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

[Jared Coughlin]

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
>>>>
>>>
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