Re: Problems with float variables in C

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#507

From: Trevor L. Jackson, III
Date: Sat, 08 Sep 2001 19:10

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Edward Rosten wrote:

> > When checking for possible difficulty with division by zero, if you don't
> > wish to get into trapping overflow and the like, you should check at least
> > for small non-zero numbers.  If all your numerators have absolute value < 4,
> > you could safely do
> > #include <float.h>
> > if(fabs(y)>FLT_MIN)
> >     proceed with division by y
> > else
> >     treat as error
>
> I'm not trying to be pedantic, I'm genuinely curious, but isn't
>
> fabs(y) > FLT_MIN
>
> equivalent to
>
> f != 0
>
> Since 0 can be represented exactly and FLT_MIN is the smallest
> possible float greater than zero?

FLT_MIN is not the smallest possible float greater than zero.  It is the smallest
float with FLT_DIG of valid mantissa digits.  Gradual underflow (to zero) may
include intermediate values between FLT_MIN and zero that have fewer than FLT_DIG
digits of precision.

Normalized numbers (>=FLT_MIN) shift the mantissa and scale the exponent until the
MSB of the mantissa is one.  So called "denormalized numbers" aren't shifted
(because there is no room to scale the exponent below FLT_MIN_EXP).  The result is
a gradual loss of precision as you decrease toward zero.  There are typically
2**(FLT_DIG-1)-1 such numbers between FLT_MIN and zero.
-- 
comp.lang.c.moderated - moderation address: clcm@plethora.net

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