Re: NaN and Inifinity

Scott Thompson wrote:

> Hrrrm... Okee dokey. I tried something like that and I get
>
> irb(main):002:0> b = "NaN".to_f
> => 0.0
> irb(main):003:0> b.nan?
> => false
>
> There must be something wacky with my Ruby. :-)

Not that wacky. I get the same.

C:\> ruby -v
ruby 1.8.0 (2003-08-04) [i386-mswin32]

Maybe this has changed recently?

Scott Thompson wrote:

>> irb(main):015:0> a = Math::log(-1)
>> => NaN
>> irb(main):016:0> a.class
>> => Float
>> irb(main):017:0> b = "NaN".to_f
>> => NaN
>> irb(main):018:0> b.class
>> => Float
>> irb(main):019:0> b.nan?
>> => true

>
>
> Hrrrm... Okee dokey. I tried something like that and I get
>
> irb(main):002:0> b = "NaN".to_f
> => 0.0
> irb(main):003:0> b.nan?
> => false
>
> There must be something wacky with my Ruby. :-)
>
> Scott

It's not just you. Happens to me, too, using Ruby 1.8.0pre9 (I believe)
mswin32 build.

Then again, Math::log(-1) throws a domain error for me.

If worse comes to worst you could always initialize with Math::log(-1),
since that returns
what you want.

- Dan

In article <B0D86431-CC65-11D7-AFAD-000393803090@mac.com>,
Scott Thompson <easco@mac.com> wrote:

>If I do something like
>
>a = Math::log(-1)
>
>then a properly takes the value NaN.
>
>Is there any way to explicitly assign the variable "a" to NaN (or
>Inifinity or -Infinity) to begin with? I tried finding "NaN" as a
>constant but couldn't locate it. It doesn't appear to be a constant in
>Float or Math.

I don't understand what you mean by 'assign the variable "a" to NaN'.

NaNs and infinite values are best tested for using the nan? and
infinite? methods. Note that infinite? returns +1 or -1 for positive
and negative infinities and nil for finite numbers:

>> a = 0.0/0.0

=> NaN

>> a.nan?

=> true

>> a.infinite?

=> nil

>> b = 0.0/0.0

=> NaN

>> b.nan?

=> true

>> a == b

=> false

>> (+1/0.0).infinite?

=> 1

>> (-1/0.0).infinite?

=> -1

>> 1.0.infinite?

=> nil

>> 1 / (-1/0.0)

=> -0.0

I'm avoiding the mathematical justification for these behaviours, they're
just what IEEE 754 math does. There are plenty of references on the web
- http://grouper.ieee.org/groups/754/reading.html is a good starting
point.

Mike
--
mike@stok.co.uk | The "`Stok' disclaimers" apply.
http://www.stok.co.uk/~mike/ | GPG PGP Key 1024D/059913DA
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Oh yeah, I'm way out of date.

I think I'm using 1.6.something.

-Kurt

On Tue, Aug 12, 2003 at 11:16:41AM +0900, Dan Doel wrote:

> Scott Thompson wrote:
>

> >>irb(main):015:0> a = Math::log(-1)
> >>=> NaN
> >>irb(main):016:0> a.class
> >>=> Float
> >>irb(main):017:0> b = "NaN".to_f
> >>=> NaN
> >>irb(main):018:0> b.class
> >>=> Float
> >>irb(main):019:0> b.nan?
> >>=> true

> >
> >
> >Hrrrm... Okee dokey. I tried something like that and I get
> >
> >irb(main):002:0> b = "NaN".to_f
> >=> 0.0
> >irb(main):003:0> b.nan?
> >=> false
> >
> >There must be something wacky with my Ruby. :-)
> >
> >Scott

>
> It's not just you. Happens to me, too, using Ruby 1.8.0pre9 (I believe)
> mswin32 build.
>
> Then again, Math::log(-1) throws a domain error for me.
>
> If worse comes to worst you could always initialize with Math::log(-1),
> since that returns
> what you want.
>
> - Dan
>
>
>======= End of Original Message =======<

The other thing you might want to consider is how useful this would be.
You can't compare NaNs, as far as I know.

irb(main):015:0> a = Math::log(-1)
=> NaN
irb(main):016:0> a.class
=> Float
irb(main):017:0> b = "NaN".to_f
=> NaN
irb(main):018:0> b.class
=> Float
irb(main):019:0> a == b
=> false
irb(main):020:0> a
=> NaN
irb(main):021:0> b
=> NaN
irb(main):022:0> a == a
=> false

-Kurt

On Tue, Aug 12, 2003 at 11:26:58AM +0900, Hal E. Fulton wrote:

> ----- Original Message -----
> From: "Mike Stok" <mike@ratdog.stok.co.uk>
> Newsgroups: comp.lang.ruby
> To: "ruby-talk ML" <ruby-talk@ruby-lang.org>
> Sent: Monday, August 11, 2003 9:21 PM
> Subject: Re: NaN and Inifinity
>
>

> > In article <B0D86431-CC65-11D7-AFAD-000393803090@mac.com>,
> > Scott Thompson <easco@mac.com> wrote:

> > >If I do something like
> > >
> > >a = Math::log(-1)
> > >
> > >then a properly takes the value NaN.
> > >
> > >Is there any way to explicitly assign the variable "a" to NaN (or
> > >Inifinity or -Infinity) to begin with? I tried finding "NaN" as a
> > >constant but couldn't locate it. It doesn't appear to be a constant in
> > >Float or Math.

>
> module Math
> Nan = log(-1)
> end
>
> Now it is. :)
>
> Hal
>
>
>======= End of Original Message =======<

Hi,

> From: "Kurt M. Dresner" <kdresner@cs.utexas.edu>
> Sent: Tuesday, August 12, 2003 11:20 AM

> Oh yeah, I'm way out of date.

Depends underlying C library.

soap4r maps "NaN", "+INF" and "-INF" by itself.
http://www.ruby-lang.org/cgi-bin/cvs...-cvsweb-markup

I wish Float to have constans for these special values.
http://blade.nagaokaut.ac.jp/cgi-bin...uby-talk/62417

Regards,
// NaHi

In article <B0D86431-CC65-11D7-AFAD-000393803090@mac.com>,
Scott Thompson <easco@mac.com> wrote:

>If I do something like
>
>a = Math::log(-1)
>
>then a properly takes the value NaN.
>
>Is there any way to explicitly assign the variable "a" to NaN (or
>Inifinity or -Infinity) to begin with? I tried finding "NaN" as a
>constant but couldn't locate it. It doesn't appear to be a constant in
>Float or Math.

If you are using IEEE 754 math then you can try

[mike@ratdog mike]$ irb --simple-prompt

>> negInf = *['FFF0000000000000'].pack('H*').unpack('G')

=> -Infinity

>> inf = *['7FF0000000000000'].pack('H*').unpack('G')

=> Infinity

>> negInf.infinite?

=> -1

>> nan = *['7FF8000000000000'].pack('H*').unpack('G')

=> NaN

>> nan.nan?

=> true

>> nan == nan

=> false

(though there are a lot of ways to initialise a nan...)

Once you have them in a variable then there's not much you can do with
them.

Hope this helps,

Mike

--
mike@stok.co.uk | The "`Stok' disclaimers" apply.
http://www.stok.co.uk/~mike/ | GPG PGP Key 1024D/059913DA
mike@exegenix.com | Fingerprint 0570 71CD 6790 7C28 3D60
http://www.exegenix.com/ | 75D2 9EC4 C1C0 0599 13DA

Hi Scott.

It might help to think of NaN meaning "some undefined value". Then it
makes sense that you can't assign it - it's undefined - and also that
you can't compare two NaNs - they're both undefined so how can you? All
you can do is use Float.nan? to ask whether the result of what you just
did, in this case log(-1), is undefined. It also helps explain why it
isn't, um, defined in Math or Float.

Cheers,
Dan

Scott Thompson wrote:

> If I do something like
>
> a = Math::log(-1)
>
> then a properly takes the value NaN.
>
> Is there any way to explicitly assign the variable "a" to NaN (or
> Inifinity or -Infinity) to begin with? I tried finding "NaN" as a
> constant but couldn't locate it. It doesn't appear to be a constant
> in Float or Math.
>
> Scott
>
>
>