Path: utzoo!utgpu!water!watmath!clyde!rutgers!sdcsvax!sdcc6!ix426
From: ix426@sdcc6.ucsd.EDU (tom stockfish)
Newsgroups: comp.lang.c
Subject: Re: Power proposal for ANSI C
Keywords: power exponentiation **
Message-ID: <3573@sdcc6.ucsd.EDU>
Date: 11 Jan 88 05:52:00 GMT
References: <38384@sun.uucp>
Reply-To: ix426@sdcc6.ucsd.edu.UUCP (tom stockfish)
Organization: University of California, San Diego
Lines: 63

In article <38384@sun.uucp> dgh%dgh@Sun.COM (David Hough) writes:
>     Recommendation: Add a power operator **, with the
>     --------------
>precedence of a cast operator, so that a cast expression can
>be
>
>        cast-expression ** cast-expression
>
>In b ** n, the base expression b may be an integral or
>floating expression, but the exponent expression n must be
>an integral expression.
>
>David Hough

This proposal will never fly as it stands.
1.	It is not in the spirit of C.
	Unlike any other C operator, a lexical analyzer will not be able to
	recognize it, since it is indistinguishable from very common
	pointer declarations, e.g., "char **argv".
2.	The meaning of C expressions is almost always independent of white
	space.  The one exception being
		a+++b
	versus
		a+ ++b.
	Even this one, however, is disambiguated by the "the longest possible
	token is the next one" rule.  You would have to invent something
	else to disambiguate
		x ** y
	from
		x * (*y).
3.	I'm not sure what precedence/associativity you have chosen for "**".
	From your posting I assume you mean that
		(double)2 ** 3
	should group as
		( (double)2 ) ** 3.
	This is disastrous.  Since unary minus has the same precedence
	as casts,
		-x**2 + x - 10
	would have to group as
		(-x) ** 2  +  x  -  10
	which is equivalent to
		x**2 + x - 10.
	(The arbitrary precision language bc(1) actually has this precedence!
	Has any other numeric user used bc for serious work?  I'd like to swap
	bug lists with you.)
	What associativity did you plan for "**"?  Left to right, or right to
	left?  All other binary operators are left to right, but this would be
	trivial for "**" since it would just collapse the exponents together:
		(x**y)**z == x**(y*z).
	Right to left associativity would be more useful, but would break the
	general rule for other C operators.  You could make it non-associative
	like fortran originally did, but then it would be C's only
	non-associative operator.

I think a good proposal would give the power operator a precedence above
unary and below primary, with right to left associativity (I believe this
is how fortran currently has it).  The only thing left to decide is how
it interacts with pre- or post-fix ++,--, and I'm not sure how that should
be done.  This should be well thought ought, and preferably implemented in
some compiler, before a proposal is submitted to X3J11.
-- 

||  Tom Stockfisch, UCSD Chemistry   tps@chem.ucsd.edu