Re: Numeric issue

[Marco Atzeri]

On 04/07/2014 11:46, JokerOne wrote:
> Hi community,
>
> I using Octave for quite a while now, however, apparently, I am not aware of
> some numeric issues going on, which is where I like to ask you for help:
>
> Basically, I noticed, that in the following dummy code, the results appear
> strange to me:
>
> clear all;
> format long;
>
> a   = [1.1:1.1:100];
> a(50)                     % expected to be == 55
> b   = 50*1.1;          % expected to be == 55

octave:7> 50*1.1
ans =  55.000


> ((a(50)-b) == 0)     % is 1
> ((a(50)-55) == 0)    % is 0 (!?)

this check is always questionable on any numerical operation,
due to rounding precision.
On my system:

octave:10> a(50)-55
ans =    7.1054e-15

> ceil(a(50))              % expected to be == 55, but results in 56 ??
> ceil(b)                    % same as above ??
> ceil(55)                   % == 55, as expected

for the reason above the ceiling is 56....

Regards
Marco