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From: franka@mmintl.UUCP
Newsgroups: comp.lang.c
Subject: Re: short circuit evaluation
Message-ID: <1971@mmintl.UUCP>
Date: Tue, 3-Feb-87 18:48:36 EST
Article-I.D.: mmintl.1971
Posted: Tue Feb  3 18:48:36 1987
Date-Received: Mon, 9-Feb-87 03:29:02 EST
References: <425@bobkat.UUCP> <102600001@datacube> <34@umich.UUCP> <14479@amdcad.UUCP> <5199@mimsy.UUCP> <3721@teddy.UUCP>
Reply-To: franka@mmintl.UUCP (Frank Adams)
Organization: Multimate International, E. Hartford, CT
Lines: 30

In article <3721@teddy.UUCP> jpn@teddy.UUCP (John P. Nelson) writes:
>60 seconds with my copy of K&R yeilds this (Appendix A):
>
>  "The & operator is associative and expressions involving & may be
>   rearranged"
>
>  "Unlike &, && guarantees left-to-right evaluation; moreover, the second
>   operand is not evaluated if the first operand is zero"
>
>True, it is not explitly stated that & and | are not short circuited.  It
>also doesn't explicitly say that + and * are not short circuited!  However
>&& and || are EXPLICITY defined as having short circuit behavier, no other
>operators are described this way, so it is safe to assume that all other
>operators do NOT short-circuit.

As a general rule, whatever is not specified in the C description (whether K&R
or ANSI) is just that: not specified.  Just because certain operators are
described as having certain behavior does not guarantee that those operators
for which the behavior is not specified behave in the opposite way.

I would not write code which assumes that & and | are not short-circuited;
nor would I write code which assumes that + and * are not short-circuited.
(Although + really can't be.)  But then, I use side-effects of expressions
only in time-critical portions of the code, and only when the side effect
is *guaranteed* to work by the language specification or in pieces of code
marked as machine dependent.  (However, my tendency is to write machine
dependent code in assembler.)

Frank Adams                           ihnp4!philabs!pwa-b!mmintl!franka
Ashton-Tate          52 Oakland Ave North         E. Hartford, CT 06108