Re: lib/44875

[Alan Barrett <apb%cequrux.com@localhost>]

The following reply was made to PR lib/44875; it has been noted by GNATS.

From: Alan Barrett <apb%cequrux.com@localhost>
To: gnats-bugs%NetBSD.org@localhost
Cc: 
Subject: Re: lib/44875
Date: Fri, 29 Apr 2011 20:46:24 +0200

 On Mon, 18 Apr 2011, Martin Husemann wrote:
 > Are there FP values that compare equal and still are 
 > distinguishable?
 
 Yes.  Positive zero and negative zero compare equal, yet are 
 distinguishable, in at least some floating point representations.
 
 When I run this program:
 
 #include <stdio.h>
 int main(void)
 {
      double x = 0.0;        /* positive zero */
      double y = -1.0 * 0.0; /* negative zero */
 
      printf("x = %g\n", x);
      printf("y = %g\n", y);
      printf("%s\n", (x == y ? "x == y" : "!(x == y)"));
      return 0;
 }
 
 I get this output:
 
 x = 0
 y = -0
 x == y
 
 I can also imagine floating point representations in which 
 many values have multiple representations, analogous to the 
 correspondence in decimal between 1.0e0, 10.0e-1, and 0.1e1.
 
 --apb (Alan Barrett)