10 Jul 1998 21:01:31 -0400

Related articles |
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Is infinity equal to infinity? erikr@iar.se (Erik Runeson) (1998-07-08) |

Re: Is infinity equal to infinity? vosse@RULS41.FSW.LEIDENUNIV.NL (1998-07-10) |

Re: Is infinity equal to infinity? rwhutch@nr.infi.net (1998-07-10) |

Re: Is infinity equal to infinity? fis@mpi-sb.mpg.de (Matthias Fischmann) (1998-07-10) |

Re: Is infinity equal to infinity? john_mitchell@intuit.com (John Mitchell) (1998-07-10) |

Re: Is infinity equal to infinity? joachim.durchholz@munich.netsurf.de (Joachim Durchholz) (1998-07-10) |

Re: Is infinity equal to infinity? bear@sonic.net (Ray Dillinger) (1998-07-11) |

Re: Is infinity equal to infinity? Kevin@quitt.net (1998-07-11) |

Re: Is infinity equal to infinity? dwcantrell@aol.com (1998-07-13) |

Re: Is infinity equal to infinity? dwcantrell@aol.com (1998-07-13) |

Re: Is infinity equal to infinity? henry@spsystems.net (1998-07-13) |

[8 later articles] |

From: | John Mitchell <john_mitchell@intuit.com> |

Newsgroups: | sci.math.num-analysis,comp.lang.c,sci.math,comp.compilers |

Date: | 10 Jul 1998 21:01:31 -0400 |

Organization: | Intuit Inc. |

Distribution: | inet |

References: | 98-07-058 |

Keywords: | arithmetic |

Erik Runeson wrote:

*> When comparing floating-point numbers, should infinity (Inf) be*

*> concidered equal to infinity?*

Inf is meant to represent "positive infinity" in a way that would be

most useful in practical computations. NaN is a bit sequence which

does not represent any type of number at all. So it makes (practical)

sense to say that "x == NaN" should be false (because "==" is a

numerical comparison). This is a bit like asking "is the alphabet

blue?". Since the alphabet is "NaC" (not a colored object), we would

answer "no" (or perhaps, "what a ridiculous question!").

The result Inf - Inf = NaN also makes sense (for practical purposes),

since the difference between two large numbers may be big or small,

and the difference between consecutive terms of two sequences which

diverge (to +infinity) may converge to a finite value, diverge to

infinity, or not converge at all.

On the other hand, results like "Inf + Inf = Inf" and "Inf == Inf is

true" do have sensible interpretations when applied to large numbers

or divergent sequences, so it's reasonable to incorporate these into

the implementation of Inf (are you sure the IEEE standard doesn't say

that Inf == Inf is true? I though it did).

John Mitchell

San Diego, California

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