[AstroPy] question on matplotlib's loadtxt

[Miguel de Val-Borro]

Hi Grigoris,

On Fri, Nov 04, 2011 at 03:56:21PM +0200, Grigoris Maravelias wrote:
> Suppose that we have a file (called "test.test") with 6 columns like:
> 
> 1496548.  862.235400937  14.008  0  20110523.201416  tth*g
> 1690289.  919.424007603  13.875  0  20110523.221527  hgf4
> 1667241.  996.262754039  13.890  0  20110524.001639  nb.mj
> 739181.6  881.1753527  14.774  0  20110524.010203  vbfhg
> 
> When I use
> x, y = loadtxt('test.test', unpack=True, usecols=(0,1), dtype=(float,float))
> 
> it prints normally the first and second column.
> If I add another column like:
> 
> x, y, z = loadtxt('test.test', unpack=True,usecols=(0,1,3), 
> dtype=(float,float,float))

This seems to work because the default dtype in loadtxt is float:
x, y, z = numpy.loadtxt('test.test', unpack=True,usecols=(0,1,3))

> Additionaly, when I am trying to load the last column as strings:
> x, y = loadtxt('test.test', delimiter='  ', unpack=True,usecols=(0,5), 
> dtype=(float,'S5'))
> 
> I get this error:
>    File "/usr/lib/python2.7/site-packages/numpy/lib/io.py", line 584, in 
> loadtxt
>      dtype = np.dtype(dtype)
> ValueError: mismatch in size of old and new data-descriptor

I'm not sure about this error message. I suggest that you try
numpy.genfromtxt which is a very convenient function to read text data
from a file into a recarray. There are several examples with mixed data
types in the documentation. For example specifying the dtypes:

data  = numpy.genfromtxt('test.test', dtype="f8,f8,f8,f8,f8,S5")

Then the last column could be accessed as data['f5']

Miguel