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

[Nathan Goldbaum]

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 block has been replaced by one containing the temperature.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
>>>
>>
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