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fof - Find groups in N-body simulations using the friends-of-friends method.


fof -e <Linking Length> [-m <nMinMembers>] [-dgs] [-v]
   [-o <Output Name>] [-p <xyzPeriod>]

   [-px <xPeriod>] [-py <yPeriod>] [-pz <zPeriod>]

Reads particles from stdin in the tipsy binary format.


-e <Linking Length>: No Default fEpsilon value!
This argument specifies the linking length used by the friends-of-friends method.

[-m <nMinMembers>]: Default nMinMembers = 8
This argument allows the user to specify what the minimum number of members (particles) per group should be. All groups with less than this value will be dicarded.

[-dgs]: Default: all particles considered (-dgs)
This set of flags allows the user to specify the particle types to be considered for grouping. If the user specifies -d, then only the dark matter particles are considered. With -dg, both dark and star particles are considered but any star particles are ignored. The default is, effectively -dgs.

[-v]: Default is no output
This flag allows the user to enable diagnostic output (on stdout). This includes the number of groups    found by the friends-of-friends method, the number of groups meeting the minimum members criterion and also the CPU time needed to find the groups.

[-o <Output Name>]: Default Output Name = "fof".
This allows the user to specify an alternate name for the output files. For example, if the user specifies -o sim5 on the command line, then fof will by default produce the file sim5.grp.

[-px <xPeriod>]: Default non-periodic in x dimension
Specifies that fof take into account periodicity in the x-dimension given by the period <xPeriod>.

[-py <yPeriod>] [-pz <zPeriod>]: As above.

[-p <xyzPeriod>]: Default non-periodic in x,y and z dimensions
Specifies that fof take into account periodicity in all three dimension given by the period <xyzPeriod>. This can be used in place of the above as a shorthand.


The fof program finds groups in N-body simulations using the friends-of-friends method. It rejects any groups found which have less than nMinMembers number of particles. A friends-of-friends group is one in which every particle in the group has a "friend" particle within a distance less than or equal to some specified "linking length". The linking length is set with the -e <Linking Length> argument.

Output File

fof.grp : This ASCII file is in TIPSY ARRAY format and contains the group number to which each particle in the input file belongs. Group number zero means this particle was not grouped. This file can be read in by tipsy or any other analysis tool able to read this format.


> fof -e 1.0e-4 -p 1 -o dark < dark.bin

This example groups all particles in the file dark.bin. The simulation is periodic with period length of 1.0 in each dimension. The minimum group members is taken to be the default value of 8 and the name of the output file is specified to be dark.grp.

> fof -e 1.0 -v -d -m 4 < gasrun.bin

This example groups only the dark matter particles in the input file gasrun.bin. The simulation state is treated as non-periodic, since there are no periods specified. Verbose (-v) ouput is requested. The number of minimum group members has been reduced to 4 and the name of the output file will be fof.grp.


If the linking length is set unreasonably large or if a large number of particles are found within a ball of radius the linking length, then the fof program will take VERY long to complete. The reason for this is that in such cases the algorithm used here degenerates to O(N^2). For typically desired linking lengths the fof program completes in a reasonable amount of time. Use AFOF (approximate friends-of-friends) when group finding with large linking lengths. The algorithm for AFOF does not suffer from this degeneracy and is an O(N) (linear scaling) method when some user defined fuzziness in the linking length can be tolerated.


Please report any!

See Also

afof(1) , denmax(1) , tipsy(1)

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