[yt-users] Creating Derived Fields and Plotting Error

Hilary Egan hilaryye at gmail.com
Wed Jul 22 12:15:51 PDT 2015


Hi Madison,

For whatever reason, yt doesn't convert units that have the same dimensions
within maximum so you'll have to manually put your zero velocity in the
same units as the data['radial_velocity']. You can either do this:

zero_vel = data.ds.quan(0.0, 'cm/s')new_field =
np.maximum(data['radial_velocity'], zero_vel.in_units('code_velocity))

or since you're just comparing to a value of zero:

zero_vel = data.ds.quan(0.0, 'code_velocity')

new_field = np.maximum(data['radial_velocity'], zero_vel)



Not sure what you mean by it reverses the order of the problem, but
hopefully that helps!

-Hilary

On Wed, Jul 22, 2015 at 10:05 AM Madison Fitzgerald <fitzg152 at msu.edu>
wrote:

> Hi Hilary and other yt users,
>
> It solved my convert_to_units problem! However, it does raise the error
> you said it would, the unit comparison problem, and even using your
> suggestion, I’m not able to resolve the issue. I’ve also tried using the
> suggestions from the FAQ (
> http://yt-project.org/docs/3.1/faq/index.html?highlight=convert%20units#how-do-i-get-the-convert-between-code-units-and-physical-units-for-my-dataset),
> but when I do that, it then reverses the order of the problem (saying
> code_velocity and then cm/s in my error output).
>
> Updated code (http://paste.yt-project.org/show/5736/) and updated error
> message (http://paste.yt-project.org/show/5735/).
>
> Thank you,
> Madison
>
>
> Madison Fitzgerald
>
> Astrophysics | Lyman Briggs College
> Women’s & Gender Studies | College of Arts & Letters
> LGBTQ and Sexuality Studies Specialization
> Women in Science, President
> Undergraduate Teaching Assistant
> Undergraduate Research Assistant
> Department of Physics and Astronomy
> Honors College
> Michigan State University
>
> On Jul 22, 2015, at 11:01 AM, Hilary Egan <hilaryye at gmail.com> wrote:
>
> Hi Madison,
>
> When using the data object within a derived field, you have to specify
> what field you want to access. So line 13 should look like
>
> new_field = np.maximum(data['radial_velocity'], 0.0)
>
>
> Right now the field is trying to find the maximum of the YTSphere object
> rather than the radial velocity data contained within the YTSphere, which
> isn't well defined.
>
> One other quick comment, once that's fixed your script is going to try and
> compare data['radial_velocity'] which has units of velocity, and 0.0 which
> is unitless. In order for that to be a valid comparison, you should give
> 0.0 in the appropriate units by doing something like this:
>
> zero_vel = data.ds.quan(0.0, 'km/s')
>
> new_field = np.maximum(data['radial_velocity'], zero_vel)
>
>
> Hope that helps!
> -Hilary
>
> On Wed, Jul 22, 2015 at 8:49 AM Madison Fitzgerald <fitzg152 at msu.edu>
> wrote:
>
>> Hello,
>>
>> I am trying to create two new derived fields and then make a simple phase
>> plot using one of these fields, but I am running into a pretty consistent
>> error involving “convert_to_units". I get basically the same error when
>> using the ds.sphere object and a ds.all_data object, so if there’s
>> something I’ve messed up on, it’s not obvious to me and I would appreciate
>> any help figuring out what’s wrong.
>>
>> I am running yt on a MacBook Pro with OS 10.10.4. I have the newest
>> version of stable yt (3.1 and changeset 665e2b6cdf78), and you can find my
>> script at this link (http://paste.yt-project.org/show/5733/) and my
>> error output at this link (http://paste.yt-project.org/show/5734/).
>>
>> Thank you very much,
>> Madison Fitzgerald
>>
>>
>>
>> Madison Fitzgerald
>>
>> Astrophysics | Lyman Briggs College
>> Women’s & Gender Studies | College of Arts & Letters
>> LGBTQ and Sexuality Studies Specialization
>> Women in Science, President
>> Undergraduate Teaching Assistant
>> Undergraduate Research Assistant
>> Department of Physics and Astronomy
>> Honors College
>> Michigan State University
>>
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