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Re: NAN Problem [message #69399 is a reply to message #43584]

Wed, 13 January 2010 12:59

penteado
Messages: 866
Registered: February 2018

Senior Member
Administrator

On Jan 13, 6:23 pm, David Fanning <n...@dfanning.com> wrote:
> Folks,
>
> Consider this sequence of commands on my IDL 7.0.1
> LINUX version.
>
> IDL> a = Findgen(11)
> IDL> a[0] = !Values.F_NAN
> IDL> Print, Min(a), Max(a)
> NaN NaN
> IDL> b = Findgen(11)
> IDL> b[1] = !Values.F_NAN
> IDL> Print, Min(b), Max(b)
> 0.00000 10.00000
>
> What do you make of that? This is giving me a great
> deal of touble today while trying to elminate bad
> values from an image. :-(
>
> I have the same results with my Windows IDL 7.1.2
> version, except I do get a warning about a floating
> illegal operand with variable b, which I don't get
> on LINUX.

Interesting. I found the same here, with 7.1.1 in Linux. I had never
noticed this behavior before.

I do not know how the min and max functions were implemented, but this
would be the result of one algorithms that could be used for it. Since
NaN does not compare larger or smaller than anything, the following
would produce the same result:

function min_test,a
res=a[0]
for i=1,n_elements(a)-1 do if a[i] lt res then res=a[i]
return,res
end

When the NaN is the first element, it will be set as the initial value
for the minimum, and since nothing is smaller, it will be kept as the
result. Conversely, if the first value is not NaN, no NaN value that
comes after will be smaller than it, so an NaN would not replace the
minimum.

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[Message index]

		NaN problems By: David Lopez Pons on Thu, 21 April 2005 06:37
		Re: NaN problems By: R.Bauer on Sat, 23 April 2005 12:48
		Re: NaN problems By: David Fanning on Fri, 22 April 2005 20:38
		Re: NaN problems By: yp on Mon, 25 April 2005 16:51
		Re: NaN problems By: yp on Mon, 25 April 2005 16:49
		Re: NAN Problem By: R.Bauer on Thu, 14 January 2010 00:49
		Re: NAN Problem By: penteado on Wed, 13 January 2010 16:15
		Re: NAN Problem By: R.G.Stockwell on Wed, 13 January 2010 16:00
		Re: NAN Problem By: Foldy Lajos on Wed, 13 January 2010 13:00
		Re: NAN Problem By: penteado on Wed, 13 January 2010 12:59
		Re: NAN Problem By: David Fanning on Wed, 13 January 2010 12:39
		Re: NAN Problem By: jeanh on Wed, 13 January 2010 12:28

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