[yt-users] cut_region: can I make an expression dimensionless?

[Matthew Turk]

Hi Stuart,

Actually the more I look at this, the more I think it's probably quite easy
to turn this into a data selector itself, avoiding the need for a cut
region.  Should be straightforward to add it to selection_routines and
selection_data_containers and have it work out of the box.

On Fri, Jan 16, 2015 at 6:20 PM, Matthew Turk <matthewturk at gmail.com> wrote:

>
>
> On Wed, Jan 14, 2015 at 11:03 AM, Stuart Levy <salevy at illinois.edu> wrote:
>
>> Using one of Sam's past e-mails to answer my own question, it at least
>> syntactically works to use the '.uq' unit-quantity attribute of a YT object
>> to get something that has the units of the object, so e.g. 0.78 *
>> obj['x'].uq is 0.78 in x's length units.  So it actually seems to work to
>> say:
>>
>>    cr = ds.cut_region( dd, ["obj['z']-(obj['x']**2 +
>> obj['y']**2)/(2.0*obj['z'].uq) < 0.78*obj['z'].uq"])
>>
>> Neato!
>>
>>
>> As it turns out, I'll look for another lower-level way to do this - with
>> the above cut_region applied, 32GB of RAM isn't enough to process a single
>> data field of about 512^3, and it takes several minutes of CPU time even to
>> start.    But it's good to know that the above is possible.
>
>
> Glad you kind of got it working ... this is kind of ridiculous that it
> takes so much, though.  What I think could cut it dowen considerably is to
> reduce the numebr of sequential numpy operations; right now it's computing
> them and storing temporary arrays like crazy.  Unfortunately I don't know
> how one might do this with the current setup of cut_region.
>
> Perhaps one way would be to make a derived field that is each component,
> and inside it do each operation inline, then apply the conditional there?
>
>
>>
>>
>> On 1/14/15 10:26 AM, Stuart Levy wrote:
>>
>>> Hello yt people,
>>>
>>> I'm hoping to write a cut_region expression that (a) depends on position
>>> and (b) does it in a nonlinear way - I'd like to do a sort of paraboloidal
>>> cut, like
>>>
>>>     ds.cut_region(dd, "obj['z'] - (obj['x']**2 + obj['y']**2)/2.0 <
>>> 0.78")
>>>
>> er, correction - cut_region()'s 2nd arg is a [] list of strings rather
>> than a string, so I'd tried
>>
>>
>>           ds.cut_region(dd, ["obj['z'] - (obj['x']**2 + obj['y']**2)/2.0
>> < 0.78"])
>>
>>
>>> But this runs afoul of the unit-checking - obj['z'] doesn't have the
>>> same units as obj['x']**2.
>>>
>>> Somehow I either need to cast all the obj[] terms to be dimensionless
>>> ("trust me, I promise it's right"), or else give dimensions of length to
>>> the constants.   Should there be a way to do either one?
>>>
>>
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>>
>
>
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