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From: randy@umn-cs.UUCP (Randy Orrison)
Newsgroups: comp.lang.c
Subject: Re: MSC C arithmetic
Message-ID: <2042@umn-cs.UUCP>
Date: Wed, 19-Aug-87 17:38:39 EDT
Article-I.D.: umn-cs.2042
Posted: Wed Aug 19 17:38:39 1987
Date-Received: Sat, 22-Aug-87 05:20:47 EDT
References: <8792@brl-adm.ARPA> <305@wrs.UUCP>
Reply-To: randy@umn-cs.UUCP (Randy Orrison)
Organization: UofM Math Department
Lines: 44

In article <305@wrs.UUCP> dg@wrs.UUCP (David Goodenough) writes:
>The second example fails because '*' groups left to right (see page 49 of the
>gospel according to Kernighan and Ritchie :-) and it does the two integer
>multiplications first, and then the float (i.e. double). This should work
>correctly if you wrap the last multiplication in parentheses:
>
>>	total_bits = repeat_cnt * (units * (blksiz * 16.0));
>
>as this then forces all multiplications to have one float operand, hence
>getting the desired result.

Interesting.  Page 49 of my K&R states that "expressions involving one of the
associative and commutative operators (*,...) can be rearranged even when
parenthesized."  Which means that your parentheses should do nothing, and that
the original expression is equivalent to:

	16.0 * repeat_cnt * units * blksiz

Which would mean that the whole expression should be evaluated in double.

Since the compiler is free to evaluate the expression in whatever order is
wants, does that mean that it is not determined what types it will use?
It seems to me that both of the following are legal:

	repeat_cnt * units * blksiz * 16.0
		--int--	       --double--
		     --double--
and
	16.0 * blksiz * units * repeat_cnt
	 --double--
		 --double--
			--double--

Which we know can give different results.  Is this right? (under K&R and ANSI)

(Side note to ANSI people:  I like to use parentheses to make my meaning clean
 to me.  If the compiler can think of a better order to do things, LET IT!!!
 I LOVE unary plus, though temporary variables are just fine.)

-- 
Randy Orrison, University of Minnesota School of Mathematics
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