[AstroPy] Constructing Angle from a tuple specified as sign string plus numbers for degrees, arcmin, arcsec

[Erik Bray]

On 08/10/2015 05:53 PM, Thomas Robitaille wrote:
> Erik Bray wrote:
>> On 08/10/2015 05:26 PM, Eric L. N. Jensen wrote:
>>> Hi all,
>>>
>>> I’m working with a table from CDS (i.e. from an ApJ paper) that specifies the declination coordinate in four separate columns:  a string for the sign of the declination, and three numerical columns for degrees, arcminutes, and arcseconds.  Is there a straightforward way to combine these into a single astropy.coordinates Angle object?  The standard Angle function (constructor?) only accepts a tuple with three entries.
>>>
>>> I guess what I’m looking for is the inverse method of ‘signed_dms’.
>>>
>>> If it’s of use, the table I’m working with is here:  http://iopscience.iop.org/0067-0049/184/1/18/fulltext/apjs300673t4_mrt.txt
>>>
>>> Thanks in advance for your help with this,
>>
>> It seems a bit strange that the table format in this case indicates the sign of
>> the declination as a completely separate field, rather than as just part of the
>> numeric string.  But I'm not intimately familiar with the quirks of the CDS
>> table format.  Is that normal?  I wonder if there's a way to get the CDS reader
>> to format those fields as a single column.  Tom Aldcroft would know better.
>>
>> You can instantiate an Angle array using a generator, so you could probably just
>> slurp in the data using a generator expression, allowing you to transform the
>> values one a time while creating the Angle rather than transforming the table
>> all at once first (please bear with the LISPiness of this example):
>>
>> dec = Angle(((int(row[0] + str(row[1])), row[2], row[3])
>>                for row in table['DE-', 'DEd', 'DEm', 'DEs'))
>>
>>
>> # (Note: originally I used row[2:] above, but there seems to be an unrelated bug
>> involved in taking slices of rows...I'll report on that.)
>
> In case you want to do this without loops, np.char.add can be used to
> concatenate string arrays element-wise. So for instance you could
> concatenate the sign column and the degree column of the declination with
>
> np.add(t['DE-'], t['DEd'])
>
> then you would end up with three arrays instead of four.

This doesn't work because t['DE-'] is a string column, and t['DEd'] is a 
(masked) integer column.

In any case, doing a row-wise transformation allows doing this with one fewer loop.

Erik