Floating point minimum values are positive?

[H. S. Teoh]

On Mon, Jul 22, 2013 at 11:02:28PM +0200, Joseph Rushton Wakeling wrote:
> Always amusing to run into those little quirks of parts of the
> language you've never worked with before ...
> 
> I just realized that while e.g. int.min gives a negative value, the
> floating point equivalent, e.g. double.min, gives a very small
> positive value -- I guess the smallest possible positive value.
> 
> I guess this is intentional, so I thought I'd ask why -- it's a
> little unintuitive after the integral type behaviour.
> 
> Also, how can I get the truly least value of a floating point
> number? I guess with e.g. -double.max ... ?

I believe double.min has been deprecated. In any case, it is a misnomer.
Basically, it's supposed to give the smallest representable "normal"
float, that is, it's the non-zero positive number with the smallest
possible exponent and smallest possible mantissa. The new name for this
IIRC is .min_normal.

There are some floats that can go even smaller than this, but they are
"denormal" and may incur a large runtime overhead (they are intended to
prevent underflow / minimize loss of precision in certain computations
involving very small quantities, and aren't supposed to be used in
normal calculations).

tl;dr: don't use double.min, use -double.max. :)


T

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