| Re: Philosophical Question about NAN [message #63742 is a reply to message #63730] |
Mon, 17 November 2008 07:32   |
Kenneth P. Bowman
Messages: 585
Registered: May 2000
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Senior Member |
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In article <MPG.238b3491ef337cc798a534@news.giganews.com>,
David Fanning <news@dfanning.com> wrote:
> Folks,
>
> I've had a couple of run-ins lately with NANs and I wonder
> why routines like TOTAL and MEAN don't have the NAN keyword
> set to 1 by default. Why does the user have to set it?
>
> I understand the argument that the NAN capability was
> added as an afterthought (or more likely when someone
> standardized the NAN bit pattern), and so the functionality
> was added as an optional addition that enhanced the function
> rather than changed it. But really...is there a reason
> why it is not the default now?
>
> One could argue, I suppose, that having a program stumble
> over a NAN alerts you to its presence in your data. That
> is useful, certainly. But, typically, once I add a NAN
> keyword to my code, I don't know (nor do I or care) if the
> argument has NANs. Is this lazy programming on my part?
>
> I am just wondering whether not setting the default value
> of the NAN keyword to 1 on routines like TOTAL, MEAN,
> et. al is the functional equivalent of not setting the
> default values of the COLOR and BITS_PER_PIXEL keywords
> to the PostScript device to something useful by default.
> That is, an act of negligence on the part of the
> manufacturer.
>
> What say you?
>
> Cheers,
>
> David
HI David,
I think they chose correctly and erred on the side of safety.
If I know there are Nans in my data, I'll take care of it.
If there are Nans in the data that I don't expect, I don't want to
have to set a keyword somewhere to find that out. That is, I don't
want IDL to automatically skip those Nans.
OTOH, I still find this to be frustrating and dangerous
IDL> PRINT, TOTAL(REPLICATE(!VALUES.F_NAN, 5), /NAN)
0.00000
There are no valid numbers in the input vector, but TOTAL
returns a valid FLOAT. This makes the NAN keyword useless
in many situations.
Ken
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