| Using Minimum operator with NaN values [message #86252] |
Mon, 21 October 2013 17:25  |
wlandsman
Messages: 743
Registered: June 2000
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Senior Member |
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Most of this information is in the IDL docs http://www.exelisvis.com/docs/Minimum_and_Maximum_Oper.html# expressions_2976722315_1032669 but it is easy to forget. The minimum and maximum operators do not work nicely with NaN values.
IDL> print,!Version
{ x86_64 darwin unix Mac OS X 8.2.3 May 2 2013 64 64}
IDL> a = findgen(5)
IDL> a[2] = !values.f_nan
IDL> print,a
0.00000 1.00000 NaN 3.00000 4.00000
IDL> print,a>2
2.00000 2.00000 2.00000 3.00000 4.00000
% Program caused arithmetic error: Floating illegal operand
I think most people would say that the proper behavior would be for the NaN value to be preserved. The IDL doc says the behavior is actually hardware dependent, and on some machines the NaN is preserved! It suggests instead using WHERE and FINITE() instead of ">", whenever NaN values are present. This also eliminates the annoying "illegal operand" error message. The ">" operator does not check for NaN values for speed reasons, and the syntax does not allow having a /NaN keyword with ">" ;-)
A related problem occurs with the HIST_EQUAL() function.
IDL> print,hist_equal(a)
0 85 0 170 255
% Program caused arithmetic error: Floating illegal operand
In this case, it is correct that the NaN is not preserved, since the output of HIST_EQUAL() is a byte array. But I think it is a bug that the illegal operand error occurs. HIST_EQUAL() *does* check for NaN values internally -- for example, HISTOGRAM() is called with the NaN keyword. It is only at the last step in HIST_EQUAL() where a ">" operator is used, that the NaN value are ignored. In this case, I think ITTVIS should follow their own advice and use WHERE and FINITE() rather than the ">" operator.
--Wayne
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