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[Octave-bug-tracker] [bug #42589] Inaccurate results with the colon oper


From: Michael Godfrey
Subject: [Octave-bug-tracker] [bug #42589] Inaccurate results with the colon operator
Date: Sun, 22 Jun 2014 14:36:24 +0000
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:30.0) Gecko/20100101 Firefox/30.0

Follow-up Comment #22, bug #42589 (project octave):

While discussing floating point precision I noticed:
(on Matlab and Octave)

e = eps; z = 1+(1+e)*e/2;d = z -1;

yields for d:

d =    2.2204e-16

This means, according to Kahan, that the FP extended bits
are turned off. Matlab has been critized for doing this.
Was this done in Octave intendionally for compatibility?

This only applies to "native" Octave arithmetic, but is
it really required?

If the extended bits were on but ignored the results 
are for z
z = 1 , not z =    1.000
and for d
d = 0 , not d =    2.2204e-16

This is not "psychological" it is about making use
of available FP precision.


d is 0.

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