I see your point. How about passing number of significant figures instead of atol.

In fact, that’s what I originally intended but I thought that it could be expressed via atol and rtol, whereas number of significant figures doesn’t seem to suffer from the ambiguity you pointed out.

From: NumPy-Discussion <numpy-discussion-bounces+aminthefresh=gmail.com@python.org> On Behalf Of Stephan Hoyer
Sent: Thursday, July 16, 2020 3:06 PM
To: Discussion of Numerical Python <numpy-discussion@python.org>
Subject: Re: [Numpy-discussion] Augment unique method

On Thu, Jul 16, 2020 at 11:41 AM Roman Yurchak <rth.yurchak@gmail.com> wrote:

One issue with adding a tolerance to np.unique for floats is say you have
  [0, 0.1, 0.2, 0.3, 0.4, 0.5] with atol=0.15

Should this return a single element or multiple ones? One once side each
consecutive float is closer than the tolerance to the next one but the
first one and the last one are clearly not within atol.

Generally this is similar to what DBSCAN clustering algorithm does (e.g.
in scikit-learn) and that would probably be out of scope for np.unique.

I agree, I don't think there's an easy answer for selecting "approximately unique" floats in the case of overlap.

np.unique() does actually have well defined behavior for float, comparing floats for exact equality. This isn't always directly useful, but it definitely is well defined.

My suggestion for this use-case would be round floats to the desired precision before passing them into np.unique().

Roman

On 16/07/2020 20:27, Amin Sadeghi wrote:
> It would be handy to add "atol" and "rtol" optional arguments to the
> "unique" method. I'm proposing this since uniqueness is a bit vague for
> floats. This change would be clearly backwards-compatible.
>
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