[yt-dev] Issue #1147: Derived quantities `max_location` and `min_location` not returning correct index values (yt_analysis/yt)

[Cameron Hummels]

This looks like it would be a welcome addition to the code if someone wants
to put it together, but I agree, that at the minimum, we should remove the
bad index value returned to the user.  Good idea!

On Mon, Nov 23, 2015 at 1:08 PM, Matthew Turk <matthewturk at gmail.com> wrote:

> I have an alternate solution to this problem, which dovetails with the
> numpy-like operations.
>
> If we were to make a different DerivedQuantity (which MaxLocation and
> MinLocation could subclass with a prescription that they return x,y,z)
> that accepted the names of the fields to obtain at the maximum value
> of a field, this could probably accomplish what you are looking for.
> This would be equivalent to the .argmax() operation, so you would be
> able to do:
>
> dd = ds.all_data()
> dd.argmax("density", ["temperature", "velocity_magnitude"])
>
> and get back the temperature and velocity_magnitude fields at the
> maximum density.
>
> Would that suffice?
>
> On Sun, Nov 22, 2015 at 4:11 PM, Matthew Turk <matthewturk at gmail.com>
> wrote:
> > I'm not sure that, as it stands, it is possible to get the "index",
> > unless we made the parallel iteration sequential.  Perhaps a better
> > solution would be to not return the index value.
> >
> > On Sun, Nov 22, 2015 at 4:06 PM, Cameron Hummels
> > <issues-reply at bitbucket.org> wrote:
> >> New issue 1147: Derived quantities `max_location` and `min_location`
> not returning correct index values
> >>
> https://bitbucket.org/yt_analysis/yt/issues/1147/derived-quantities-max_location-and
> >>
> >> Cameron Hummels:
> >>
> >> Something is going awry in the `max_location` and `min_location`
> derived quantities such that they return an incorrect index for where the
> extrema occur.  Here's an example script.  I tried correcting the code, but
> I got caught up in the chunk processing for derived quantities.
> >>
> >>
> >> ```
> >> #!python
> >>
> >> import yt
> >> import numpy as np
> >>
> >> ds = yt.load('IsolatedGalaxy/galaxy0030/galaxy0030')
> >> ad = ds.all_data()
> >> val, index, x, y, z = ad.quantities.max_location('density')
> >> print val == ad['density'][int(index)]  # false
> >>
> >> val2 = ad['density'].max()
> >> index2 = np.argmax(ad['density'])
> >> print val2 == ad['density'][index2] # true
> >>
> >> print val == val2 # true
> >> print index == index2 # false
> >>
> >> # Thus, something is going awry in the max_location (and the
> min_location)
> >> # derived quantities where it calculates the index values for the
> extrema
> >> ```
> >>
> >>
> >>
> >>
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-- 
Cameron Hummels
NSF Postdoctoral Fellow
Department of Astronomy
California Institute of Technology
http://chummels.org
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