| Re: Strange behaviour of Uniq static method [message #93390 is a reply to message #93386] |
Wed, 29 June 2016 12:09   |
Dick Jackson
Messages: 347
Registered: August 1998
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Senior Member |
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On Wednesday, 29 June 2016 02:14:02 UTC-7, Johan Gustafsson wrote:
> This is my first post here, so hello everybody.
>
> I've encountered a strange behaviour of the static method Uniq (not the old Uniq function, more about that later). To give a short example:
>
> IDL> x = [FltArr(5), -FltArr(5), RandomN(seed, 5)]
> IDL> Print, x.Uniq()
> -1.73792 -1.55209 -0.0861842 0.000000 -0.000000 0.000000 -0.000000 0.000000 -0.000000 0.000000 0.0552376 0.835585
>
> The problem is the repeated zeros in array with supposed unique elements. It seems like the Uniq method treats 0. and -0. as two different values, which I believe is a bit unlogical. Also, according to the help page x.Uniq() should be equivalent to x[Uniq(x, Sort(x))], but
>
> IDL> Print,x[Uniq(x, Sort(x))]
> -1.73792 -1.55209 -0.0861842 0.000000 0.0552376 0.835585
>
> which is the result I would expect.
>
> I don't know if I really have a question, but it would be nice if someone could confirm that x.Uniq() in the example indeed does not give the expected output. Is this a known bug?
>
> I use IDL 8.5.1 under Windows 10
>
> /Johan
Welcome aboard, Johan!
That is indeed strange… it seems that -0.0 and 0.0 are considered equal:
IDL> -0.0 eq 0.0
1
… yet they are distinct IEEE floating point values (showing the conversion to byte values):
IDL> byte(0.0, 0, 4)
0 0 0 0
IDL> byte(-0.0, 0, 4)
0 0 0 128
… and it would depend on the sorting algorithm how the ten "equal but distinct" values get sorted in your array of fifteen values. What you show is that the static x.Uniq() method may be using a sorting method, which handles these differently from Sort(). I'd call it a bug, one that comes only with the unusual occurrence of -0.0.
Of course, you can work around this with an extra step:
IDL> x = [FltArr(5), -FltArr(5), RandomN(seed, 5)]
IDL> x[Where(x EQ -0.0, /NULL)] = 0.0
IDL> Print, x.Uniq()
-0.109547 -0.0809556 -0.0519432 0.000000 0.209843 0.807860
IDL> Print,x[Uniq(x, Sort(x))]
-0.109547 -0.0809556 -0.0519432 0.000000 0.209843 0.807860
May I ask, how did you come across this? Most arithmetic operations that result in zero do not give -0.0. If you convert from a string or text read from a file that is '-0.0', or if you negate 0.0 explicitly, IDL results in -0.0, but I wonder if there was another tricky case we should be aware of.
Cheers,
-Dick
Dick Jackson Software Consulting Inc.
Victoria, BC, Canada --- http://www.d-jackson.com
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