Define a system of units

In order to define a system of units, we need at least to define three numbers, which fix the units of length, the units of mass and the units of time.

In pNbody, there is two way to define a system of units, either constructing it directely from the object constructor UnitSystem(), or using the function Set_SystemUnits_From_Params(),

Define a system of unit using UnitSystem()

In this first possibility ,we decide explicitely what are the basic units of our system of units. We need here to use units defined in the pNbody.units module as pNbody.units.Units objects

TODO: SOMETHING MISSING HERE?!

This gives:

>>> from pNbody import units
>>> system_of_units = units.UnitSystem('local',[units.Unit_kpc,  10**10*units.Unit_Ms, units.Unit_Myr])

Here, the first argument is the name of the unit system (useless) and the second, a list of basic units. In this list, the first argument must be a unit length, the second a mass unit and the third a time unit.

To check we can call the UnitSystem.info() method:

>>> system_of_units.info()
units info
>>> UnitLength_in_cm         = 3.085e+21
>>> UnitVelocity_in_cm_per_s = 9.78247e+07
>>> UnitMass_in_g            = 1.9891e+43

With gives the values of our units as a function of the basic units: centimeters, gramm and centimeter per second.

Define a system of unit using Set_SystemUnits_From_Params()

Another way to define a system of unit is defined giving at least three numbers, corresponding to the value of respectively centimeter (UnitLength_in_cm), gramm (UnitMass_in_g) and centimeter per second (UnitVelocity_in_cm_per_s) in the desired system of unit. All are passed to the function Set_SystemUnits_From_Params() using a dictionary:

>>> from pNbody import units
>>> params = {}
>>> params['UnitLength_in_cm']         = 3.085e+21
>>> params['UnitVelocity_in_cm_per_s'] = 9.78247e+07
>>> params['UnitMass_in_g']            = 1.9891e+43
>>> system_of_units = units.Set_SystemUnits_From_Params(params)
>>> system_of_units.info()
units info
  UnitLength_in_cm         =3.085e+21
  UnitVelocity_in_cm_per_s =9.78247e+07
  UnitMass_in_g            =1.9891e+43

Define a system of unit using Set_SystemUnits_From_File()

Here, we use the information from either a Gadget parameter files containing similar lines:

UnitLength_in_cm                              3.085678e+21
UnitMass_in_g                                 1.989e+43
UnitVelocity_in_cm_per_s                      100000.0

or a pNbody units parameter file, containing the following lines:

UnitLength_in_cm
UnitLength in cm
Float
3e+21

UnitMass_in_g
UnitMass in g
Float
3e+43

UnitVelocity_in_cm_per_s
UnitVelocity in cm per s
Float
20000000.

In both case, we use the following command:

>>> from pNbody import units
>>> system_of_units = units.Set_SystemUnits_From_File(unitfile)