[Octave-bug-tracker] [bug #41894] mod output ML incompatible

[Juan Pablo Carbajal]

URL:
  <http://savannah.gnu.org/bugs/?41894>

                 Summary: mod output ML incompatible
                 Project: GNU Octave
            Submitted by: juanpi
            Submitted on: Tue 18 Mar 2014 10:41:14 AM GMT
                Category: Octave Function
                Severity: 3 - Normal
                Priority: 5 - Normal
              Item Group: Matlab Compatibility
                  Status: None
             Assigned to: None
         Originator Name: juanpi
        Originator Email: 
             Open/Closed: Open
         Discussion Lock: Any
                 Release: dev
        Operating System: Any

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Details:

In the mailing list we have a report about MOD. The matlab implementation
indeed corrects the result when roundoff error generates an answer different
than expected. Example follows


Gamma = 1.62e7;
duration = 10/Gamma;
dt = 0.0025/Gamma;
t   = 0:dt:duration;
y = mod (t, 0.2/Gamma);
find (y==0,3,'first')

octave
1   241   401

Matlab r2008b
1    81   161


Reading the help of mod in matlab it says that the result of MOD
is x - n.*Y with n = floor(x./Y). If Y is not an integer and the quotient x./Y
is within roundoff error of an integer, then n is that integer.

So indeed matlab is giving a result considering roundoff error, I assume they
do something like


function m = mod_ml(x,y)
  if fix(y) != y
   err      = abs (x./y - round(x./y)) < sqrt (eps);
   m       = mod (x,y);
   m(err) = 0;
  endif
endfunction


We could issue a warning as well.

This bug is related to bug #32924




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