Re: Is this a numerical problem?

[Jaroslav Hajek]

2010/8/19 Jordi Gutiérrez Hermoso <address@hidden>:
> 2010/8/19 Judd Storrs <address@hidden>:
>> 2010/8/19 Jordi Gutiérrez Hermoso <address@hidden>
>>>
>>> I believe the inconsistency is how 0.999995 is displayed in each with
>>> "format short". Octave displays 0.99999 and Matlab displays 1.0000,
>>> from what I can read in the bug report.
>>
>> I don't understand this comparison. Matlab is displaying four digits after
>> the decimal whereas octave is displaying five. If octave displayed four
>> digits it would also display 1.0000.
>
> It's five digits total. Format short displays five digtis.
>
>> Why is Matlab more correct? The inaccuracy of the Matlab representation is
>> +0.000005 and the inaccuracy of the Octave representation is -0.000005. Why
>> is one "correct" and the other wrong?
>
> Because 5 should round up. Precisely for reasons like this.
>

But remember that the value is rounded already at the point it is
entered, so it's not exactly what you think it is.

And watch this:

octave:1> x = 0.999995
x =  0.99999
octave:2> x - 0.99999 < 1.0 - x
ans =  1

however:

octave:3> round(1e5*x)/1e5
ans =  1

Hmmmm...
That's why I invited Lukas to submit a bug report. This may require
some investigation, and ML threads are always in danger of being
forgotten.

regards

-- 
RNDr. Jaroslav Hajek, PhD
computing expert & GNU Octave developer
Aeronautical Research and Test Institute (VZLU)
Prague, Czech Republic
url: www.highegg.matfyz.cz