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Comparison operators and floating-point errors [message #70560]

Mon, 19 April 2010 11:31

MarioIncandenza
Messages: 231
Registered: February 2005

Senior Member

IDL> nan=1/0.0
% Program caused arithmetic error: Floating divide by 0
IDL> print, -0.1 > nan
Inf
IDL> print, -0.1 < nan
-0.100000
IDL> print, 1 gt nan
0
IDL> print, 1 lt nan
1

I thought the rule was "Any calculation involving a NaN will produce
NaN."
Has it been this way since the dawn of time? I'm sure it has.

I have found a few uses for NaN in IDL over the years, and now I have
to come up with new ones.

For instance (the one that led me to track this down), I use NaN to
exclude areas from contour plots. There are other ways, but none quite
as simple and flexible.

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