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From: tps@sdchem.UUCP
Newsgroups: comp.lang.c
Subject: Re: short circuit evaluation
Message-ID: <621@sdchema.sdchem.UUCP>
Date: Sun, 25-Jan-87 15:58:46 EST
Article-I.D.: sdchema.621
Posted: Sun Jan 25 15:58:46 1987
Date-Received: Mon, 26-Jan-87 02:20:47 EST
References: <425@bobkat.UUCP> <102600001@datacube> <34@umich.UUCP> <5178@mimsy.UUCP>
Sender: news@sdchem.UUCP
Reply-To: tps@sdchemf.UUCP (Tom Stockfisch)
Organization: UC San Diego
Lines: 57

In article <5178@mimsy.UUCP> chris@mimsy.UUCP (Chris Torek) writes:
>...
>    [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:
>      ------	------------------------------
>	&	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)

I don't think so.  "&" and "|" are like "+" and "-" in that you are not
guaranteed that the left side will be evaluated before the right.  Read
K&R sections 7.8-10, p. 190:
	"The [&|^] operator is associative and expressions involving [&|^]
	may be rearranged." 
Otherwise, how could an optimizing compiler change

	(a | b)  &  (b | c)

to

	(a & c) | b

or fold constants together as in changing

	(a | CONST1) & (a | CONST2)

to

	a | CONST3

where CONST3 is (CONST1 & CONST2).
	
I agree with you that the evaluation might be short circuited if the result
is already known.  However, I don't think this is guaranteed,
it might happen in the reverse order (right operand evaluated,
left operand skipped),
and it is guaranteed not to happen if the to-be-skipped operand has a
side effect.

|| Tom Stockfisch, UCSD Chemistry	tps%chem@sdcsvax.UCSD