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

[Jared Coughlin]

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 block has been replaced by one containing the temperature.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