[AstroPy] Changing table column from float64 to float32

[Peter Weilbacher]

Hi Ivan,

I have no particular insight into this, but it seems to me that since
you created the Column objects with a specific format, assigning any
data to it will cause it to be casted to match that. (Otherwise the
underlying memory buffer does not match.)

Instead, I guess you could create a new column with the desired type and
format and recreate the table using that:

c1_single = fits.Column(name='a', array=table_hdu.data['a'].astype(np.single), format='E')
table_hdu2 = fits.BinTableHDU.from_columns(c1_single + table_hdu.columns[1:])

(It appears that you can even remove the .astype() from this and it
still works.)

   Peter.

On Mon, 11 Jul 2022, Ivan Valtchanov wrote:

> Hi all,
> I have been trying to change a FITS BinTableHDU column from float64 to
> float32. I tried three obvious paths to do so and they all failed. Here is
> a very simple code to check.
>
> ####################
> import numpy as np
> from astropy.io import fits
>
> # create a simple table with float64 and float32
> c1 = fits.Column(name='a', array=np.array([1,
> 2],dtype=np.double),format='D')
> c2 = fits.Column(name='b', array=np.array([3,
> 4],dtype=np.single),format='E')
> table_hdu = fits.BinTableHDU.from_columns([c1, c2])
> #
> # check columns types
> for j in table_hdu.columns:
>     print (j.name,j.dtype)
> #a float64
> #b float32
> #=================
> # first attempt to change float64 to float32
> #
> table_hdu.data['a'] = table_hdu.data['a'].astype(np.single)
> #
> for j in table_hdu.columns:
>     print (j.name,j.dtype)
> # Nope
> #a float64
> #b float32
> #=================
> # second attempt, also obvious
> #table_hdu.data['a'].dtype = np.single
> #
> # Value error:  To change to a dtype of a different size, the array must be
> C-contiguous
> # not sure what this means...
> #==================
> # third attempt, list comprehension and creating a numpy array of type
> float32
> xx = np.array([float(v) for v in table_hdu.data['a']],dtype=np.single)
> print (xx.dtype)
> # float32
> # Good, it's float32
> table_hdu.data['a'] = xx
> #
> for j in table_hdu.columns:
>     print (j.name,j.dtype)
> # Nope, still a is float64
> #a float64
> #b float32
> ############
>
> Is there something I'm missing?
>
> Using python 3.10, astropy 5.1, numpy 1.22.4
>
> Thanks,
> Ivan
>

-- 
Dr. Peter M. Weilbacher           https://www.aip.de/Members/pweilbacher
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