"nan".to_f ?

[GOTO Kentaro]

Hi,

In message "Re: "nan".to_f ?"

| def aNaN
| s, e, m = rand(2), 2047, rand(2**52-1)+1
| [sprintf("%1b%011b%052b", s,e,m)].pack("B*").unpack("G").first
| end
|
|I believe this will generate NaN on environments where
|pack/unpack works and NaN exists.

If the platform uses IEEE floating number, right?

Agreed! And if there exists NaN on a non IEEE platform,
that NaN must be another concept because NaN is defined in
IEEE 754.


Gotoken

 

[Markus]

My mistake is clear. I was assuming Float == IEEE.

I still see the original poster's desire to be able to serialize
floats as text without gross changes, but do not at this point see a
clear and portable way of implementing it. At best (using the code I
posted yesterday morning) you could maintain the distinction between
error-values and numbers, but this might well silently morph (say) Inf
to NaN, etc.

-- Markus

Hi,

In message "Re: "nan".to_f ?"

| def aNaN
| s, e, m = rand(2), 2047, rand(2**52-1)+1
| [sprintf("%1b%011b%052b", s,e,m)].pack("B*").unpack("G").first
| end
|
|I believe this will generate NaN on environments where
|pack/unpack works and NaN exists.

If the platform uses IEEE floating number, right?

Agreed! And if there exists NaN on a non IEEE platform,
that NaN must be another concept because NaN is defined in
IEEE 754.


Gotoken

 

[GOTO Kentaro]

In message Re: "nan".to_f ?

My mistake is clear. I was assuming Float == IEEE.

Mistake? I don't think you mistook. or I also mistake.
When we are talking about Ruby, NaN means IEEE 754's NaN, isn't it?

I still see the original poster's desire to be able to serialize
floats as text without gross changes, but do not at this point see a
clear and portable way of implementing it. At best (using the code I
posted yesterday morning) you could maintain the distinction between
error-values and numbers, but this might well silently morph (say) Inf
to NaN, etc.

NaN was designed to represent abnormal result of an arithmetic operation.
So, IMHO, String#to_f is not arithmetic and should not returns NaN.
By the way, we sometimes want to define a method return NaN value.
In such situations I believe pack/unpack will help.


Gotoken

 

[Yukihiro Matsumoto]

Hi,

In message "Re: "nan".to_f ?"

|> My mistake is clear. I was assuming Float == IEEE.
|
|Mistake? I don't think you mistook. or I also mistake.
|When we are talking about Ruby, NaN means IEEE 754's NaN, isn't it?

C99 defines isnan() and isinf(). I'm not sure C99 assumes IEEE 754.

matz.

 

[GOTO Kentaro]

Hi,

In message Re: "nan".to_f ?

|When we are talking about Ruby, NaN means IEEE 754's NaN, isn't it?

C99 defines isnan() and isinf(). I'm not sure C99 assumes IEEE 754.

That is very IEEE 754 support.

ISO/IEC 9899:1999 Annex F says:

F.1 Introduction

This annex specifies C language support for the IEC 60559
floating-point standard. The IEC 60559 floating-point standard is
specifically Binary floating-point arithmetic for microprocessor
systems, second edition (IEC 60559:1989), previously designated IEC
559:1989 and as IEEE Standard for Binary Floating-Point Arithmetic
(ANSI/IEEE 754-1985). ...

Gotoken

 

[Ryo Furue]

Hi,

In message "Re: "nan".to_f ?"

|I would say that 'NaN' is a the string representation of a float, so
|logically it should be parsed to such, that's why I'm wondering why
|this behaviour was *removed*.

It's not valid string representation of a float.

[...]

Maybe I'm too late to jump in to this old thread
(three-days old is old enough for people to forget about
a thread . Anyway, . . .

I think that symmetry is important, or if not important, it's nice
to have. So, *if*

x = 0.0/0.0 #=> NaN
x.to_s #=> "NaN"

*then*

y = "NaN".to_f # y *should* be NaN.

In other words, I'd expect (x.to_s).to_f to be an identity
operation (apart from potential truncation associated with to_s)
for a Float x.

I'm not saying that 0.0/0.0 *should* evaluate to NaN. That depends
on the platform. What I'm saying is that whatever x.to_s evaluates
to, (x.to_s).to_f "should" recover the (more-or-less) original value.
By "should", I mean "I wish".

Same goes for "Inf" and "-Inf".

Cheers,
Ryo

 

[Yukihiro Matsumoto]

Hi,

In message "Re: "nan".to_f ?"
on Sat, 16 Oct 2004 05:54:36 +0900, (e-mail address removed)-tokyo.ac.jp (Ryo Furue) writes:

| x = 0.0/0.0 #=> NaN
| x.to_s #=> "NaN"
|
|*then*
|
| y = "NaN".to_f # y *should* be NaN.

I agree that it's good to be so. The point is how I can implement
that in portable way. I'm waiting someone to enlighten me the way to
generate NaN portably, or that I can assume IEEE 754 for all the
platforms. Maybe Gotoken is saying the latter in [ruby-talk:116791],
but I'm not sure yet.

matz.

 

[Yukihiro Matsumoto]

Hi,

In message "Re: "nan".to_f ?"

|That is very IEEE 754 support.
|
|ISO/IEC 9899:1999 Annex F says:

<snip>

Does this mean that I can assume IEEE 754 on every platform?
If so, I'm glad that life is THAT simple.

matz.