[yt-users] How to use if, elif, else statements in creating a derived field

[Suoqing JI]

Hi Reju,

The first thing occurs to me is that the area of white patch contains no positive field value, so it can not be plotted in log scale.

Also, about the way of defining a derived field, I might prefer the following way, for example:

@derived_field(name = "MyField")
def my_new_field(field, data):
    tmp_data = (data["MachNumber"] <= 2) * (.5 * data["MachNumber"]**2. - 1)
    tmp_data = tmp_data + (data["MachNumber"] > 2) * (data["MachNumber"] <= 100) * ((.5* (data["MachNumber"])**4) + (.5* (data["MachNumber"])**3))
    tmp_data = tmp_data + (data["MachNumber"] > 100) * 5.446361E-01
    return tmp_data

Best wishes,
Suoqing

On Oct 26, 2013, at 10:27 PM, Reju Sam John <rejusamjohn at gmail.com> wrote:

> Dear Matt,
> Thanks for the help. Now my problem with the code solved. But the slice plot of the new field has some problem -- white patch are coming as shown below. What may be the reason? 
> <output.png> 
> 
> 
> On Sat, Oct 26, 2013 at 7:59 PM, Matthew Turk <matthewturk at gmail.com> wrote:
> Hi Reju,
> 
> On Sat, Oct 26, 2013 at 2:50 AM, Reju Sam John <rejusamjohn at gmail.com> wrote:
> > Dear Kacper,
> >
> > My logic was wrong, and yours is right. But with the following code
> >
> > @derived_field(name = "MyField")
> > def my_new_field(field, data):
> >     temp = data["MachNumber"].copy().fill(5.446361E-01)
> >     #print temp
> >
> >     ind = np.where(data["MachNumber"] < 2)
> >     temp[ind] = 1.9564E-3*((data["MachNumber"][ind]**2) - 1)
> >     ind = np.where((data["MachNumber"] >= 2) and (data["MachNumber"]) < 100)
> >     temp[ind] =
> > (5.699327E-1*((data["MachNumber"][ind]-1)**4))/(data["MachNumber"][ind]**4)
> > - (3.337557E-1*
> > ((data["MachNumber"][ind]-1)**3))/((data["MachNumber"][ind])**4) +
> > (4.173271E+0*
> > ((data["MachNumber"][ind]-1)**2))/((data["MachNumber"][ind])**4) -
> > (9.775620E+0* ((data["MachNumber"][ind]-1)))/((data["MachNumber"][ind])**4)+
> > 5.455605E+0/(data["MachNumber"][ind])**4
> >     return temp
> >
> > I am getting this  Traceback
> >
> >  line 41, in my_new_field
> >     temp[ind] = 1.9564E-3*((data["MachNumber"][ind]**2) - 1)
> > TypeError: 'NoneType' object does not support item assignment
> >
> > Would you suggest any solution?
> 
> I believe the problem is in your .fill() statement.  That doesn't
> return an array, so you need to do it after the assignment.  For
> instance:
> 
> temp = data["MachNumber"].copy()
> temp.fill(5.446361E-01)
> 
> -Matt
> 
> >
> >
> >
> > On Mon, Oct 21, 2013 at 2:58 PM, Kacper Kowalik <xarthisius.kk at gmail.com>
> > wrote:
> >>
> >> On 10/21/2013 11:10 AM, Reju Sam John wrote:
> >> > Dear all,
> >> >
> >> > I would like to create a derived field which should return a value
> >> > according to specified conditions. My definition of new field is shown
> >> > below.. But it is giving error.
> >> >
> >> >
> >> > @derived_field(name = "MyField")
> >> > def my_new_field(field, data):
> >> >     if data.pf["MachNumber"] < 2 :
> >> >         return (.5* (data["MachNumber"])**2) - 1
> >> >     elif data.pf["MachNumber"] < 100 :
> >> >         return (.5* (data["MachNumber"])**4) + (.5*
> >> > (data["MachNumber"])**3)
> >> >     else:
> >> >        return 5.446361E-01
> >> >
> >> >
> >> >
> >> > Please suggest me how to implement if, elif, else statements in creating
> >> > a
> >> > derived field.
> >>
> >> Hi,
> >> your derived fields looks fine (except for missing space in the
> >> indentation of the last statement). Could you attach the backtrace?
> >>
> >> One thing that's confusing me is 'data.pf["MachNumber"]'. Do you have a
> >> global parameter with such name defined or you'd rather create a MyField
> >> based on local value of "MachNumber" field?
> >> If the latter you could try something like this:
> >>
> >> @derived_field(name = "MyField")
> >> def my_new_field(field, data):
> >>     temp = data["MachNumber"].copy().fill(5.446361E-01)
> >>     ind = np.where(data["MachNumber"] < 2)
> >>     temp[ind] = 0.5 * data["MachNumber"][ind] ** 2 - 1.0
> >>     ind = np.where((data["MachNumber"] >= 2) &
> >>                    (data["MachNumber"]) < 100))
> >>     temp[ind] = 0.5 * data["MachNumber"][ind] ** 4 + \
> >>                 0.5 * data["MachNumber"][ind] ** 3
> >>     return temp
> >>
> >>
> >> Cheers,
> >> Kacper
> >>
> >>
> >>
> >> _______________________________________________
> >> yt-users mailing list
> >> yt-users at lists.spacepope.org
> >> http://lists.spacepope.org/listinfo.cgi/yt-users-spacepope.org
> >>
> >
> >
> >
> > --
> > Reju Sam John
> >
> > _______________________________________________
> > yt-users mailing list
> > yt-users at lists.spacepope.org
> > http://lists.spacepope.org/listinfo.cgi/yt-users-spacepope.org
> >
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> 
> 
> 
> -- 
> Reju Sam John
> _______________________________________________
> yt-users mailing list
> yt-users at lists.spacepope.org
> http://lists.spacepope.org/listinfo.cgi/yt-users-spacepope.org

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