From bug-request at octave dot org Mon Oct 17 08:07:35 2005
Subject: Re: comparison error (approssimation?)
From: Miroslaw Kwasniak <mirek at zind dot ikem dot pwr dot wroc dot pl>
To: bug at octave dot org
Date: Mon, 17 Oct 2005 15:07:01 +0200

On Mon, Oct 17, 2005 at 03:48:54AM -0400, John W. Eaton wrote:
> On 16-Oct-2005, Marco Iora wrote:
> 
> | To: bug at octave dot org
> | Cc: marcoiora
> | Subject: comparison error (approssimation?)
> | 
> | Bug report for Octave 2.1.71 configured for powerpc-apple-darwin7.9.0
> | 
> | Description:
> | -----------
> | 
> | Octave does an uncorrectly comparison, i think due to approssimation.
> | 
> | Repeat-By:
> | ---------
> | 
> | t = [-1:0.1:1]
> | t(16) == 1/2
> | t(16) == 0.5

For Marco, see what Maxima says:

 a:0.1; b:1/10; c:a-b; [is(a=a), is(b=b), is(a=b), is(c=0)];

                                0.1

                                1
                                --
                                10

                                0.0

                    [TRUE, TRUE, FALSE, FALSE]

;)

[...] 
 
> Here, cache is the matrix of values corresponding to range elements
> (in some cases, the conversion to a matrix is not done).
> 
> So how could this be improved?

I though than doing increments in integers and scaling them is much better
but this example:

octave:1> n=1000;a=-1*n:.1:1*n; b=(-n*10:10*n)/10; ea=(diff(a)/.1)-1;
eb=(diff(b)/.1)-1;
octave:2> [ statistics (ea) statistics (eb)]
ans =

   -9.0949e-13   -9.0949e-13
   -5.6843e-14   -5.6843e-14
    2.2737e-13    2.2737e-13
    2.2737e-13    2.2737e-13
    2.2737e-13    2.2737e-13
    5.6821e-17   -2.2204e-20 <-----------------
    3.5038e-13    3.5039e-13
   -1.6464e+00   -1.6460e+00
    1.6251e+00    1.6237e+00

shows only improvment in mean error.



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