From:	Erik Max Francis <max AT alcyone DOT com>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: [Q] expression evaluation order
Date:	Thu, 07 Aug 1997 08:53:22 -0700
Organization:	Alcyone Systems
Message-ID:	<33E9EF72.74D5AA78@alcyone.com>
References:	<5sbb1h$kvr$1 AT newton DOT pacific DOT net DOT sg> <33E98313 DOT 6250 AT pentek DOT com>
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To:	djgpp AT delorie DOT com
DJ-Gateway:	from newsgroup comp.os.msdos.djgpp

Charles Krug wrote:

> > Is it safe to assume that the expression
> >
> > if(first_function()==OK && second_function()==OK &&
> > third_function()==OK) return(OK);
> >
> > will be evaluated from left to right?
>
> No.
> 
> I think that ANSI tries to demand left-to-right evalutaion of &&.  But
> not all compilers do this correctly.  To guarantee left-to-right
> evaluation, you need:

This is just plain wrong.

ANSI C guarantees that && and || are short-circuiting; that is, the
arguments are evaluated in order from left to right, and if an expression
returns a value that determines the value of the logical expression, then
the other arguments will not be evaluated.  You really should get a good
book on ANSI C to allay you fears.  Short-circuiting of && and || was
present in traditional C, much less K&R C.  (In fact, bitwise & and | used
to substitute as logical operators as well -- the && and || operators were
chosen to simplify the logical evaluation _and_ introduce
short-circuiting.)

I have not seen _one_ ANSI C implementation (and I've tried dozens) that
do not do this correctly.

In the expression 

    if (f() && g() && h()) return 0;

f is called; if it returns zero then the others aren't called.  But if it
isn't, then g is called, with the same criteria:  if it returns zero, then
h isn't called.  If f, g, and h all return nonzero values, then this
fragment returns 0.

This works for other expressions with side effects:

    if (i >= 0 && a[i - 1] && j++) /* ... */

Note the implication here; we first check to make sure that i is
nonnegative (that is, a valid array offset), and then with the
short-circuited && expression, index into the array.  There's no danger of
this crashing, because if i is negative, the if expression here will stop
evaluating before it gets to the array subscription.

> The professional in me would also like to see comments explaining what
> first(), second(), and third() do, just in case you put this project
> away for a year.  This is much easier to accomplish when they are on
> seperate lines.

Not an issue if they are given real names, rather than hypothetical names
given here.

> The best explanation of the rules of precedence I've ever heard:
> 
>         "Multiply and divide before you add and subtract"
> 
>         "Put parentheses around everything else"

Then again, people who are too uninterested to learn the rules of
precedence get stomped on the head with a big LAZY.

-- 
       Erik Max Francis, &tSftDotIotE / email / mailto:max AT alcyone DOT com
                     Alcyone Systems /   web / http://www.alcyone.com/max/
San Jose, California, United States /  icbm / 37 20 07 N  121 53 38 W
                                   \
   "Love is not love which alters / when it alteration finds."
                                 / William Shakespeare, _Sonnets_, 116

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