Re: [BLACKBOX] Seasons Greetings from Linuxland

From: [at]} <Chris>
Date: Wed, 29 Dec 2010 09:35:22 +1030

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>-----Original Message-----
>From: BlackBox [mailto:BLACKBOX{([at]})nowhere.xy
>Sent: Wednesday, 29 December 2010 2:01 AM
>To: BLACKBOX{([at]})nowhere.xy
>Subject: [BLACKBOX] Seasons Greetings from Linuxland
>
>A C programmer can make exactly the same argument that he/she
>has to evaluate an expression and then read two additional
>lines to understand what the outcome was if the initial
>complex expressions are expanded (a.k.a. made long winded).
>
>
> // z = x++ + ++y;
> ++y;
> z = x + y;
> x++;
>
>But which forms best express what the program writer intended?
>

The original point made by Wirth was related to the problem of parsing:

  x+++++y

It was your interpretation that it must have been intended to be:

  z = x++ + ++y;

However, if additional blanks are to be admitted, the C compiler I tested it
with (GCC 4.4.2) is also happy to compile:

        z = x+ + + ++y;
        z = x+ + + + +y;
        z = x++ + + +y;
        z = x+++ ++y;

so who can say what the program writer intended?

--------------------------------

However, my initial complaint about the ++ operator was not related to the
brevity of expressions. I was reporting a real life example of the
unintended consequences of the order of evaluation of expressions and the
involvement of side effects. I said that a few days ago a C programmer was
complaining in another group that the following "does not work, gives
unexpected results":

  digdata[digNr] = (bitdata[bitNr++] * 8) + (bitdata[bitNr++] * 4) +
(bitdata[bitNr++] * 2) + (bitdata[bitNr++]);

As it says in my ancient copy of K&R C (maybe C has been tightened up since
then but I don't think so?):

"In particular the compiler considers itself free to compute subexpressions
in the order it believes most efficient, even if the subexpressions involve
side effects. The order in which side effects take place is unspecified.
Expressions involving a commutative and associative operator (*, +, &, |, ^)
may be rearranged arbitrarily, even in the presence of parentheses; to force
a particular order of evaluation an explicit temporary must be used."

That is just one of many reasons why I believe that programming in C is akin
to running with chainsaws.

>Good program readability is important whatever the language
>which is being used and anything which enhances it is to be
>welcomed and anything which detracts from it is to be discouraged.
>

I agree with you 100%.

Regards,
Chris Burrows
CFB Software
http://www.cfbsoftware.com

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Received on Wed Dec 29 2010 - 00:05:22 UTC