ParticleIDs in Illustris-1-Dark are too large

Mohammed Khedr
  • 17 Dec '16

I'm doing calculations on some of the particles belonging to specific halos and to do so I need to make an array with size comparable to the ID number of those particles but they are too large. Is there any way I can reduce them without losing their uniqueness?

Dylan Nelson
  • 2
  • 20 Dec '16

Hi Mohammed,

You are right, the general idea of a "ID to Index Map" is made difficult by the large range the IDs span, as most approaches need to allocate an array at least of size max(ids)-min(ids).

The typical alternative is to sort. You can always sort the global ID list, and the sort indices are then such a mapping. Indeed the sort indices are exactly a re-numbering of the particles in the range [0,N-1], and you can then allocate an array covering them (e.g. 22.5GB for int32 of this size for Illustris-1).

If this is too vague perhaps you can say more about what you're trying to do and I can give a more specific suggestion.

Mohammed Khedr
  • 14 Jan '17

Hi Dylan,

I'm trying to trace certain FoF group's particles IDs and Coordinates throughout the snapshots, is there a simple way to do that directly?


Dylan Nelson
  • 14 Jan '17

Following the tutorial in the Example Scripts page you load the main progenitor branch, including the SnapNum and SubfindID fields. Then just below you load particles from a FoF group. Putting these two together you can load the associated particles back in time following the tree.

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