Hi Tazkera,

I think yt should be able to understand your code units and do the unit conversion for you, or easily configured to interpret your code units. See an example here. This I feel is the better solution.

But if you want to the conversion by hand, a quick fix to your code is to define new variables for domain width and domain center:

```

new_domain_width = ds.domain_width*((3.086e18*6e6*0.909090893796)/(0.7))

new_domain_center = ds.domain_center*((3.086e18*6e6*0.909090893796)/(0.7))

yt.ProjectionPlot(ds, 'z', ('gas', 'ElectronPressure'), width=new_domain_width[0],

center=new_domain_center)

```

The issue with your code is that ds.domain_width and ds.domain_center are not fields themselves. If you try to add new fields for those constant values, you'll get a long list of those values as you could read from the error message. You could print out dd['gas', 'PhysicalDomainWidth'] or dd['gas', 'PhysicalDomainCenter'] in your code to see what they look like. That might help you understand what was wrong.

Best,

Bili

On Thu, Oct 5, 2017 at 2:59 PM, tazkera haque <h.tazkera@gmail.com> wrote:

Dear yt people,

I am working on a tipsy output file from a n-body simulation. My file contains some scaled units for fields like density, coordinates, mass etc. To make a physical sense of the data, I had to convert those fields units to some physical units (c.g.s) and I derived new fields having cgs unit. for example:

dd = ds.all_data()

# derive a new domain width with conversion factor (6Mpc*current_time/hubble_constant)
def _NewDomainWidth(field, data):
    return ds.domain_width*((3.086e18*6e6*0.909090893796)/(0.7))

ds.add_field(('gas', 'PhysicalDomainWidth'), function=_NewDomainWidth, units="code_length")

# derive a new domain center with conversion factor (6Mpc*current_time/hubble_constant)
def _NewDomainCenter(field, data):
    return ds.domain_center*((3.086e18*6e6*0.909090893796)/(0.7))

ds.add_field(('gas', 'PhysicalDomainCenter'), function=_NewDomainCenter, units="code_length")

note that here "code_length" is actually in "cm". but I could not just do dd["Gas",'Coordinates'].in_units('cm'), because this conversion does not take into account some constant corrections and returns the wrong length magnitude.

My goal was to make sliceplot and projectionplot of a newly derived field :

# derive a new electron pressure field
def _ElectronPressure(field, data):
    return data['Gas','Density']*(5.49e-7)*data['Gas','Temperature']

ds.add_field(('gas', 'ElectronPressure'), function=_ElectronPressure, units="(code_mass*K)/(code_length**3)")

# integration of pressure field along line of sight
yt.ProjectionPlot(ds, 'z', ('gas', 'ElectronPressure'), width = dd['gas', 'PhysicalDomainWidth'],
                  center = dd['gas', 'PhysicalDomainCenter'] )

and it is returning me
ValueError                                Traceback (most recent call last)
<ipython-input-8-fa351b6c09d1> in <module>()
      1 yt.ProjectionPlot(ds, 'z', ('gas', 'ElectronPressure'), width = dd['gas', 'PhysicalDomainWidth'], 
----> 2                   center = dd['gas', 'PhysicalDomainCenter'] )
ValueError: operands could not be broadcast together with shapes (39678,) (54608,) 
Now I cannot use width =ds.domain_width and center = ds.domain_center for the same unit error. I was wondering how I can make the projectionplot for my newly derived field and newly derived domain width and domain center.
Your suggestion is most welcome
Best
Tazkera
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