[Numpy-discussion] loadtxt and usecols

[Sebastian Berg]

On Di, 2015-11-10 at 10:24 +0100, Irvin Probst wrote:
> On 10/11/2015 09:19, Sebastian Berg wrote:
> > since a scalar row (so just one row) is read and not a 2D array. I tend
> > to say it should be an array-like argument and not a generalized
> > sequence argument, just wanted to note that, since I am not sure what
> > matlab does.
> 
> Hi,
> By default Matlab reads everything, silently fails on what can't be 
> converted into a float and the user has to guess what was read or not.
> Say you have a file like this:
> 
> 2010-01-01 00:00:00         3.026
> 2010-01-01 01:00:00         4.049
> 2010-01-01 02:00:00         4.865
> 
> 
>  >> M=load('CONCARNEAU_2010.txt');
>  >> M(1:3,:)
> 
> ans =
> 
>     1.0e+03 *
> 
>      2.0100         0    0.0030
>      2.0100    0.0010    0.0040
>      2.0100    0.0020    0.0049
> 
> 
> I think this is a terrible way of doing it even if newcomers might find 
> this handy. There are of course optionnal arguments (even regexps !) but 
> to my knowledge almost no Matlab user even knows these arguments are there.
> 
> Anyway, I made a PR here https://github.com/numpy/numpy/pull/6656 with 
> usecols as an array-like.
> 

Actually, it is the "sequence special case" type ;). (matlab does not
have this, since matlab always returns 2-D I realized).

As I said, if usecols is like indexing, the result should mimic:

arr = np.loadtxt(f)
arr = arr[usecols]

in which case a 1-D array is returned if you put in a scalar into
usecols (and you could even generalize usecols to higher dimensional
array-likes).
The way you implemented it -- which is fine, but I want to stress that
there is a real decision being made here --, you always see it as a
sequence but allow a scalar for convenience (i.e. always return a 2-D
array). It is a `sequence of ints or int` type argument and not an
array-like argument in my opinion.

- Sebastian


> Regards.
> _______________________________________________
> NumPy-Discussion mailing list
> NumPy-Discussion at scipy.org
> https://mail.scipy.org/mailman/listinfo/numpy-discussion
> 

-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 819 bytes
Desc: This is a digitally signed message part
URL: <http://mail.python.org/pipermail/numpy-discussion/attachments/20151110/b11438a3/attachment.sig>