[Yt-dev] pHOP in YT question

[Geoffrey So]

Hi this question is mainly for Stephen,

I was working on using the ellipsoid branch of YT to do some analysis on the
3200 cube haloes found.  I came across the same problem you described
earlier where calculating for tB_index I get nan sometimes, but this is
happening even after your fix.  I dug through and found out that this
happens because pHOP found some halo that have only 1 particle.

This becomes problematic with my geometric ellipsoid, it requires at least 4
particles distributed in different planes.  When the ellipsoid method is
trying to find a particle to satisfy the equation of an ellipse in 2D, it
will go through the list (of 1 element in this case), and find that it
returns nan for the length it calculates.  So even though the function
nanargmax() is used to pick out biggest value of non-nan element index, with
a list of only 1 nan, it still gets selected and the method fails.
 Currently my temporary solution is to tell the function that, if it
encounters a halo with less than 4 particles, to returns all 0s for the
ellipsoid attribute (A,B,C,e1x,y,z,tilt).

I was under the impression from our previous talk that there's a lowest
number threshold that pHOP requires to form a halo, I thought the number was
10 particles at least.  Am I just remembering things wrong?  It has been a
couple of years...

From
G.S.
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