Re: max() and min() of nan/inf

[Olli Saarela]

Hello,

> Is this a correct behaviour?
> 
> max() or min() return nan is a nan is one of its arguments? max() or
> min() considering +/-Inf looks OK.

Yes, it is correct. Of course this is a matter of definition (defined in
the IEEE Standard 754), but the following example tries to give some
rationale:

  John is 186 cm long.
  Mark is 176 cm long.
  I don't know how long Philip is (= NaN).
  How long is the longest of them (= max)? Can't be answered (= NaN).

A NaN value (Not a Number) is not a real number. It doesn't have a
magnitude which could be compared. The min and max functions return the
smallest and the largest number of the set of numbers given. When the
set contains a NaN, the numbers don't any more have a minimum or a
maximum value. Hence, the value NaN is returned, indicating that there
is no number equal to the minimum or maximum value of the set.

> arithmetic or comparition operations
> are of course not defined on nans...

But they are defined. Any arithmethic operation with a NaN operand is
defined to evaluate to NaN. Any comparison with a NaN operand is defined
to evaluate to FALSE (except the ~= operation to TRUE).

For example:
  Is NaN bigger than 3? No.   (NaN > 3) --> FALSE
  Is NaN smaller than 3? No.  (NaN < 3) --> FALSE
  Is NaN equal to 3? No.      (NaN == 3) --> FALSE

For more complete description, please see:
http://www.lpthe.jussieu.fr/DOC_HTML/WorkShop/common-tools/numerical_comp_guide/goldberg1.ps


Best regards,
        Olli
-- 
Olli Saarela                       KCL Development Oy
address@hidden                tel. +358-9-4371538 (office)
Tekniikantie 2, Espoo-Otaniemi     fax. +358-9-464305
P.O. Box 70, FIN-02151 Espoo, Finland



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