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From: cik@l.cc.purdue.edu (Herman Rubin)
Newsgroups: comp.lang.c
Subject: Re: Power operator?
Message-ID: <646@l.cc.purdue.edu>
Date: 7 Jan 88 12:21:04 GMT
References: <47000029@uxe.cso.uiuc.edu> <10063@mimsy.UUCP>
Organization: Purdue University Statistics Department
Lines: 39
Summary: Unsatisfactory

In article <10063@mimsy.UUCP>, chris@mimsy.UUCP (Chris Torek) writes:
> In article <47000029@uxe.cso.uiuc.edu> mcdonald@uxe.cso.uiuc.edu writes:
> >I've been wondering why there is no power operator in C.
> 
> There is:
> 
> 	#include <math.h>
> 	...
> 		double a, x;
> 		...
> 		a = pow(x, 7.);
> 
> Unless you write `#undef pow' after `#include <math.h>', you may
> get in line code (fpushf (fp-4); fftod; fpushd $0d7.0; fexpd etc).

I think everyone should consider this unsatisfactory. 

One reason is that requiring the writing of a subroutine call is that
this requires people to use that notation instead of writing 

	x = y ** z	or x = y ! z.

Similar thinking would require one to write x = sub(y, z) instead of
x = y - z.  This is a major design flaw in all HLLs which I know.

Another is that there are _several_ power functions; the one in the standard
math library is the slowest one, which must be used if the exponent is non-
integral.  Standard FORTRAN treats y ** z to be of the type of y if z is
typed integer, and most implementations use the binary representation of z to
compute the result in that case, and even to have several of the most important
cases "precoded", i.e, y ** 2 is usually done as y * y; even if this is not
done, in Torek's example, if it is known that the 7 is an integer the resulting
code will run at least an order of magnitude faster than the pow function.


-- 
Herman Rubin, Dept. of Statistics, Purdue Univ., West Lafayette IN47907
Phone: (317)494-6054
hrubin@l.cc.purdue.edu (ARPA or UUCP) or hrubin@purccvm.bitnet