Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.milw.wisc.edu!bionet!ames!xanth!kremer
From: kremer@cs.odu.edu (Lloyd Kremer)
Newsgroups: comp.lang.c
Subject: doubles used as booleans
Keywords: double,float,boolean
Message-ID: <8269@xanth.cs.odu.edu>
Date: 30 Mar 89 19:48:53 GMT
Distribution: na
Organization: Old Dominion University, Norfolk, Va.
Lines: 48


	Can a double be used directly as a boolean?

	A few years ago, when DOS C compilers were in their infancy, I
	was hand optimizing the code of one of our products so that it
	would still fit in the standard 256K of an IBM XT.
	(I said it was a few years ago :-)  )

	The optimizing in the compilers I was using was minimal, and I
	found that substitutions such as 'if(i)' instead of 'if(i != 0)',
	and 'if(!n)' in place of 'if(n == 0)' produced smaller code for
	integral types.  When I tried to extend this technique to floating
	types where the potential gains were even greater (floating point
	comparisons were expensively implemented), the code broke completely.

	For example, if I had

		double a, b;
		.
		.
		if ( a && b )  /* meaning if(a != 0.0 && b != 0.0) */
			statement;

	the entire code fragment silently miscompiled.

	But if I changed the condition to

		( !!a && !!b )
	or to
		( !( !a || !b ) )

	it compiled fine!

	Even K&R 1 states that the ! operator can be applied to any type,
	so I believe the latter examples are required to work, but what
	about the first one?  Would these compilers be officially broken
	by today's standards?

	Pray, what sayeth the pANS?

	If it means anything, doubles were implemented as IEEE 754
	64-bit reals, where 0.0 was represented by a zero-bit pattern.
	Maybe that's the only reason the latter cases worked!  :-)


					Lloyd Kremer
					Brooks Financial Systems
					{uunet,sun,...}!xanth!brooks!lloyd