[Octave-bug-tracker] [bug #56692] Second output of ismember (S_IDX) is equivalent to "legacy" output in Matlab

[Rik]

URL:
  <https://savannah.gnu.org/bugs/?56692>

                 Summary: Second output of ismember (S_IDX) is equivalent to
"legacy" output in Matlab
                 Project: GNU Octave
            Submitted by: rik5
            Submitted on: Sun 28 Jul 2019 09:42:09 PM PDT
                Category: Octave Function
                Severity: 2 - Minor
                Priority: 5 - Normal
              Item Group: Matlab Compatibility
                  Status: None
             Assigned to: None
         Originator Name: 
        Originator Email: 
             Open/Closed: Open
         Discussion Lock: Any
                 Release: dev
        Operating System: Any

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Details:

For the following code, Matlab returns [0, 0, 2, 1]


A = [5 3 4 2]; 
B = [2 4 4 4 6 8];
[~,ib] = ismember(A,B)
ib = [0, 0, 2, 1]


Octave, however, returns [0, 0, 4, 1].

The same result can be had in Matlab if the "legacy" option is used with
ismember (which uses the algorithm from Matlab versions prior to 2012b).

The difference is that when there are repeated, non-unique values in the set
B, Matlab returns the index of the first occurrence while Octave returns the
index of the last occurrence.

Debugging further in the ismember.m file, it is the call to lookup() which
returns the last index rather than the first index.

Changing lookup is a lot of work, and likely to have other consequences.

The problem could be worked around in the ismember.m file by sorting the input
set in descending order before calling lookup() and then returning 


s_idx(tf) = (numel (tf) + 1) - is(s_idx(tf));


But, all of the manipulation seems like a lot of work just to get Matlab
compatibility on indices which still work for indexing.





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