| Re: TOTAL() and NaNs, again [message #83735 is a reply to message #83730] |
Mon, 25 March 2013 19:06   |
ben.bighair
Messages: 221
Registered: April 2007
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Senior Member |
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Hi,
On Monday, March 25, 2013 5:30:30 PM UTC-4, Fabien wrote:
> Dear IDLers,
>
>
>
> I know my problems with NaNs do bother just my little person. Maybe I am
>
> the only one having crappy data to deal with, or I am the only one using
>
> NaNs to mask out things in my 3D data arrays... Anyways, I'll try to
>
> make this is my last post about NaNs.
>
>
>
> I've been complaining lately about MEAN throwing a math error in this case:
>
> IDL> array = [!VALUES.F_NAN, !VALUES.F_NAN]
>
> IDL> print, MEAN(array, /NAN)
>
> -NaN
>
> % Program caused arithmetic error: Floating illegal operand
>
>
>
> I think, personally, that the result of mean in this case should be a
>
> NaN, and that this should not throw a math error. Because, afterwards,
>
> if I do:
>
> IDL> print, (array[0] + array[1]) / 0.
>
> NaN
>
>
>
> This is an ugly divide by zero but there is no math warning here. But
>
> the problems comes from TOTAL:
>
> IDL> print, TOTAL(array, /NAN)
>
> 0.00000
>
> And of course:
>
> IDL> print, TOTAL(array, /NAN) / TOTAL(FINITE(array))
>
> -NaN
>
> % Program caused arithmetic error: Floating illegal operand
>
>
>
> To be honest, this is described in the Doc: "Since the value NaN is
>
> treated as missing data, if Array contains only NaN values the TOTAL
>
> routine will return 0."
>
>
>
> But I don't see why it is so. This is not coherent with what one would
>
> expect TOTAL to do: the sum of all the elements in the array...
>
>
>
> Anyone to convince me that TOTAL() *has* to return 0 in this case?
>
Well, it seems to be conventional...
http://en.wikipedia.org/wiki/Empty_sum
Cheers,
Ben
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