0 if zero, 1 if nonzer.

	||	Left side evaluated.  If nonzero, result is 1,
		and evaluation stops.  If zero, right side evaluated;
		result is 1 if nonzero, 0 if zero.

	&	1.  Left side evaluated.  If OFF, evaluation stops;
		    result is OFF.  If not, right side evaluated,
		    and both results ANDed.
		2.  Left side evaluated.  Right side evaluated.
		    Results ANDed.
		3.  Right side evaluated.  If OFF, evaluation stops;
		    result is OFF.  If not, left side evaluated, and
		    both results ANDed.

	|	1.  Left side evaluated.  If ON, evaluation stops;
		    result is ON.  If not, right side evaluated,
		    and both results ORed.
		2.  Left side evaluated.  Right side evaluated.
		    Results ORed.
		3.  Right side evaluated.  If ON, evaluation stops;
		    result is ON.  If not, left side evaluated, and
		    both results ORed.
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7690)
UUCP:	seismo!mimsy!chris	ARPA/CSNet:	chris@mimsy.umd.edu