[Numpy-discussion] unique() should return a sorted array

Norbert Nemec Norbert.Nemec.list at gmx.de
Sun Jul 2 04:47:11 CDT 2006

I agree.

Currently the order of the output of unique is undefined. Defining it in
such a way that it produces a sorted array will not break any compatibility.

My idea would be something like

def unique(arr,sort=True):
    if hasattr(arr,'flatten'):
        tmp = arr.flatten()
        idx = concatenate([True],tmp[1:]!=tmp[:-1])
        return tmp[idx]
    else: # for compatibility:
        set = {}
        for item in inseq:
            set[item] = None
        if sort:
            return asarray(sorted(set.keys()))
            return asarray(set.keys())

Does anybody know about the internals of the python "set"? How is
.keys() implemented? I somehow have really doubts about the efficiency
of this method.

David Huard wrote:
> Hi,
> Numpy's unique(x) returns an array x with repetitions removed.
> However, since it returns asarray(dict.keys()), the resulting array is
> not sorted, worse, the original order may not be conserved. I think
> that unique() should return a sorted array, like its matlab homonym.
> Regards,
> David Huard
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