[yt-users] Problem with Units in a Derived Field

[Andrew Myers]

Hi Jared,

You probably don't want to loop over the entire array in pure python; that
could be very slow for large datasets. One thing you can do is use slicing
to select the elements of the array that meet your criterion:

indices = data['gas','InternalEnergy'] <= 1e4
data['gas', 'InternalEnergy'][indices] = *something*

that would set all the elements where the InternalEnergy field is less than
or equal to 1e4 to *something*. Usually you can express most operations
you'd want to do this way without explicitly looping over the elements.

-Andrew

On Tue, Aug 25, 2015 at 2:44 PM, Jared Coughlin <Jared.W.Coughlin.29 at nd.edu>
wrote:

> That seemed to have worked, thanks!
>
> I just have another quick question, if you wouldn't mind? In my function
> to generate my new field, there's an if statement that looks at the value
> of the temperature: if data['gas','InternalEnergy'] <= 1e4:
> However, I get the error: "The truth value of an array with more than one
> element is ambiguous. Use a.any() or a.all()." I wasn't sure how to loop
> over the elements of the array to access a specific element, or if that's
> even possible? Using a.any() or a.all() don't seem to fit the bill for this
> particular problem, either. Thanks!
> -Jared
>
> On Tue, Aug 25, 2015 at 3:19 PM, Nathan Goldbaum <nathan12343 at gmail.com>
> wrote:
>
>>
>>
>> On Tue, Aug 25, 2015 at 2:17 PM, Jared Coughlin <
>> Jared.W.Coughlin.29 at nd.edu> wrote:
>>
>>> Great, thanks! I tried that, and it gave me the error: "YTQuantity
>>> instances must be scalars." I'm not sure what to do about that, since all
>>> of these quantities depend on either temperature or density, both of which
>>> are arrays. I tried looking at the doc page for YTQuantity, but I didn't
>>> see anything that looked like it could solve this problem, though it's
>>> possible I missed it.
>>>
>>
>> You need to use YTArray instead. YTQuantity is a subclass of YTArray that
>> only holds scalars. YTArray can hold arrays of data with units.
>>
>>
>>> -Jared
>>>
>>> On Mon, Aug 24, 2015 at 8:13 PM, Nathan Goldbaum <nathan12343 at gmail.com>
>>> wrote:
>>>
>>>>
>>>>
>>>> On Monday, August 24, 2015, Cameron Hummels <chummels at gmail.com> wrote:
>>>>
>>>>> Hi Jared,
>>>>>
>>>>> The problem is one of units.  In order to make common arithmetic work
>>>>> like addition, yt requires both arguments going into the addition operator
>>>>> to have the same units.
>>>>>
>>>>> In your line,
>>>>>
>>>>> fHI = (recomb * ne) / (gamma_HI + (gamma_c *ne))
>>>>>
>>>>> you're adding two things (gamma_HI) and (gamma_c*ne) that have
>>>>> different units--in this case, something with (code_mass/code_length**3)
>>>>> units and something with no units defined as (1).
>>>>>
>>>>
>>>> Just a quick clarification: they need not have the same units, just the
>>>> same dimensions.  You can add two quantities with units of grams and solar
>>>> masses (for example) but not grams and liters.
>>>>
>>>>
>>>>>
>>>>> In order to fix this, you can re-assign the appropriate units to
>>>>> different arrays or quantities with the YTArray and YTQuantity classes.  In
>>>>> this case, make sure that the two arguments going into the addition have
>>>>> the same units.  If you want to recast "ne" to have number density units,
>>>>> you can do this:  from yt.units.yt_array import YTQuantity; ne =
>>>>> YTQuantity(ne, 'cm**-3').  For more info on units, check out
>>>>> http://yt-project.org/docs/dev/analyzing/units/index.html
>>>>>
>>>>> I hope this helps!
>>>>>
>>>>> Cameron
>>>>>
>>>>> On Mon, Aug 24, 2015 at 4:12 PM, Jared Coughlin <
>>>>> Jared.W.Coughlin.29 at nd.edu> wrote:
>>>>>
>>>>>> Hello! I have a gadget snapshot for which the standard internal
>>>>>> energy block has been replaced by one containing the temperature.  I'm
>>>>>> trying to calculate a derived field for the neutral hydrogen number
>>>>>> density,  but I'm getting an error when I try to do ds.add_field():
>>>>>>
>>>>>> http://paste.yt-project.org/show/5833/
>>>>>>
>>>>>> However, I get an error about being unable to add quantities with
>>>>>> differing units:
>>>>>>
>>>>>> http://paste.yt-project.org/show/5834/
>>>>>>
>>>>>> The docs say not to do any unit conversion because that is apparently
>>>>>> taken care of behind the scenes, so I didn't.  I was just wondering if
>>>>>> there was a way around this? Thanks!
>>>>>> -Jared
>>>>>>
>>>>>>
>>>>>> _______________________________________________
>>>>>> yt-users mailing list
>>>>>> yt-users at lists.spacepope.org
>>>>>> http://lists.spacepope.org/listinfo.cgi/yt-users-spacepope.org
>>>>>>
>>>>>>
>>>>>
>>>>>
>>>>> --
>>>>> Cameron Hummels
>>>>> NSF Postdoctoral Fellow
>>>>> Department of Astronomy
>>>>> California Institute of Technology
>>>>> http://chummels.org
>>>>>
>>>>
>>>> _______________________________________________
>>>> yt-users mailing list
>>>> yt-users at lists.spacepope.org
>>>> http://lists.spacepope.org/listinfo.cgi/yt-users-spacepope.org
>>>>
>>>>
>>>
>>> _______________________________________________
>>> yt-users mailing list
>>> yt-users at lists.spacepope.org
>>> http://lists.spacepope.org/listinfo.cgi/yt-users-spacepope.org
>>>
>>>
>>
>> _______________________________________________
>> yt-users mailing list
>> yt-users at lists.spacepope.org
>> http://lists.spacepope.org/listinfo.cgi/yt-users-spacepope.org
>>
>>
>
> _______________________________________________
> yt-users mailing list
> yt-users at lists.spacepope.org
> http://lists.spacepope.org/listinfo.cgi/yt-users-spacepope.org
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.spacepope.org/pipermail/yt-users-spacepope.org/attachments/20150825/d1524961/attachment.htm>